essay about the importance of homeostasis within the human body

Homeostasis and Regulation in the Human Body Essay

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Homeostasis is the ability of the body system to maintain a balance or equilibrium internally against external forces. It is an organism attempt to persistently monitor and adjust internally as the external environment changes. Both animals and human beings require this process to maintain desirable body temperature, blood pressure, and proper levels of nutrients in their body system. Maintaining homeostasis requires that the body correspond with the mechanism and system to establish a favorable and stable internal environment.

The endocrine system performs a significant role in homeostasis since hormones control the operation of body cells. The maintenance and stability of body cells occurs by means of feedback loops that regulate the body’s internal environment. It helps restore back any important deviation from a set point value. The feedback loop basic components include the receptor; also known as a sensor, detects changes in the environment (Suchacki et al., 2017). The control center integrates the information from the receptor, and later sends appropriate signals to the effector. The sensor and control center helps to identify the role of feedback loop to homeostasis.

To maintain homeostasis, an effector organ, causes a significant change, to either increase the change to the system, or decrease the situation, and return it to desirable level. The best example is the kidney which retains water when blood pressure is low in the body. Ways of communication amidst the basic components of feedback loop are essential in order for it to operate effectively which usually happen through hormones or nerves. The maintenance and stability of body cells occur through feedback loops that regulate the body’s internal environment. The feedback loop essential components include the receptor, also known as a sensor, which detects changes in the body

Remember that homeostasis is the ability of the body to maintain stable and balanced internal conditions. When alter in the environment or a stimuli is detected, feedback loops act to keep body system functioning at the ideal point. Feedback is a result or a reaction of a loop that impact or affects the stimulus or input. Typically feedback loops are divided into two types that control physiological system in the human body.

To start with, negative feedback loops are important mechanism to maintain homeostasis in the body. Negative feedback helps reduce or drop an excessive stimulus and maintain the variable within the set point. In other words, if a level increases, the body reacts to bring it down, and if a level decreases, the body reacts to bring it up. For instance, in the maintenance blood glucose level, specific endocrine cells of pancreas sense the level of glucose in the body.

The cells; alpha and beta, react appropriately to maintain an ideal level of blood glucose. If the level of blood glucose is high, pancreatic cells produce hormone insulin into the blood. The insulin relays signals to other organs such as the liver cells, to remove excess glucose, until blood glucose drops to the normal level (Roskoski, 2017). On the other hand, if blood glucose level drops below normal point, pancreatic cells produce hormone glucagon. This hormone helps stored glycogen to be broken down to glucose and increase blood glucose level to the normal range. Negative feedback helps explain why they are predominant mechanism to control blood glucose.

Other example of negative feedback is thermoregulation; the human set point in temperature is 37 degrees Celsius and anything below or beyond this level can lead to disease. Luckily, human body is a self-regulatory system where if body temperatures are high, the brain stimulates vasodilation which allows the sweat glands to release more heat, and blood vessels near the skin surface dilate thus cooling the epidermis (Samanta et al., 2017). In contrast, if the body temperature drops below the normal range and is exposed to cold, the blood stops flowing near the skin cover and allows the heat to be trapped in the body core and prevent heat loss.

Positive feedback increases a change in the physiological process of the body rather than decreasing it. Positive feedback is less common in homeostasis because it accelerates the direction of the stimuli until an endpoint is arrived. For instance, when one is injured, blood from the injured site produces chemical that attracts platelets to aid in clotting. Positive feedback speeds the process of clotting because if more chemicals are released, more platelets will be attracted, and a mass a large clot to cease the bleeding. when mothers’ breastfeeds, the hormone prolactin is triggered by the baby suckles resulting to more milk production.

Positive feedback also occurs during childbearing; oxytocin triggers a labor contraction which pushes the baby against the cervix which eventually dilates to allow the baby to pass. Therefore, positive feedback loop aids to evaluate how the mechanism plays an important role to control physiological systems. In conclusion, homeostasis is important to human survival and functionality; together with feedback loops and their components, body system is able to remain stable and work well.

Suchacki, K. J., Roberts, F., Lovdel, A., Farquharson, C., Morton, N. M., MacRae, V. E., & Cawthorn, W. P. (2017). Skeletal energy homeostasis: a paradigm of endocrine discovery . J Endocrinol , 234 (1), R67-79. Web.

Samanta, D., Prabhakar, N. R., & Semenza, G. L. (2017). Systems biology of oxygen homeostasis . Wiley Interdisciplinary Reviews: Systems Biology and Medicine , 9 (4), e1382. Web.

Roskoski Jr, R. (2017). Vascular endothelial growth factor (VEGF) and VEGF receptor inhibitors in the treatment of renal cell carcinomas . Pharmacological Research , 120 , 116-132. Web.

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Home — Essay Samples — Nursing & Health — Homeostasis — The importance of homeostasis within the human body

The Importance of Homeostasis Within The Human Body

• Categories: Homeostasis

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Published: Jan 4, 2019

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Works Cited

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell (4th ed.). Garland Science.
• Guyton, A. C., & Hall, J. E. (2006). Textbook of Medical Physiology (11th ed.). Saunders.
• Widmaier, E. P., Raff, H., & Strang, K. T. (2008). Vander's Human Physiology: The Mechanisms of Body Function (11th ed.). McGraw-Hill.
• Silverthorn, D. U. (2010). Human Physiology: An Integrated Approach (5th ed.). Pearson.
• Saladin, K. S. (2018). Anatomy & Physiology: The Unity of Form and Function (8th ed.). McGraw-Hill Education.
• Sherwood, L. (2015). Human Physiology: From Cells to Systems (9th ed.). Cengage Learning.
• Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). Wiley.
• Pocock, G., Richards, C., & Richards, D. (2006). Human Physiology: The Basis of Medicine (3rd ed.). Oxford University Press.
• Johnson, L. R. (Ed.). (2000). Essential Medical Physiology. Academic Press.
• Berne, R. M., Levy, M. N., Koeppen, B. M., & Stanton, B. A. (2017). Physiology (7th ed.). Elsevier.

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High school biology

Course: high school biology   >   unit 8, homeostasis.

• Tissues, organs, & organ systems
• Body structure and homeostasis review
• Body structure and homeostasis

• Homeostasis is the tendency to resist change in order to maintain a stable, relatively constant internal environment.
• Homeostasis typically involves negative feedback loops that counteract changes of various properties from their target values, known as set points .
• In contrast to negative feedback loops, positive feedback loops amplify their initiating stimuli, in other words, they move the system away from its starting state.

Introduction

Maintaining homeostasis.

• One is activated when a parameter—like body temperature—is above the set point and is designed to bring it back down.
• One is activated when the parameter is below the set point and is designed to bring it back up.

Homeostatic responses in temperature regulation

Disruptions to feedback disrupt homeostasis..

• Muscle and fat cells don't get enough glucose, or fuel. This can make people feel tired and even cause muscle and fat tissues to waste away.
• High blood sugar causes symptoms like increased urination, thirst, and even dehydration. Over time, it can lead to more serious complications. 4 , 5 ‍  

Positive feedback loops

Attribution.

• " Homeostasis " by OpenStax College, Biology, CC BY 4.0 ; download the original article for free at http://cnx.org/contents/[email protected]
• " Homeostasis " by OpenStax College, Anatomy & Physiology, CC BY 4.0 ; download the original article for free at http://cnx.org/contents/[email protected] .
• " The endocrine pancreas " by OpenStax College, Anatomy & Physiology, CC BY 4.0 ; download the original article for free at http://cnx.org/contents/[email protected]

Works cited

• "Human Body Temperature," Wikipedia, last modified June 18, 2016, https://en.wikipedia.org/wiki/Human_body_temperature .
• "Circadian Rhythm," WIkipedia, last modified June 29, 2016, https://en.wikipedia.org/wiki/Circadian_rhythm .
• David E. Sadava, David M. Hillis, H. Craig Heller, and May Berenbaum, "Physiology, Homeostasis, and Temperature Regulation," in Life: The Science of Biology , 9th ed. (Sunderland: Sinauer Associates, 2009), 847.
• "Causes of Diabetes," National Institute of Diabetes and Digestive and Kidney Diseases, last modified June 2014, http://www.niddk.nih.gov/health-information/health-topics/Diabetes/causes-diabetes/Pages/index.aspx .
• Mayo Clinic Staff, "Hyperglycemia in Diabetes," last modified April 18, 2015, Mayo Clinic, http://www.mayoclinic.org/diseases-conditions/hyperglycemia/basics/definition/con-20034795 .

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What Is Homeostasis in Biology? Definition and Examples

Homeostasis is a fundamental concept in biology that refers to the self-regulating process by which biological systems maintain stability while adjusting to changing conditions. This stability, or equilibrium, is essential for organisms to function effectively and efficiently.

Simple Definition of Homeostasis

Homeostasis is the ability of an organism to maintain a stable internal environment despite changes in external conditions. This process involves various biological mechanisms that detect changes, trigger responses, and restore balance. Examples of things that homeostasis controls include body temperature, chemical energy, pH levels, oxygen levels, blood pressure, and blood sugar.

Origin and History of Discovery

The word “homeostasis” originates from the Greek words ‘homeo,’ meaning similar, and ‘stasis,’ meaning standing still. Walter Cannon, an American physiologist, coined the term in the early 20th century. He built upon the work of Claude Bernard, a French physiologist who first recognized the concept of an internal milieu in the mid-19th century.

Components of Homeostasis

Homeostasis involves three primary components:

• Receptors : These are structures that detect changes in the environment (internal or external) and send this information to the control center.
• Control Center : Usually the brain or endocrine system, it processes the information and determines the appropriate response.
• Effectors : These are organs or cells that enact the response determined by the control center, thereby restoring balance.

A classic example of homeostasis involving receptors, control center, and effectors is the regulation of blood glucose levels in the human body. This process maintains the energy supply to cells and is tightly controlled.

1. Receptors: Detecting Blood Glucose Levels

In this context, receptors are specialized cells in the pancreas that monitor glucose levels in the blood. These cells are known as pancreatic beta cells. When blood glucose levels rise (such as after eating), these cells detect the increased glucose.

2. Control Center: Pancreas as the Decision-Maker

Upon detecting high glucose levels, the beta cells of the pancreas serve as the control center. They assess the information from the receptors and determine the necessary response to restore glucose levels to a normal range. The pancreas then synthesizes and releases the hormone insulin into the bloodstream.

3. Effectors: Actions to Lower Blood Glucose

The effectors in this process are primarily the liver and muscle cells, which respond to the insulin released by the pancreas. Insulin signals these cells to increase the uptake of glucose from the blood. Muscle cells use glucose for energy, especially during physical activity. The liver converts excess glucose into glycogen for storage, effectively lowering the blood glucose level and restoring equilibrium.

Positive and Negative Feedback in Homeostasis

Feedback mechanisms maintain the stability in the body’s internal environment. There are two types of regulatory mechanisms: negative feedback and positive feedback.

Negative Feedback

Negative feedback is the most common feedback mechanism in homeostasis. It counteracts or negates a change, bringing the system back to its set point or equilibrium. When a deviation from a set point is detected, negative feedback mechanisms initiate responses that reverse the change and restore balance. Key characteristics include:

• Self-limiting : Once the desired level is reached, the response diminishes or stops.
• Examples : Body temperature regulation (sweating to cool down when hot, shivering to warm up when cold), blood glucose regulation (insulin and glucagon balancing glucose levels).

Positive Feedback

Positive feedback is less common in homeostasis. This type of feedback amplifies a change or deviation, pushing the system further away from its set point. This mechanism is useful in situations where a rapid, decisive change is beneficial. Characteristics of positive feedback include:

• Self-amplifying : The response enhances the change, leading to an even greater response.
• Controlled and Temporary : Usually, positive feedback is part of a larger negative feedback system and is short-lived.
• Examples : Blood clotting (where each step in the clotting process triggers the next), the release of oxytocin during childbirth to intensify labor contractions.

Both negative and positive feedback mechanisms are crucial for maintaining homeostasis, though they operate differently. Negative feedback maintains stability and balance, while positive feedback aids specific, often critical, functions that require a rapid or substantial change.

More Examples of Homeostasis

Examples in humans.

• Water Balance : The body regulates water balance through mechanisms like thirst, urine production, and sweating to prevent dehydration or overhydration.
• Temperature Regulation : The body maintains an internal temperature around 37°C. When body temperature rises, mechanisms like sweating and increased blood flow to the skin help cool the body.
• Blood pH Regulation : The body maintains the pH of blood (around 7.35-7.45) through the respiratory system (by altering breathing rates) and kidneys (by excreting H + ions).
• Calcium Levels : Regulation of calcium levels in the blood is controlled by hormones like parathyroid hormone and calcitonin, affecting bone, kidney, and intestinal activities.
• Oxygen and Carbon Dioxide Levels : The respiratory system maintains a balance in oxygen and carbon dioxide levels in the blood through changes in breathing rate and depth.
• Electrolyte Balance : Sodium, potassium, and chloride ions are regulated to maintain nerve and muscle function, fluid balance, and acid-base balance.

Examples in Other Organisms

• Thermoregulation in Birds and Mammals : Many birds and mammals maintain a constant body temperature through mechanisms like shivering, sweating, panting, and adjusting their metabolic rate.
• Osmoregulation in Fish : Fish maintain the balance of water and salts in their bodies, despite the salt concentration in their environment. Freshwater fish actively excrete water and retain salts, while marine fish do the opposite.
• Stomatal Regulation in Plants : Plants open and close stomata to balance CO 2 intake for photosynthesis with water loss through transpiration.
• pH Regulation in Marine Life : Marine organisms like corals and mollusks regulate the pH within their cells and bodily fluids to counteract the acidification of ocean water.
• Hibernation in Bears and Other Animals : Hibernation is a form of long-term homeostasis where animals slow their metabolism, reduce body temperature, and conserve energy during scarce food availability in winter.

Microbial Homeostasis

Even microorganisms like bacteria exhibit homeostasis. For instance, they regulate their internal pH, ion concentrations, and respond to osmotic stress by synthesizing or importing compatible solutes.

Importance of Homeostasis

Homeostasis is crucial for the survival of organisms. It ensures optimal operating conditions for cells and organs, facilitates physiological processes, and maintains a balance despite environmental changes. Disruption in homeostasis often lead to diseases or disorders, reflecting its importance in health and disease.

• Aronoff, Stephen L.; Berkowitz, Kathy; et al. (2004). “Glucose Metabolism and Regulation: Beyond Insulin and Glucagon”. Diabetes Spectrum . 17 (3): 183–190. doi: 10.2337/diaspect.17.3.183
• Betts, J. Gordon; Desaix, P.; et al. (2013) Anatomy and Physiology (1st ed.). OpenStax. ISBN: 9781947172043.
• Boron, W.F.; Boulpaep, E.L. (2009). Medical Physiology: A Cellular and Molecular Approach (2nd International ed.). Philadelphia, PA: Saunders/Elsevier. ISBN 9781416031154.
• Kalaany, N.Y.; Mangelsdorf, D.J. (2006). “LXRS and FXR: the yin and yang of cholesterol and fat metabolism”. Annual Review of Physiology . 68: 159–91. doi: 10.1146/annurev.physiol.68.033104.152158
• Marieb, E.N.; Hoehn, K.N. (2009). Essentials of Human Anatomy & Physiology (9th ed.). San Francisco: Pearson/Benjamin Cummings. ISBN 978-0321513427.

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What Is Homeostasis?

Homeostasis keeps the internal milieu of the body stable and balanced

How It Works

Significance.

Homeostasis is a physiological process that keeps the internal environment of a living organism stable and balanced. The constant equilibrium created by homeostasis is vital to the survival of every species. Even when the external environment is rapidly changing, homeostasis keeps the body's internal environment constant and steady.

This article defines the meaning of homeostasis, describes how the concept originated, explains how these physiological processes work via negative feedback loops , and details why maintaining a "milieu intérieur" (constant internal environment) is essential for all living things. 

Westend61 / Getty Images

Homeostasis is a term derived from the Greek words "homeo" (meaning similar to) and "stasis" (meaning standing still). In the 1920s, an American physiologist named Walter B. Cannon invented the word "homeostasis."

Cannon described homeostasis as "coordinated physiological processes" that maintain "steady states" in a living organism. He clarified that the word doesn't imply something set in stone or stagnant but instead describes an automated, self-regulating process that varies but is "relatively constant."

Cannon's concept of homeostasis can be traced back to the principles of milieu intérieur developed by a French physiologist Claude Bernard in the 1860s.

"Homeostasis" and "milieu intérieur" are similar terms that highlight the importance of living organisms having internal mechanisms that can maintain inner balance and "relatively constant" stability. Even if the external environment changes rapidly, homeostasis makes it possible for the body to keep its internal environment steady via dynamic equilibrium reactions.

Body Temperature Exemplifies Homeostasis

The body's ability to maintain a constant internal temperature of 98.6 degrees F (37 degrees C) is a perfect example of homeostasis. When the external environment is hot, the body sweats to cool itself down and maintain a steady internal temperature. When it's cold outside, the body shivers to stay warm.

Homeostasis typically involves negative feedback loops. Within these loops, negative stimuli automatically trigger mechanisms to help homeostasis's dynamic equilibrium process. "Dynamic equilibrium" describes the process and reactions that occur as the body makes adjustments in response to negative feedback warnings of imbalance.

Homeostasis's ultimate goal is to help the body maintain a constant and relatively stable internal environment whenever possible. For example, being in hot weather triggers cooling mechanisms, such as sweat, that keep the body's core temperature around 98.6 degrees F.

When someone is in a cold environment, this negative stimulus activates homeostatic mechanisms designed to hold onto the body's heat, like shivering, in an attempt to maintain body temperature.

The human body has a phenomenal ability to maintain internal balance in response to disruptive outside forces and different types of internal or external negative stimuli. 

Examples of Homeostasis

Several types of homeostatic regulation maintain homeostasis in healthy human bodies, including:

• Body temperature homeostasis (thermoregulation) : Keeps the body's temperature stable at around 98.6 degrees F and helps offset the risks of heat exhaustion or hypothermia (low body temperature)
• Water and electrolyte homeostasis (osmoregulation) : Keeps fluid levels and electrolyte balance stable within the body by filtering and removing excess fluids via the kidneys (electrolytes are minerals that carry a charge dissolved in water, such as in the bloodstream, body fluids, and cells, including sodium, potassium, calcium, phosphorus, chloride, and magnesium)
• Oxygen (O₂) homeostasis : Keeps oxygen levels within a healthy range and helps the body avoid low oxygen levels ( hypoxia )
• Blood sugar homeostasis (pancreatic regulation) : Keeps blood sugar ( glucose ) at stable and healthy levels of between 70 and 120 milligrams per deciliter (mg/dL) via the pancreas
• Blood pressure homeostasis (baroreflex regulation) : Sends homeostatic signals from the hypothalamus to the heart, blood vessels, and kidneys that keep blood pressure in a healthy range that isn't too high ( hypertension ) or too low ( hypotension )
• Calcium homeostasis : Maintains serum calcium homeostasis via the synthesis and release of parathyroid hormone (PTH) from the parathyroid gland
• Potassium homeostasis : Keeps potassium levels within a healthy range of 3.6 to 5.2 milliequivalents per liter (mEq/L) and helps the body avoid having too much potassium in the blood ( hyperkalemia ) or too little ( hypokalemia )

It's impossible to overstate the importance of homeostasis. Every living thing relies on homeostatic processes for its survival. When homeostasis fails to function properly, internal imbalances can lead to sickness and even death.

For example, when the body can't maintain blood sugar homeostasis due to low production of insulin or poor response to insulin, diabetes results. The inability to maintain potassium homeostasis can affect the heart's rhythm and nervous system activity, which can lead to death.

George E. Billman, a U.S. physiologist, describes homeostasis as "the central organizing principle upon which the discipline of physiology is built."

Centuries ago, Claude Bernard identified the importance of living organisms having the ability to maintain a stable inner environment, which he called milieu intérieur, meaning constant internal environment. In the early 20th century, Walter B. Cannon built on Bernard's concept of milieu intérieur and coined the term "homeostasis," which combines the Greek words "homeo" (similar to) and "stasis" (standing still).

Homeostatic mechanisms are triggered by negative feedback or any stimuli that throw off the body's inner balance. There are seven types of homeostasis that maintain a stable and constant internal environment regardless of external environmental changes.

Every living thing relies on homeostasis for its real-time and long-term survival. It helps organisms regulate themselves in response to changes in their environment. Even something as simple as sweating on a hot day is key to maintaining this balance and staying healthy.

Journal of Medical Physiology and Therapeutics. Role of homeostasis in human physiology: a review .

Davies KJA. Adaptive homeostasis.   Molecular Aspects of Medicine . 2016;49:1-7. doi:10.1016/j.mam.2016.04.007

Cannon WB.  The Wisdom of the Body.  W.W. Norton & Company Inc; New York: 1932.

Hoenig MP, Zeidel ML. Homeostasis, the milieu intérieur, and the wisdom of the nephron .  CJASN . 2014;9(7):1272-1281. doi:10.2215/CJN.08860813

Prabhakar NR, Semenza GL. Oxygen sensing and homeostasis .  Physiology . 2015;30(5):340-348. doi:10.1152/physiol.00022.2015

Röder PV, Wu B, Liu Y, Han W. Pancreatic regulation of glucose homeostasis .  Exp Mol Med . 2016;48(3):e219-e219. doi:10.1038/emm.2016.6

Armstrong M, Kerndt CC, Moore RA. Physiology, baroreceptors . In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.

Yu E, Sharma S. Physiology, calcium . In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.

Simon LV, Farrell MW.  Hyperkalemia . In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.

Centers for Disease Control and Prevention. What is diabetes?

Udensi UK, Tchounwou PB.  Potassium homeostasis, oxidative stress, and human disease .  Int J Clin Exp Physiol . 2017;4(3):111–122. doi:10.4103/ijcep.ijcep_43_17

Billman GE. Homeostasis: the underappreciated and far too often ignored central organizing principle of physiology .  Front Physiol . 2020;11:200. doi:10.3389/fphys.2020.00200

By Christopher Bergland Bergland is a retired ultra-endurance athlete turned medical writer and science reporter. He is based in Massachusetts.

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What Is Homeostasis?

The body's need to maintain a state of equilibrium

Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Karen Cilli is a fact-checker for Verywell Mind. She has an extensive background in research, with 33 years of experience as a reference librarian and educator.

AleksandarNakic / Getty Images

• Maintaining Homeostasis

Homeostasis and Mental Health

Homeostasis refers to the body's need to reach and maintain a certain state of equilibrium. The term was first coined by a physiologist named Walter Cannon in 1926. More specifically, homeostasis is the body's tendency to monitor and maintain internal states, such as temperature and blood sugar, at fairly constant and stable levels.

Homeostasis refers to an organism's ability to regulate various physiological processes to keep internal states steady and balanced. These processes take place mostly without our conscious awareness.

How Is Homeostasis Maintained?

Your body has set points for a variety of states—including temperature, weight, sleep, thirst, and hunger. When the level is off (in either direction, too much or too little), homeostasis will work to correct it. For example, to regulate temperature, you will sweat when you get too hot or shiver when you get too cold.

Another way to think of it is like the thermostat in your house. Once set at a certain point, it works to keep the internal state at that level. When the temperature drops in your house, your furnace will turn on and warm things up to the preset temperature.

In the same way, if something is out of balance in your body, a physiological reaction will kick in until the set point is once again reached. Here's how the primary components of homeostasis work:

• Stimulus : A stimulus from a change in the environment kicks something out of balance in the body.
• Receptor : The receptor reacts to the change by informing the control unit.
• Control unit : The control unit then communicates the change needed to bring the body back into balance.
• Effector : The effector receives this information and acts on the change that is needed.

A negative feedback loop will work to decrease the effect of the stimulus, whereas a positive feedback loop will increase it. In homeostasis, negative feedback loops are most common, as the body is typically attempting to decrease the effect of the stimulus to get the body back to equilibrium.

Types of Homeostatic Regulation

There are three main types of homeostatic regulation that happen in the body. Though their names might be unfamiliar, you probably experience them every day.

Thermoregulation

When you think about homeostasis, temperature might come to mind first. It is one of the most important and obvious homeostatic systems. Regulating body temperature is called thermoregulation.

All organisms, from large mammals to tiny bacteria, must maintain an ideal temperature in order to survive. Some factors that influence this ability to maintain a stable body temperature include how these systems are regulated as well as the overall size of the organism.

• Endotherms : Some creatures, known as endotherms or "warm-blooded" animals, accomplish this via internal physiological processes. Birds and mammals (including humans) are endotherms.
• Ectotherms : Other creatures are ectotherms (aka "cold-blooded") and rely on external sources to regulate their body temperature. Reptiles and amphibians are both ectotherms.

The colloquial terms "warm-blooded" and "cold-blooded" do not actually mean that these organisms have different blood temperatures. These terms simply refer to how these creatures maintain their internal body temperatures.

Thermoregulation is also influenced by an organism's size, or more specifically, the surface-to-volume ratio. 

• Large organisms : Larger creatures have a much greater body volume, which causes them to produce more body heat.
• Small organisms : Smaller animals, on the other hand, produce less body heat but also have a higher surface-to-volume ratio. They lose more body heat than they produce, so their internal systems must work much harder to maintain steady body temperature. This is even true of babies, especially those born prematurely.

Osmoregulation

Osmoregulation strives to maintain the right amount of water and electrolytes inside and outside cells in the body. The balance of salt and water across membranes plays an important role, as in osmosis, which explains the name "osmoregulation." In this process, the kidneys are responsible for getting rid of any excess fluid, waste, or electrolytes. Osmoregulation also affects blood pressure.

Chemical Regulation

Your body regulates other chemical mechanisms as well to keep systems in balance. These use hormones as chemical signals—for example, in the case of blood sugar levels. In this situation, the pancreas would release either insulin, when blood sugar levels are high, or glucagon, when blood sugars are low, to maintain homeostasis.

Impact of Homeostasis

Homeostasis involves both physiological and behavioral responses. In terms of behavior, you might seek out warm clothes or a patch of sunlight if you start to feel chilly. You might also curl your body inward and keep your arms tucked in close to your body to keep in the heat.

As endotherms, people also have a number of internal systems that help regulate body temperature. When your body temperature dips below normal, a number of physiological reactions respond to help restore balance. Blood vessels in the body's extremities constrict in order to prevent heat loss. Shivering also helps the body produce more heat.

The body also responds when temperatures go above normal. Have you ever noticed how your skin becomes flushed when you are very warm? This is your body trying to restore temperature balance. When you are too warm, your blood vessels dilate in order to give off more body heat. Perspiration is another common way to reduce body heat, which is why you often end up flushed and sweaty on a very hot day.

Like the body, the mind seeks its own type of homeostasis and attempts to compensate when out of balance. For example, one prominent theory of human motivation , known as drive-reduction theory , suggests that homeostatic imbalances create needs. These needs, in turn, motivate behavior in an attempt to restore homeostasis.

Davies KJ. Adaptive homeostasis . Mol Aspects Med. 2016;49:1-7. doi:10.1016/j.mam.2016.04.007

APA Dictionary of Psychology. Osmoregulation .

Samuel SA, Francis AO, Anthony OO. Role of the kidneys in the regulation of intra- and extra-renal blood pressure . Ann Clin Hypertens . 2018;2:048-058. doi:10.29328/journal.ach.1001011

Röder PV, Wu B, Liu Y, Han W. Pancreatic regulation of glucose homeostasis . Exp Mol Med . 2016;48(3):e219. doi:10.1038/emm.2016.6

Tansey EA, Johnson CD. Recent advances in thermoregulation .  Advances in Physiology Education . 2015;39(3):139-148. doi:10.1152/advan.00126.2014

Deckers L. Motivation: Biological, Psychological, and Environmental . Taylor & Francis; 2018.

Molnar C, Gair J. Homeostasis and osmoregulation . In: Concepts of Biology - 1st Canadian Editio n. BCcampus; 2015.

By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

1.5 Homeostasis

Learning objectives.

By the end of this section, you will be able to:

• Discuss the role of homeostasis in healthy functioning
• Contrast negative and positive feedback, giving one physiologic example of each mechanism

Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A set point is the physiological value around which the normal range fluctuates. A normal range is the restricted set of values that is optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37°C (98.6°F) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. Negative feedback is a mechanism that reverses a deviation from the set point. Therefore, negative feedback maintains body parameters within their normal range. The maintenance of homeostasis by negative feedback goes on throughout the body at all times, and an understanding of negative feedback is thus fundamental to an understanding of human physiology.

Negative Feedback

A negative feedback system has three basic components ( Figure 1.10 a ). A sensor , also referred to a receptor, is a component of a feedback system that monitors a physiological value. This value is reported to the control center. The control center is the component in a feedback system that compares the value to the normal range. If the value deviates too much from the set point, then the control center activates an effector. An effector is the component in a feedback system that causes a change to reverse the situation and return the value to the normal range.

In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is, beyond homeostasis). This stimulus is “heard” by a specific sensor. For example, in the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. The insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. As glucose concentration in the bloodstream drops, the decrease in concentration—the actual negative feedback—is detected by pancreatic alpha cells, and insulin release stops. This prevents blood sugar levels from continuing to drop below the normal range.

Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain ( Figure 1.10 b ). When the brain’s temperature regulation center receives data from the sensors indicating that the body’s temperature exceeds its normal range, it stimulates a cluster of brain cells referred to as the “heat-loss center.” This stimulation has three major effects:

• Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the surface of the skin allowing the heat to radiate into the environment.
• As blood flow to the skin increases, sweat glands are activated to increase their output. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it.
• The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.

In contrast, activation of the brain’s heat-gain center by exposure to cold reduces blood flow to the skin, and blood returning from the limbs is diverted into a network of deep veins. This arrangement traps heat closer to the body core and restricts heat loss. If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering. The muscle contractions of shivering release heat while using up ATP. The brain triggers the thyroid gland in the endocrine system to release thyroid hormone, which increases metabolic activity and heat production in cells throughout the body. The brain also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes the breakdown of glycogen into glucose, which can be used as an energy source. The breakdown of glycogen into glucose also results in increased metabolism and heat production.

Interactive Link

Water concentration in the body is critical for proper functioning. A person’s body retains very tight control on water levels without conscious control by the person. Watch this video to learn more about water concentration in the body. Which organ has primary control over the amount of water in the body?

Positive Feedback

Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point. Childbirth and the body’s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed.

Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired. Enormous changes in the mother’s body are required to expel the baby at the end of pregnancy. And the events of childbirth, once begun, must progress rapidly to a conclusion or the life of the mother and the baby are at risk. The extreme muscular work of labor and delivery are the result of a positive feedback system ( Figure 1.11 ).

The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). These nerve cells send messages to the brain, which in turn causes the pituitary gland at the base of the brain to release the hormone oxytocin into the bloodstream. Oxytocin causes stronger contractions of the smooth muscles in of the uterus (the effectors), pushing the baby further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts, stopping the release of oxytocin.

A second example of positive feedback centers on reversing extreme damage to the body. Following a penetrating wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion (penetration of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting substances. This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.

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Osmosis and Its Role in Human Biology and Health

Human kidney cross-section (Mohammed Haneefa Nizamudeen, iStockphoto)

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Learn how and where osmosis takes place in the digestive system and excretory system and the role of osmosis in kidney dialysis.

Introduction

Imagine playing basketball with your friends on a hot summer day. By the end of the game, you feel thirsty. You decide to drink some water. But have you ever wondered how your body absorbs it? 

It happens because of osmosis . We will look at how osmosis happens and why it is important for our bodies.

What is a semipermeable membrane?

Before we jump into osmosis, we need to understand some important things about cells . The cells in our bodies are surrounded by a wall-like structure called a cell membrane. This membrane is special because only water and very small molecules can pass through it. We use the word semipermeable to describe the ability to only let certain things pass through a membrane.

Why is this membrane important? It’s because water must pass through semipermeable membranes to travel from one place in our body to another. 

What are solutes and solvents? 

Osmosis is when water molecules travel from a place with low solute concentration to a place with high solute concentration. To understand this better, we need to talk about solutes and solvents. A solute is a chemical that can dissolve in a solvent. Chemicals that can do this are called soluble . When you dissolve one or more solutes into a solvent, you get a solution . Sugar and salt are both chemicals that are soluble in water.

Did you know?  Water is often called “the universal solvent”  because so many things can dissolve in it.

What is concentration?

You may have heard some products described as concentrated - like laundry detergent or orange juice. Concentrated refers to the amount of solute compared to the amount of solvent in a solution. When there is a lot of solute compared to solvent, a solution is said to be  concentrated . When there is a small amount of solute compared to solvent, then a solution is said to be  dilute . 

Did you know? Tap water is actually a solution! This is because tap water is not just H 2 O (water). It also contains minerals like calcium. In tap water, water is the solvent and the minerals are the solutes. 

Did you know? Osmolarity  refers to the total concentration of all solutes in the solution. 

When solutions of different osmolarities are separated by a membrane that lets water but not solutes pass through, water will move from the side with lower osmolarity (high concentration of water) to the side with higher osmolarity (lower concentration of water).

Moving water from place to place is what allows plants and animals to keep their levels of water and of nutrients in balance or equilibrium . In living things, all processes involved in maintaining conditions necessary for survival is called homeostasis . It’s kind of like the story of Goldilocks and the Three Bears. Organisms like to keep everything not too hot, not too cold, but just right!

Osmosis and our Gastro-intestinal System 

So now that you know more about osmosis, it’s time to talk about your gastro-intestinal (GI) system - the parts of your body that deals with food and drink. 

When you eat food or drink water, it travels from your mouth, down your esophagus and into your stomach. In the stomach, the food is broken into tiny pieces that are mixed with stomach liquids. This mush of food and stomach liquids is called chyme . The chyme travels into the small intestine. This is where osmosis takes place. 

The chyme has a higher concentration than the epithelial cells that line your intestines. So, in order to reach homeostasis, water moves into these cells through their semipermeable membranes, taking small nutrients along with it. Near the epithelial cells are capillaries . The water and nutrients move through the cells of the capillaries and into the bloodstream. 

Did you know? About nine litres of fluid goes into your GI system each day and the  small intestine absorbs 8 of those litres !

Chyme passes from the stomach into the small intestines. Within the small intestine are folds of tissue called villi. Water passes through the epithelial cells on the villi and into capillaries which carry blood.

Osmosis and our Kidneys

The water in your blood then travels to your kidneys. Kidneys are some of the most complex parts of the body, and they use osmosis as well. 

Kidneys are made up of two parts - the cortex and medulla . The cortex is the outer part and the medulla is the inner part of the kidney. 

The kidneys are made up of groups of cells called renal pyramids . Each pyramid contains little units called nephrons . Nephrons look like a bunch of tubes connected to each other. Nephrons are important because they help filter waste out of your blood and put it into your urine. 

The largest part of each nephron is inside the medulla. The environment of the medulla has a higher osmolarity than the inside of the nephron. You know what that means - osmosis time! Water travels from inside the nephron tubes, through a semipermeable membrane, out into the medulla. Eventually, concentrated urine is left in the nephron. The urine travels through the ureter to the bladder .

Dialysis 

So you can see that the kidneys have a vital role in your body. But what happens if one of your kidneys can’t do its job anymore? This is when doctors have to use dialysis to help. 

A dialysis machine also uses a semipermeable membrane. It works in a similar way to a nephron. Blood is pumped next to a membrane that has dialysis fluid on the other side. Because of osmosis, the water in the blood, and very small molecules of waste, move across the membrane into the dialysis fluid. Eventually the dialysis fluid will remove all of the waste materials it can from the blood. That’s why dialysis can be life-saving for people who don’t have healthy kidneys.

Does osmosis cause your fingers to wrinkle in water? 

Have you ever noticed how your fingers become wrinkly after sitting in the bath too long? You might think this is because osmosis is causing water to leave your finger cells and into the bath water. That’s what doctors used to think.

In the 1930s, Dr. T. Lewis and Dr. G.W. Pickering noted that people with nerve damage in their fingers did not experience this wrinkling. If your fingers only wrinkled because of osmosis, then nerve damage would not change it at all! Thus, it was discovered that osmosis is not responsible for the wrinkling, but it’s actually due to our sympathetic nerves . 

Sympathetic nerves are a special type of nerves that help with vasoconstriction , which is the narrowing of the blood vessels. The nerves make the skin on our fingers wrinkle. The wrinkles act like threads on tires. This allows us to get a better grip on things in water. Our ancestors may have benefited from wrinkly feet to good footing in wet areas.

Let’s summarize what we have learned: 

• Osmosis is when water moves from an area of LOW solute concentration (low osmolarity) to an area of HIGH solute concentration (high osmolarity) through a semipermeable membrane.
• Osmosis is one of the most important ways that plants and animals achieve homeostasis. Keeping the body's conditions stable makes it possible for living things to survive. 
• Osmosis plays an important role in the human body, especially in the gastro-intestinal system and the kidneys. Osmosis helps you get nutrients out of food. It also gets waste products out of your blood.

I admit, that was quite a bit of information! Maybe it’s time to drink some water and eat a snack!

Starting Points

Connecting and relating.

• What clue tells you that your body needs water? 
• Do you have any products at home that are concentrated? Do you have to make the product more dilute before using it? Why or why not?

Relating Science and Technology to Society and the Environment

• What are the advantages and benefits of selling a product that is concentrated?
• What are the social and economic advantages and disadvantages of having dialysis technology available for people who have impaired kidney function? 
• Does your province or territory have a voluntary organ donation? Is this program really necessary if there is existing technology that can help people with kidney failure? Explain.

Exploring Concepts

• What is the difference between osmosis and osmolarity?
• What is a semipermeable membrane? In the human body, what moves through a semipermeable membrane?
• What is the role of osmosis in the small intestines? In the kidneys?
• Describe how a kidney dialysis machine works to support or replace kidney function?

Nature of Science/Nature of Technology

• How has knowledge of osmosis and semi-permeable membranes had applications in the food industry? (Note: This question requires some additional research)

Teaching Suggestions

• This article supports teaching and learning of biology and health related to animal cells, digestive system, excretory system, osmosis & diffusion. Concepts introduced include cells, semipermeable membrane, solutes, solvents, equilibrium, homeostasis, chyme, cortex, medulla, nephrons, dialysis,“fight or flight” response and vasoconstriction.
• Prior to reading this article, teachers could provide students with a Vocabulary Preview to engage prior knowledge and introduce new terminology. Ready-to-use reproducibles using the Vocabulary Preview learning strategy are available in [ Google doc ] and [ PDF ] formats. 
• To consolidate learning after reading the article, teachers could have students complete a Concept Definition Web  learning strategy for the concept of osmosis . Ready-to-use reproducibles for this article are available in [ Google doc ] and [ PDF ] formats.

Diffusion: Supplying the cell

BBC Bitesize Wxplains the three ways substances move around cells: diffusion, active transport, and of course osmosis!

Homeostasis and Negative/Positive Feedback (2017)

This video by the Amoeba Sisters (6:24 min) explains the importance of homeostasis in the human body, with examples of positive feedback and negative feedback.

How do your kidneys work? (2015)

This video by Emma Bryce (3:54 min) details how kidneys balance the amount of fluid in your body, detect waste in your blood, and know when to release the vitamins, minerals, and hormones you need to stay alive.

Why Skin Wrinkles In Water (2014)

Today I Found Out goes into more detail about the different theories there were about why this happens and how they were tested.

Eden. (2017) A Comprehensive Break Down of Nephron Functioning into Six Easy Steps! http://blog.cambridgecoaching.com/a-comprehensive-break-down-of-nephron-functioning-into-six-easy-steps

Molnar, C. and Gair, J. (2019) 11.1 Homeostasis and Osmoregulation . CONCEPTS OF BIOLOGY – 1ST CANADIAN EDITION. https://opentextbc.ca/biology/chapter/11-1-homeostasis-and-osmoregulation/

National Institute of Diabetes and Digestive and Kidney Diseases Health Information Center. (2017) Your Digestive System & How it Works https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works

Reasoner, A. (n.d.) Teaching Osmosis and Diffusion through Kidney Dialysis . https://teachers.yale.edu/curriculum/viewer/initiative_11.07.07_u

Thompson, V. (2016, September 29) How Does Osmosis Occur in the Digestive System? https://education.seattlepi.com/osmosis-occur-digestive-system-4874.html

University of California Los Angeles - Chemistry & Biochemistry. (n.d.) Aqueous Solutions - Molarity . https://www.chem.ucla.edu/~gchemlab/soln_conc_web.htm

Related Topics

An introduction to Homeostasis

Researched and Written by Jonjo Minns

Submitted to biologyonline.com on February 25, 2009. Published in biologyonline.com on March 29, 2009.

Introduction

Homeostasis is defined as “the condition of equilibrium (balance) in the bodies internal environment due to the consistent interaction of the body’s main regulatory processes” Tortora and Derrickson [2009:8].   The scope of this essay is that it will describe the concept of homeostasis, in addition to the homeostatic mechanisms of which regulate heart rate, breathing rate, body temperature, and blood glucose levels.   In addition to this, the importance of homeostasis in maintaining healthy functioning of the body will be explained.

The maintenance of body temperature is the responsibility of a team of structures within the body.   Temperature control is vital to the maintenance of homeostasis within the body.   Heat is sensed by thermo-regulators in both the skin and the hypothalamus.   The difference is, internal temperature (temperature inside the body) is sensed by thy hypothalamus, and external temperature (temperature outside the body) is sensed by the skin.

When the external temperature outside is too cold, messages are sent from the many thermo-receptors located within the skin (or from thermo-receptors located either deep in the muscle or in the blood), to the cerebellum leading to the hypothalamus.   The role of the cerebellum is to make the individual aware of feeling cold, of which may cause voluntary behavioural changes such as putting on more layers of clothing or a coat.  

Once the message is received by the hypothalamus, a series of reactions follow.   The first of which is by the hypothalamus, of which secretes thyroid releasing hormone (TRH).   This hormone’s target is the anterior lobe of the pituitary gland.   When the TRH reaches its target, it releases Thyroid Stimulating Hormone (TSH) of when then enters the blood stream.   The target of this hormone is the thyroid gland.  

Once the TSH is received by the thyroid gland, thyroxin is produced.   The role of thyroxin is to increase cellular metabolism in order to generate heat.   This hormone also inhibits vasoconstriction, the process whereas blood is diverted from the skin in order to conserve heat by keeping it deep within the body.   Sweating is also reduced to keep the surface of the skin dry, thus preventing heat loss.   In addition to all of these processes, the erector pilli muscles contract, causing the skin hairs to stand erect.   This traps air between the hairs and the skin and creates a layer of insulation, therefore keeping the body warmer.   In addition, the phenomenon of shivering is displayed and the bodies’ metabolic rate is increased.

One of the effects of the body becoming too cold is hypothermia.   This occurs when the body’s core temperature falls from the norm, 37 degrees (98F) to an abnormal temperature below 35 degrees (95F).   This is usually the response to prolonged exposure to cold temperatures.   As was mentioned above, the normal response of the body in such situations is to take preventative action for example applying more layers or going out indoors.   However if this is not possible, such as in hill walking, then hypothermia can ensue.   When an individual is presented with a cold environment, the normal response would be shivering, vasoconstriction, and endocrine activity (where the body releases hormones in order to promote the generation of heat), however in hypothermia, these are not substantial enough to maintain the normal core temperature of the body.

There are multiple symptoms to hypothermia; these include excessive shivering, feeling cold, and lethargy, less tolerable of the cold, pale skin with any accompanying cyanosis (blue skin).   These are the symptoms of a mild case of hypothermia.   In the moderate case, the symptoms are as follows; extremely violent shivering of which cannot be controlled, cognitive difficulties, confusion, loss of fine motor skills, sleepiness, shallow, slow breathing rate.   These are just a selection of the symptoms typically seen from a moderate case of hypothermia, of course there are more.   These extend more seriously into a severe case if hypothermia symptoms include loss of gross motor skills, cessation of shivering, unconsciousness, dilated pupils, weak pulse, weak breathing rate, and cardio-respiratory arrest.   There may also be a degree of cyanosis present due to the lack of blood to the superficial layers of the skin.

Hypothermia is treated by slow re-warming of the individual.   This is done within the acute care setting for the moderate and severe cases.   The warming of the individual’s body takes place from the inside, mainly by using warm intravenous fluids.

When the body is too warm, messages are sent in the same way as if the body is cold to the hypothalamus, this causes an increase in the amount of sweating, this is releasing heat via water, and the water on the skin evaporates, cooling the body down.   Vasodilatation is also apparent, in this instance, blood is diverted to the skin in order to loose heat, the erector pilli muscles relax, allowing the skin hairs to be lowered, and the bodies’ metabolic rate is reduced.   The reactions are different for each of the environmental states as the messages of which are sent are different.   There is one message for cold and a different one for hot.

Water balance is another very important aspect of homeostasis of which needs to be controlled within narrow limits.   The control of water balance is conducted using the following series of events.   The osmoreceptors located within the hypothalamus detect the condition of fluid balance within the body.   In the event of the fluid balance dropping too low, then the hypothalamus will act to bring the level back up by keeping more water within the body.  

If the concentration of water within the body is too high, then the hypothalamus will react to excrete more water from the body.   In the event of the hypothalamus sensing a change in fluid balance, messages are sent to the cerebellum, of where a feeling of thirst is produced, this is only when there is there is not enough water within the body.   In addition, the hypothalamus sends a message also to the posterior pituitary gland to induce the secretion of ADH, the action of the ADH in this instance is to increase the permeability of the kidney’s collecting duct.   Therefore, increasing the amount of water of which is reabsorbed into the body.   On the contrary, if there is too much water within the body then the pituitary gland secretes no ADH, therefore more water leaves the body in the urine.

Blood glucose is another contributing factor to homeostasis.   The blood glucose concentration in the blood is vital to the functioning of cells within the body and is controlled by a number of internal structures and external influence (food and drink).   If too much glucose is present within the blood, then specific receptors located within the pancreas detect this.   These receptors then send messages to the cerebellum, feelings of satiety (feeling full) are induced, and therefore the individual’s intake of food is decreased.   Messages are also sent to the islets of Langerhans for the production of insulin to commence.   Once insulin is produced, it is secreted into the capillary circulation and eventually into the systemic blood stream.   The insulin has many effects, mainly consisting of increasing the intake of glucose by all the cells of the body.   This action uses up surplus glucose and brings back a stable equilibrium.   The insulin also aids in the conversion of glucose into a substance called glycogen in the liver, thus lowering the level of glucose in the blood and restoring equilibrium.  

On the other hand, if there is not enough glucose in the bloodstream, then the very same receptors, of which are located in the pancreas detect the change.   Once again, a message is sent to the cerebellum, of which brings around feelings of hunger, therefore increasing the consumption of food and drink.   Messages are also sent to cells in the islets of langerhans to start the production of glucagon.   This glucagon is released by the islets of langerhans into the capillary circulation.   In turn the systemic blood stream, and stimulates the liver to convert stored glycogen into glucose.   In addition, the liver is stimulated also to start the conversion of amino acids into glucose, therefore the levels of glucose in the bloodstream rise and equilibrium is achieved.

Homeostasis is also heavily involved with the control of the respiratory rate.   In the norm, individuals are not conscious of their respiration.   This is because the act of respiration is involuntary.   Respiration is under involuntary control through an area of the brain termed the medulla.   Within the medulla is an area known as the breathing centre.   The breathing centre is composed into sections, allowing each to tackle an alternate aspect to respiration.   Both the dorsal and the lateral areas assist with inspiration and provide stimulation for respiration.   In addition, the ventral area increases both the depth and rate of respiration.   The centre is linked with the intercostals nerves and the phrenic nerves, leading to the diaphragm.   Theses routes provide a method of communication between the thorax, the respiratory system, and the medulla.  

The medulla is chief in maintaining a constant rate of respiration and depth.   However, both external and internal stimuli can alter the rate of respiration, making it higher or lower than the norm.   The main influence to this is the level of carbon dioxide in the blood stream.   If the concentration of carbon dioxide in the blood stream increases, then chemoreceptors located within both aortic and carotid bodies become aroused.   This causes messages to be sent to the medulla of which send nerve impulses back down the phrenic and intercostals nerves to the intercostals muscles and the diaphragm.   This causes them to contract and relax more quickly and therefore increasing the breathing rate.   In order to introduce more oxygen to the blood stream and bring back equilibrium of both oxygen and carbon dioxide levels in the blood stream.   This process is an example of negative feedback.

As for the control of the breathing rate, the medulla also controls the heart rate.   The set process for the regulation of the heart rate is rather complex and is as follows.   As an individual exercises, special receptors located within the muscles send impulses to the medulla.   Once these messages are received, the medulla secretes epinephrine and norepinephrine.   The combination of these two chemicals proceed through pathways within the nervous system until they reach the Sino-atrial node, located within the myocardium it acts like a pacemaker, controlling its electrical activity.   These chemicals arouse the Sino-atrial node, making it produce more electrical energy, thus making the heart rate increase.

On the other hand, when exercise is ceased, the muscles send additional impulses to the medulla of which responds by secreting the hormone acetylcholine, this hormones decreases the heart rate by slowing down the electrical impulses from the Sino-atrial node and therefore, decreasing the heart rate.   In addition, the medulla can also recognize other factors of which cause an increase in heart rate.   These include emotional stress.   In this instance, the medulla also takes information from the thalamus, which informs the medulla of the stressor.   This is with the addition of information received from the nervous system.   The combination of the two would enable the best response possible to be triggered.  

Bibliography

D.J Taylor, N.P.O Green, G.W Stout, 1997, Biological Science 1 & 2, 3 rd Ed, Cambridge University Press, Cambridge,

Tortora G. T, Derrickson B. H, 2009, Principles of Anatomy and Physiology: Volume 1: Organisation, Support, Movement, and Control Systems of the Human Body, 12 th Ed, John Wiley and Sons, Pte.   Ltd, Asia

W Gordon Sears, R S Winwood, 1974, Anatomy and Physiology for Nurses and other students of human biology, 5 th Ed, Edward Arnold Publishers, London

Stretch B and Whitehouse M, 2007, BTEC National Health and Social Care Book 1, Heinemann, Oxford

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http://www.bbc.co.uk/schools/gcsebitesize/ – BBC Bite size

http://www.revision-notes.co.uk/ – Revision Notes (.co.uk)

http://www.google.co.uk/ – Google

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http://springerlink.metapress.com/home/main.mpx – Springer Link – Home

http://dir.yahoo.com/ – Yahoo! Directory

http://www3.interscience.wiley.com – Wiley Interscience

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http://www.nhs.uk/ – NHS Choices

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http://www.biolsci.org/ – International Journal of Biological Sciences

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Microsoft® Encarta 2002 – CD-ROM

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Tortora G. T, Derrickson B. H, 2009, Principles of Anatomy and Physiology: Volume 1: Organisation, Support, Movement, and Control Systems of the Human Body, 12 th Ed, John Wiley and Sons, Pte.   Ltd, Asia [Page 8]

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Homeostasis and Regulation in the Human Body

Article objectives.

• To identify the process by which body systems are kept within certain limits.
• To explain the role of feedback mechanisms in homeostasis.
• To distinguish negative feedback from positive feedback.
• To identify and example of two organ systems working together to maintain homeostasis.
• To summarize the role of the endocrine system in homeostasis.
• To outline the result of a disturbance in homeostasis of a body system.

The human body is made up of trillions of cells that all work together for the maintenance of the entire organism. While cells, tissues, and organs may perform very different functions, all the cells in the body are similar in their metabolic needs. Maintaining a constant internal environment by providing the cells with what they need to survive (oxygen, nutrients, and removal of waste) is necessary for the well-being of individual cells and of the entire body. The many processes by which the body controls its internal environment are collectively called homeostasis. The complementary activity of major body systems maintains homeostasis.

Homeostasis

Homeostasis refers to stability, balance, or equilibrium within a cell or the body. It is an organism’s ability to keep a constant internal environment. Homeostasis is an important characteristic of living things. Keeping a stable internal environment requires constant adjustments as conditions change inside and outside the cell. The adjusting of systems within a cell is called homeostatic regulation. Because the internal and external environments of a cell are constantly changing, adjustments must be made continuously to stay at or near the set point (the normal level or range). Homeostasis can be thought of as a dynamic equilibrium rather than a constant, unchanging state.

Feedback Regulation Loops

The endocrine system plays an important role in homeostasis because hormones regulate the activity of body cells. The release of hormones into the blood is controlled by a stimulus. For example, the stimulus either causes an increase or a decrease in the amount of hormone secreted. Then, the response to a stimulus changes the internal conditions and may itself become a new stimulus. This self-adjusting mechanism is called feedback regulation.

Feedback regulation occurs when the response to a stimulus has an effect of some kind on the original stimulus. The type of response determines what the feedback is called. Negative feedback occurs when the response to a stimulus reduces the original stimulus. Positive feedback occurs when the response to a stimulus increases the original stimulus.

Thermoregulation: A Negative Feedback Loop

Negative feedback is the most common feedback loop in biological systems. The system acts to reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of homeostatic balance. For instance, when the concentration of carbon dioxide in the human body increases, the lungs are signaled to increase their activity and exhale more carbon dioxide, (your breathing rate increases). Thermoregulation is another example of negative feedback. When body temperature rises, receptors in the skin and the hypothalamus sense the temperature change. The temperature change (stimulus) triggers a command from the brain. This command, causes a response (the skin makes sweat and blood vessels near the skin surface dilate), which helps decrease body temperature. Figure 1 shows how the response to a stimulus reduces the original stimulus in another of the body’s negative feedback mechanisms.

Figure 1: Control of blood glucose level is an example of negative feedback. Blood glucose concentration rises after a meal (the stimulus). The hormone insulin is released by the pancreas, and it speeds up the transport of glucose from the blood and into selected tissues (the response). Blood glucose concentrations then decrease, which then decreases the original stimulus. The secretion of insulin into the blood is then decreased.

Positive feedback is less common in biological systems. Positive feedback acts to speed up the direction of change. An example of positive feedback is lactation (milk production). As the baby suckles, nerve messages from the mammary glands cause the hormone prolactin, to be secreted by the pituitary gland. The more the baby suckles, the more prolactin is released, which stimulates further milk production.

Not many feedback mechanisms in the body are based on positive feedback. Positive feedback speeds up the direction of change, which leads to increasing hormone concentration, a state that moves further away from homeostasis.

System Interactions

Each body system contributes to the homeostasis of other systems and of the entire organism. No system of the body works in isolation and the well-being of the person depends upon the well-being of all the interacting body systems. A disruption within one system generally has consequences for several additional body systems. Most of these organ systems are controlled by hormones secreted from the pituitary gland, a part of the endocrine system. Table 1 summarizes how various body systems work together to maintain homeostasis.

Main examples of homeostasis in mammals are as follows:

• The regulation of the amounts of water and minerals in the body. This is known as osmoregulation. This happens primarily in the kidneys. • The removal of metabolic waste. This is known as excretion. This is done by the excretory organs such as the kidneys and lungs. • The regulation of body temperature. This is mainly done by the skin. • The regulation of blood glucose level. This is mainly done by the liver and the insulin and glucagon secreted by the pancreas in the body.

Table 1: Types of Homeostatic Regulation in the Body

Endocrine System

The endocrine system, shown in Figure 2, includes glands which secrete hormones into the bloodstream. Hormones are chemical messenger molecules that are made by cells in one part of the body and cause changes in cells in another part of the body. The endocrine system regulates the metabolism and development of most body cells and body systems through feedback mechanisms. For example, Thyrotropin-Releasing Hormone (TRH) and Thyroid Stimulating Hormone (TSH) are controlled by a number of negative feedback mechanisms. The endocrine glands also release hormones that affect skin and hair color, appetite, and secondary sex characteristics of males and females.

Figure 2: The endocrine system controls almost every other body system through feedback mechanisms. Most of the mechanisms of the endocrine system are negative feedback.

The endocrine system has a regulatory effect on other organ systems in the human body. In the muscular system, hormones adjust muscle metabolism, energy production, and growth. In the nervous system, hormones affect neural metabolism, regulate fluid and ion concentration and help with reproductive hormones that influence brain development.

Urinary System

Toxic wastes build up in the blood as proteins and nucleic acids are broken down and used by the body. The urinary system rids the body of these wastes. The urinary system is also directly involved in maintaining proper blood volume. The kidneys also play an important role in maintaining the correct salt and water content of the body. External changes, such as a warm weather, that lead to excess fluid loss trigger feedback mechanisms that act to maintain the body’s fluid content by inhibiting fluid loss. The kidneys also produce a hormone called erythropoietin, also known as EPO, which stimulates red blood cell production.

Reproductive System

The reproductive system does little for the homeostasis of the organism. The reproductive system relates instead to the maintenance of the species. However, sex hormones do have an effect on other body systems, and an imbalance in sex hormones can lead to various disorders. For example, a woman whose ovaries are removed early in life is at higher risk of developing osteoporosis, a disorder in which bones are thin and break easily. The hormone estrogen, produced by the ovaries, is important for bone growth. Therefore, a woman who does not produce estrogen will have impaired bone development.

Disruption of Homeostasis

Many homeostatic mechanisms keep the internal environment within certain limits (or set points). When the cells in your body do not work correctly, homeostatic balance is disrupted. Homeostatic imbalance may lead to a state of disease. Disease and cellular malfunction can be caused in two basic ways: by deficiency (cells not getting all they need) or toxicity (cells being poisoned by things they do not need). When homeostasis is interrupted, your body can correct or worsen the problem, based on certain influences. In addition to inherited (genetic) influences, there are external influences that are based on lifestyle choices and environmental exposure. These factors together influence the body’s ability to maintain homeostatic balance. The endocrine system of a person with diabetes has difficulty maintaining the correct blood glucose level. A diabetic needs to check their blood glucose levels many times during the day, as shown in Figure 3, and monitor daily sugar intake.

Figure 3: A person with diabetes has to monitor their blood glucose carefully. This glucose meter analyses only a small drop of blood.

Internal Influences: Heredity

Genetics: Genes are sometimes turned off or on due to external factors which we have some control over. Other times, little can be done to prevent the development of certain genetic diseases and disorders. In such cases, medicines can help a person’s body regain homeostasis. An example is the metabolic disorder Type 1 diabetes, which is a disorder where the pancreas is no longer producing adequate amounts of insulin to respond to changes in a person’s blood glucose level. Insulin replacement therapy, in conjunction with carbohydrate counting and careful monitoring of blood glucose concentration, is a way to bring the body’s handling of glucose back into balance. Cancer can be genetically inherited or be due to a mutation caused by exposure to toxin such as radiation or harmful drugs. A person may also inherit a predisposition to develop a disease such as heart disease. Such diseases can be delayed or prevented if the person eats nutritious food, has regular physical activity, and does not smoke.

External Influences: Lifestyle

Nutrition: If your diet lacks certain vitamins or minerals your cells will function poorly, and you may be at risk to develop a disease. For example, a menstruating woman with inadequate dietary intake of iron will become anemic. Hemoglobin, the molecule that enables red blood cells to transport oxygen, requires iron. Therefore, the blood of an anemic woman will have reduced oxygen-carrying capacity. In mild cases symptoms may be vague (e.g. fatigue), but if the anemia is severe the body will try to compensate by increasing cardiac output, leading to weakness, irregular heartbeats and in serious cases, heart failure.

Physical Activity: Physical activity is essential for proper functioning of our cells and bodies. Adequate rest and regular physical activity are examples of activities that influence homeostasis. Lack of sleep is related to a number of health problems such as irregular heartbeat, fatigue, anxiety, and headaches. Being overweight and obesity, two conditions that are related to poor nutrition and lack of physical activity greatly affect many organ systems and their homeostatic mechanisms. Being overweight or obese increases a person’s risk of developing heart disease, Type 2 diabetes, and certain forms of cancer. Staying fit by regularly taking part in aerobic activities such as walking, shown in Figure 4, has been shown to help prevent many of these diseases.

Figure 4: Adding physical activity to your routine can be as simple as walking for a total of 60 minutes a day, five times a week.

Mental Health: Your physical health and mental health are inseparable. Our emotions cause chemical changes in our bodies that have various effects on our thoughts and feelings. Negative stress (also called distress) can negatively affect mental health. Regular physical activity has been shown to improve mental and physical well-being, and helps people to cope with distress. Among other things, regular physical activity increases the ability of the cardiovascular system to deliver oxygen to body cells, including the brain cells. Medications that may help balance the amount of certain mood-altering chemicals within the brain are often prescribed to people who have mental and mood disorders. This is an example of medical help in stabilizing a disruption in homeostasis.

Environmental Exposure

Any substance that interferes with cellular function and causes cellular malfunction is a cellular toxin. There are many different sources of toxins, for example, natural or synthetic drugs, plants, and animal bites. Air pollution, another form of environmental exposure to toxins is shown in Figure 5. A commonly seen example of an exposure to cellular toxins is by a drug overdose. When a person takes too much of a drug that affects the central nervous system, basic life functions such as breathing and heartbeat are disrupted. Such disruptions can results in coma, brain damage, and even death.

Figure 5: Air pollution can cause environmental exposure to cellular toxins such as mercury.

The six factors described above have their effects at the cellular level. A deficiency or lack of beneficial pathways, whether caused by an internal or external influence, will almost always result in a harmful change in homeostasis. Too much toxicity also causes homeostatic imbalance, resulting in cellular malfunction. By removing negative health influences and providing adequate positive health influences, your body is better able to self-regulate and self-repair, which maintains homeostasis.

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USFG. http://commons.wikimedia.org/wiki/File:Illu_endocrine_system.png. Public Domain.

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Jame. http://www.flickr.com/photos/jamehealy/461578738/. CC-SA-BY 2.0.

http://www.flickr.com/photos/pingnews/450243814/. Public Domain.

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Chapter 11: Introduction to the Body’s Systems

11.1 Homeostasis and Osmoregulation

Learning objectives.

By the end of this section, you will be able to:

• Explain the concept of homeostasis
• Describe thermoregulation of endothermic and ectothermic animals
• Explain how the kidneys serve as the main osmoregulatory organs in the human body

Homeostasis refers to the relatively stable state inside the body of an animal. Animal organs and organ systems constantly adjust to internal and external changes in order to maintain this steady state. Examples of internal conditions maintained homeostatically are the level of blood glucose, body temperature, blood calcium level. These conditions remain stable because of physiologic processes that result in negative feedback relationships. If the blood glucose or calcium rises, this sends a signal to organs responsible for lowering blood glucose or calcium. The signals that restore the normal levels are examples of negative feedback. When homeostatic mechanisms fail, the results can be unfavorable for the animal. Homeostatic mechanisms keep the body in dynamic equilibrium by constantly adjusting to the changes that the body’s systems encounter. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium. Two examples of factors that are regulated homeostatically are temperature and water content. The processes that maintain homeostasis of these two factors are called thermoregulation and osmoregulation.

Homeostasis

The goal of homeostasis is the maintenance of equilibrium around a specific value of some aspect of the body or its cells called a set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor; the response of the system is to adjust the activities of the system so the value moves back toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If glucose levels in the blood rise after a meal, adjustments are made to lower them and to get the nutrient into tissues that need it or to store it for later use.

When a change occurs in an animal’s environment, an adjustment must be made so that the internal environment of the body and cells remains stable. The receptor that senses the change in the environment is part of a feedback mechanism. The stimulus—temperature, glucose, or calcium levels—is detected by the receptor. The receptor sends information to a control center, often the brain, which relays appropriate signals to an effector organ that is able to cause an appropriate change, either up or down, depending on the information the sensor was sending.

Thermoregulation

Animals can be divided into two groups: those that maintain a constant body temperature in the face of differing environmental temperatures, and those that have a body temperature that is the same as their environment and thus varies with the environmental temperature. Animals that do not have internal control of their body temperature are called ectotherms. The body temperature of these organisms is generally similar to the temperature of the environment, although the individual organisms may do things that keep their bodies slightly below or above the environmental temperature. This can include burrowing underground on a hot day or resting in the sunlight on a cold day. The ectotherms have been called cold-blooded, a term that may not apply to an animal in the desert with a very warm body temperature.

An animal that maintains a constant body temperature in the face of environmental changes is called an endotherm. These animals are able to maintain a level of activity that an ectothermic animal cannot because they generate internal heat that keeps their cellular processes operating optimally even when the environment is cold.

Concept in Action

Watch this Discovery Channel video on thermoregulation to see illustrations of the process in a variety of animals.

Animals conserve or dissipate heat in a variety of ways. Endothermic animals have some form of insulation. They have fur, fat, or feathers. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals can increase body heat production by shivering, which is an involuntary increase in muscle activity. In addition, arrector pili muscles can contract causing individual hairs to stand up when the individual is cold. This increases the insulating effect of the hair. Humans retain this reaction, which does not have the intended effect on our relatively hairless bodies; it causes “goose bumps” instead. Mammals use layers of fat as insulation also. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.

Ectotherms and endotherms use their circulatory systems to help maintain body temperature. Vasodilation, the opening up of arteries to the skin by relaxation of their smooth muscles, brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, cooling the body. Vasoconstriction, the narrowing of blood vessels to the skin by contraction of their smooth muscles, reduces blood flow in peripheral blood vessels, forcing blood toward the core and vital organs, conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins that are flowing next to each other, warming blood returning to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. The countercurrent adaptation is found in dolphins, sharks, bony fish, bees, and hummingbirds.

Some ectothermic animals use changes in their behavior to help regulate body temperature. They simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks in the evening to capture heat on a cold desert night before entering their burrows.

Thermoregulation is coordinated by the nervous system ( Figure 11.2 ). The processes of temperature control are centered in the hypothalamus of the advanced animal brain. The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation or vasoconstriction and shivering or sweating. The sympathetic nervous system under control of the hypothalamus directs the responses that effect the changes in temperature loss or gain that return the body to the set point. The set point may be adjusted in some instances. During an infection, compounds called pyrogens are produced and circulate to the hypothalamus resetting the thermostat to a higher value. This allows the body’s temperature to increase to a new homeostatic equilibrium point in what is commonly called a fever. The increase in body heat makes the body less optimal for bacterial growth and increases the activities of cells so they are better able to fight the infection.

When bacteria are destroyed by leukocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body temperature to rise?

<!–Pyrogens increase body temperature by causing the blood vessels to constrict, inducing shivering, and stopping sweat glands from secreting fluid.–>

Osmoregulation

Osmoregulation is the process of maintaining salt and water balance (osmotic balance) across membranes within the body. The fluids inside and surrounding cells are composed of water, electrolytes, and nonelectrolytes. An electrolyte is a compound that dissociates into ions when dissolved in water. A nonelectrolyte, in contrast, does not dissociate into ions in water. The body’s fluids include blood plasma, fluid that exists within cells, and the interstitial fluid that exists in the spaces between cells and tissues of the body. The membranes of the body (both the membranes around cells and the “membranes” made of cells lining body cavities) are semipermeable membranes. Semipermeable membranes are permeable to certain types of solutes and to water, but typically cell membranes are impermeable to solutes.

The body does not exist in isolation. There is a constant input of water and electrolytes into the system. Excess water, electrolytes, and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance. Insufficient fluid intake results in fluid conservation by the kidneys. Biological systems constantly interact and exchange water and nutrients with the environment by way of consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences.

Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, interstitial fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Since blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure.

Excretory System

The human excretory system functions to remove waste from the body through the skin as sweat, the lungs in the form of exhaled carbon dioxide, and through the urinary system in the form of urine. All three of these systems participate in osmoregulation and waste removal. Here we focus on the urinary system, which is comprised of the paired kidneys, the ureter, urinary bladder and urethra ( Figure 11.3 ). The kidneys are a pair of bean-shaped structures that are located just below the liver in the body cavity. Each of the kidneys contains more than a million tiny units called nephrons that filter blood containing the metabolic wastes from cells. All the blood in the human body is filtered about 60 times a day by the kidneys. The nephrons remove wastes, concentrate them, and form urine that is collected in the bladder.

Internally, the kidney has three regions—an outer cortex, a medulla in the middle, and the renal pelvis, which is the expanded end of the ureter. The renal cortex contains the nephrons—the functional unit of the kidney. The renal pelvis collects the urine and leads to the ureter on the outside of the kidney. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.

Blood enters each kidney from the aorta, the main artery supplying the body below the heart, through a renal artery. It is distributed in smaller vessels until it reaches each nephron in capillaries. Within the nephron the blood comes in intimate contact with the waste-collecting tubules in a structure called the glomerulus. Water and many solutes present in the blood, including ions of sodium, calcium, magnesium, and others; as well as wastes and valuable substances such as amino acids, glucose and vitamins, leave the blood and enter the tubule system of the nephron. As materials pass through the tubule much of the water, required ions, and useful compounds are reabsorbed back into the capillaries that surround the tubules leaving the wastes behind. Some of this reabsorption requires active transport and consumes ATP. Some wastes, including ions and some drugs remaining in the blood, diffuse out of the capillaries into the interstitial fluid and are taken up by the tubule cells. These wastes are then actively secreted into the tubules. The blood then collects in larger and larger vessels and leaves the kidney in the renal vein. The renal vein joins the inferior vena cava, the main vein that returns blood to the heart from the lower body. The amounts of water and ions reabsorbed into the circulatory system are carefully regulated and this is an important way the body regulates its water content and ion levels. The waste is collected in larger tubules and then leaves the kidney in the ureter, which leads to the bladder where urine, the combination of waste materials and water, is stored.

The bladder contains sensory nerves, stretch receptors that signal when it needs to be emptied. These signals create the urge to urinate, which can be voluntarily suppressed up to a limit. The conscious decision to urinate sets in play signals that open the sphincters, rings of smooth muscle that close off the opening, to the urethra that allows urine to flow out of the bladder and the body.

Dialysis Technician

Dialysis is a medical process of removing wastes and excess water from the blood by diffusion and ultrafiltration. When kidney function fails, dialysis must be done to artificially rid the body of wastes and fluids. This is a vital process to keep patients alive. In some cases, the patients undergo artificial dialysis until they are eligible for a kidney transplant. In others who are not candidates for kidney transplants, dialysis is a lifelong necessity.

Dialysis technicians typically work in hospitals and clinics. While some roles in this field include equipment development and maintenance, most dialysis technicians work in direct patient care. Their on-the-job duties, which typically occur under the direct supervision of a registered nurse, focus on providing dialysis treatments. This can include reviewing patient history and current condition, assessing and responding to patient needs before and during treatment, and monitoring the dialysis process. Treatment may include taking and reporting a patient’s vital signs, preparing solutions and equipment to ensure accurate and sterile procedures.

Section Summary

Homeostasis is a dynamic equilibrium that is maintained in body tissues and organs. It is dynamic because it is constantly adjusting to the changes that the systems encounter. It is an equilibrium because body functions are kept within a normal range, with some fluctuations around a set point. The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and maintain the dissolved ion concentrations of body fluids. They are made up internally of three distinct regions—the cortex, medulla, and pelvis. The blood vessels that transport blood into and out of the kidneys arise from and merge with the aorta and inferior vena cava, respectively. The nephron is the functional unit of the kidney, which actively filters blood and generates urine. The urine leaves the kidney through the ureter and is stored in the urinary bladder. Urine is voided from the body through the urethra.

Concepts of Biology - 1st Canadian Edition Copyright © 2015 by Charles Molnar and Jane Gair is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Importance of pH Homeostasis in Metabolic Health and Diseases: Crucial Role of Membrane Proton Transport

1 Laboratory of Health Science, Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Kyoto 606-8522, Japan

Yoshinori Marunaka

2 Departments of Molecular Cell Physiology and Bio-Ionomics, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto 602-8566, Japan

3 Japan Institute for Food Education and Health, St. Agnes' University, Kyoto 602-8013, Japan

Protons dissociated from organic acids in cells are partly buffered. If not, they are transported to the extracellular fluid through the plasma membrane and buffered in circulation or excreted in urine and expiration gas. Several transporters including monocarboxylate transporters and Na + /H + exchanger play an important role in uptake and output of protons across plasma membranes in cells of metabolic tissues including skeletal muscle and the liver. They also contribute to maintenance of the physiological pH of body fluid. Therefore, impairment of these transporters causes dysfunction of cells, diseases, and a decrease in physical performance associated with abnormal pH. Additionally, it is known that fluid pH in the interstitial space of metabolic tissues is easily changed due to little pH buffering capacitance in interstitial fluids and a reduction in the interstitial fluid pH may mediate the onset of insulin resistance unlike blood containing pH buffers such as Hb (hemoglobin) and albumin. In contrast, habitual exercise and dietary intervention regulate expression/activity of transporters and maintain body fluid pH, which could partly explain the positive effect of healthy lifestyle on disease prognosis.

1. Introduction

Body fluid pH is determined by the content of protons (H + ) generated from organic acids produced in living cells. Lactic acid (lactate − /H + ) is a typical proton source and is involved in the regulation of physiological pH. In metabolic tissues such as skeletal muscle and adipose tissue, the glycolytic anaerobic metabolism mediates the conversion of glucose and glycogen into lactic acid. Because the pKa of lactic acid is 3.80, it is immediately dissociated into lactate (lactate − ) and protons under physiological conditions, resulting in reduced intracellular pH. Pyruvic acid (pyruvate − /H + ), an intermediate metabolite in the glycolytic system, is also a source of protons, although it generates much less protons compared to lactic acid. In addition, metabolites such as ketone bodies also act as proton sources. Beta-hydroxybutyric acid (beta-hydroxybutyrate − /H + ), a typical ketone body, is generated as a result of fatty acid metabolism in the liver and is also dissociated into beta-hydroxybutyrate anions and protons, leading to the reduction of intracellular pH.

The intracellular pH in most living cells is alkaline compared to the pH generated by protons that are transported passively through the plasma membrane by electrochemical forces. In addition to buffering systems such as the bicarbonate-carbonate system, protein-proton binding, and phosphoric acid, several membrane transporters are responsible for proton removal from the cytosol and play important roles in maintaining the alkaline pH in cells ( Figure 1 ). In most mammalian cells, H + -monocarboxylate cotransporters (MCTs) participate in the transport of monocarboxylic acids such as lactate, pyruvate, beta-hydroxybutyrate, and acetoacetate across the cellular membrane by cotransporting protons and monocarboxylate anions [ 1 – 3 ]. Other transporters such as the Na + /H + exchanger (NHE) and bicarbonate-dependent exchanger also contribute to proton extrusion from the cytosol to the extracellular space [ 4 , 5 ]. This review focuses on the critical role of the membrane transport system of protons in regulation of intracellular and extracellular fluid pH and its importance in maintaining physiological homeostasis and preventing diseases development.

Proton production and its transporting kinetics in intracellular and extracellular fluid in metabolic tissues. The production of organic acids such as lactic acid and ketone bodies is accelerated by elevating glycolytic anaerobic metabolism and β -oxidation in metabolic cells. Body fluid pH is strictly maintained by buffering systems, efflux across plasma membrane, and acid excretion. Monocarboxylate transporter (MCT) and Na + /H + exchanger (NHE) contribute to proton extrusion from the cytosol to the extracellular space. In contrast to intracellular fluid and blood containing pH buffers such as Hb (hemoglobin) and albumin, the interstitial fluid pH could be easily reduced by acid stress owing to the limited availability of the buffering factors such as proteins.

2. Proton Transport across the Plasma Membrane in pH Regulation

Regulation of body fluid pH is one of the most important physiological functions of homeostasis, because activity of most chemical reactions via enzyme proteins is dependent on fluid pH. To maintain homeostasis of body fluid pH, various buffering systems are utilized in addition to proton excretion from the cytosol to the extracellular space and ultimately outside of the body. However, if production of organic acid is elevated or the buffering and excretion systems are impaired, body fluid turns acidic, leading to abnormal conditions. A typical example is elevation of lactic acid production in skeletal muscle in response to strenuous exercise, which leads to body fluid acidosis, preventing muscle contraction [ 6 , 7 ]. Proton transport across the plasma membrane of muscle cells is important for maintaining the appropriate intracellular pH. Skeletal muscle is a major metabolic organ that generates acids, in particular during contraction. Strenuous muscle contractions can cause a drastic reduction in intramuscular pH to −6.5 with accumulation of more than 40 mM lactate [ 6 – 8 ], regardless of cellular buffering capacity. Several studies have shown that intracellular pH is reduced during muscle contraction and has a delayed recovery to basal conditions during the recovery phase in the absence of proton transporters [ 9 ]. This delay suggests that proton transporters play a key role in maintaining pH homeostasis. Indeed, the function of proton transporters is involved in the capacity for pH maintenance [ 9 , 10 ]. In particular, over 80% of intracellular proton is transported through lactate cotransport in contracting muscle, although remaining parts are transported through NHE and bicarbonate-depending transport [ 8 , 11 ]. The liver, another organ that is closely associated with the metabolism of organic acids, generates ketone bodies (i.e., acetoacetic and β -hydroxybutyric acids), metabolizes lipids, and converts lactate to glucose via gluconeogenesis. Therefore, this organ generates acidic conditions [ 12 – 14 ] and intracellular pH should be maintained by proton extrusion along with buffering function.

MCTs, a part of the solute carrier (SLC) 16 that contains 14 members in total, play a crucial role in proton transport across the plasma membrane by cotransporting monocarboxylates. Each isoform has different transport kinetics and is specifically located on a particular subcellular site. It has been shown that MCT1–MCT4 transport aliphatic monocarboxylates such as lactate, pyruvate, and ketone bodies [ 2 ] and that the direction of transport across the plasma membrane in a 1 : 1 manner is determined by the concentration gradients of protons and monocarboxylate both inside and outside of the cell [ 15 – 17 ]. Thus, these isoforms play important roles in proton transport maintaining intracellular pH. In particular, the expression of two MCT isoforms (MCT1 and MCT4) is associated with lactate disposal in muscles. MCT1 is highly expressed and located in both the sarcolemmal and the mitochondrial membranes of oxidative muscles [ 18 – 20 ]; on the other hand, MCT4 is predominantly located on the plasma membrane of glycolytic muscle and is assumed to contribute to lactate efflux [ 19 , 21 ]. In contrast, MCT2 is mainly located on the membranes of liver cells and contributes to the extrusion of ketone bodies [ 22 ]. Other members of the family have different substrate specificities. For example, MCT6 has been shown to transport bumetanide, a diuretic drug [ 23 ], MCT 8 acts as a thyroid hormone transporter [ 24 ], MCT9 is a potential carnitine extrusion transporter [ 25 ], and MCT10 is identified as a low-affinity transporter of aromatic amino acids along with iodothyronines [ 26 ]. In addition, NHE is known as another major proton transporter that plays an important role in intracellular pH homeostasis by exchanging intracellular proton with extracellular Na + using the chemical gradient between intra- and extracellular Na + concentrations [ 4 , 27 ]. Currently, 10 isoforms are known to exist in mammals. NHE1–NHE5 are located on the plasma membrane of their specific tissues, while NHE6–NHE9 are located on the membrane of subcellular organelles [ 27 – 29 ]. In particular, NHE1 has been recognized as a ubiquitous isoform and plays an important role in maintaining homeostasis in metabolic organs.

Proton transport across the plasma membrane is important for maintenance of intracellular and extracellular fluid pH. In particular, proton excretion and bicarbonate reabsorption are recognized as important function of renal tubules. Proton excretion into urine is mainly mediated by both proton-ATPase and NHE3 located on the apical plasma membrane of the proximal convoluted tubule participating in approximately 80% of bicarbonate reabsorption occurring in the whole kidney, acting as the major buffering system in blood [ 30 , 31 ], which has also pH buffers such as Hb (hemoglobin) and albumin. Bicarbonate reacts with protons via catalytic carbonic anhydrase on the apical membrane and generates CO 2 . Then, it is transported into the blood by sodium-bicarbonate cotransporters on the basolateral side [ 32 ].

3. pH Disturbance and Disease Development

The normal physiological pH of mammalian arterial blood is strictly maintained at 7.40; blood has pH buffers such as Hb (hemoglobin) and albumin. A decrease of more than 0.05 units from the normal pH results in acidosis. The body fluids of diabetic patients are chronically acidic and exhibit characteristic ketoacidosis caused by an increased level of ketone bodies in the blood [ 33 , 34 ]. Insulin resistance in metabolic tissues such as skeletal muscle, adipose tissue, and the liver accelerates the utilization of lipids as an energy substrate instead of glucose. Excess lipolysis caused by impaired glucose metabolism leads to free fatty acids in circulation, which facilitate hepatic gluconeogenesis by the oxidation of fatty acids resulting in large quantities of ketone bodies. This further accelerates proton overloads, leading to the metabolic ketoacidosis found in diabetic patients. Such acidic conditions prevent the activity of metabolic enzymes such as phosphofructokinase and further accelerate the progression of pathological conditions [ 33 – 35 ]. Acidic conditions can also result in physical fatigue of diabetic patients. Therefore, maintaining normal pH is important for physiological homeostasis.

It has been suggested that loss of function of MCTs causes a change of body fluid pH. Several point mutations of the MCT gene have been shown to affect both specificity and transport activity. The spontaneously occurring mutation of arginine 306 to threonine in domain 8 of MCT1 resulted in reduced transport activity [ 36 ]. In addition, it has been shown that subjects who have mutations in MCT1 cDNA have drastically lower transport rates and a delayed decline of blood lactate after exercise [ 37 , 38 ]. Healthy subjects feel severe chest pain and muscle cramping after strenuous exercise, along with a defect in lactate efflux from muscle. Furthermore, many amino acid differences that are not attributable to polymorphisms are found in MCT1 obtained from muscle tissues in these subjects [ 37 , 39 ]; thus, mutations in MCT1 are related to physical fatigue and exercise performance. MCT dysfunction may lead to metabolic disorder. Indeed, lower level expression of MCT1 and MCT4 is found in the skeletal muscle of obese rats compared to normal rats [ 40 ]. In addition, the activity of lactate transport in muscle is also decreased by both denervation and aging [ 41 , 42 ]. A significant negative correlation between the level of circulating lactate and degree of insulin sensitivity is found in humans [ 43 ], suggesting that lower lactic acid disposal caused by reduction of MCT function is associated with insulin resistance.

4. Interstitial Fluid pH and Disease Development

Body fluid acidosis could also contribute to the development of metabolic diseases. Our recent study indicates that before the development of diabetic symptoms the interstitial fluid pH in ascites and metabolic tissues of Otsuka Long-Evans Tokushima Fatty (OLETF) rats is lower than the normal pH (7.40) [ 44 ]. The buffering capacity is relatively high in the cytosol and blood but low in the interstitial fluid due to limited buffering factors such as proteins [ 45 , 46 ]. Therefore, interstitial fluid pH in metabolic tissues easily changes ( Figure 1 ) and may contribute to the onset of insulin resistance. We have shown the inhibitory effect of extracellular pH on the insulin signaling pathway in the L6 rat myotube. The phosphorylation level and binding affinity to insulin of insulin receptors were significantly diminished in media with low pH [ 47 ]. In addition, the levels of Akt phosphorylation, a downstream of the insulin receptor, are also decreased in low pH media, along with a reduction in glucose uptake. These in vitro observations support the hypothesis that lower extracellular pH may cause insulin resistance in skeletal muscle cells. Other studies [ 48 – 50 ] have suggested a close correlation between organic acid production and insulin sensitivity in both type 2 diabetes patients and healthy subjects. In a cross-sectional study of over 1,000 subjects [ 48 ], it has been demonstrated that body weight and waist circumference have a negative correlation with both insulin sensitivity and urine pH. Patients with metabolic syndrome have also reported a significantly lower pH of 24 h urine compared to the normal subjects and a negative correlation between the mean 24 h urine pH and the number of metabolic syndrome abnormalities [ 49 , 50 ]. It has been suggested that lower levels of serum bicarbonate and higher levels of anion gap resulting from metabolic acidosis are associated with lower insulin sensitivity [ 51 ]. Hyperlactacidemia is found in patients with obesity and type 2 diabetes [ 43 ], which supports the strong relationship between acidic condition and insulin sensitivity. Even in healthy subjects, acids level could be an independent risk factor for the development of type 2 diabetes [ 52 ].

Insulin resistance is one of the major symptoms of metabolic disorders and is frequently associated with hypertension, high blood glucose levels, visceral obesity, and dyslipidemia. Insulin resistance also causes type 2 diabetes and plays a key role in developing cancer and cardiovascular disease. Thus, pH abnormalities can cause abnormal metabolic regulation in a predisease state. We recently found an observation that the interstitial pH around the hippocampus, an important region for memory [ 53 ], is lower in diabetic OLETF rats (26 weeks of age) than in normal Wistar rats [ 54 ]. It has been reported that diabetic patients have a high risk of developing dementia and Alzheimer's disease [ 55 ] and may experience defective memory functions. The insulin action is required for neuronal survival within the central nervous system [ 56 ]. Fluctuating glucose levels resulting from defective insulin have been suggested to lead to apoptosis, energy starvation, formation of neuritic plaques and neurofibrillary tangles, hallmark lesions of Alzheimer's disease, and altered acetylcholine levels in the hippocampus [ 57 , 58 ]. Therefore, we indicate that maintenance of the interstitial fluid pH within the normal range or the recovery of the interstitial pH to the normal range could be one of the most important factors in developing molecular and cellular therapies for metabolic brain disorders.

5. pH Regulation by Diet and Exercise Intervention

The maintenance of pH in metabolic organs is achieved through various regulatory systems. Physical exercise and appropriate diet contribute to pH homeostasis. Habitual exercise adaptively accelerates the entry of fatty acids both from the plasma into the muscle cell and from the cytosol into the mitochondria, while also enhancing Krebs cycle function in the resting state. Their actions are caused by elevation of activity and expression of related enzymes in skeletal muscles [ 59 – 61 ]. Since the energy consumed in muscle during exercise is mainly supplied by carbohydrates and lipids, the exercise-induced lipid utilization may decrease the energy obtained from carbohydrates, further decreasing the lactate/proton production, or lactic acidosis. In addition, circulating and intramuscular buffering capacities are improved via habitual exercise increasing proteins, amino acids, and phosphate [ 62 – 64 ]. Peripheral circulation is also improved through vasodilation caused as a physiological adaptation to exercise [ 65 ], which further facilitates the proton washout. In particular there is evidence suggesting that excretion of protons from the cytosol to the extracellular space or into circulation via transporters located on the plasma membrane contributes to the prevention of intracellular acidosis. It has been reported that exercise training increases the MCT1 and MCT4 levels in the skeletal and cardiac muscle of humans and animals [ 66 – 68 ]. Although the regulation of MCT expression levels is not clearly understood, it has been suggested that protein kinases A and B are involved in the regulation of MCT expression [ 69 ] as an adaptation mechanism, which may be mediated by an increase in lactate movement across the membrane. In addition, our recent study has reported that MCT1 content in erythrocyte membranes is elevated by exercise training in rats [ 70 , 71 ]. A proportion of the lactate released from skeletal muscles into the plasma is taken up by erythrocytes. The mature erythrocytes generate ATP only through the glycolytic pathway, since they have no mitochondrial machinery. Thus, erythrocytes cannot utilize lactate produced as a respiratory fuel and this necessitates the release of lactate into the plasma via MCT1 [ 72 ]. However, one of the most important roles of erythrocytes is to distribute released monocarboxylates by taking up monocarboxylates, since erythrocytes produce much less lactate than other tissues. Based on the results of our in vitro study, the skeletal muscle may be entirely dependent on MCT1-mediated lactate uptake by erythrocytes to maintain pH homeostasis [ 71 ]. In addition, there is a high correlation between the athletic performance of horses and their erythrocyte lactate concentrations after racing [ 73 ]. Therefore, efficient proton transport via MCTs induced by habitual exercise may contribute to the improvement of insulin sensitivity and muscle fatigue caused by lowered pH.

It is well known that adequate diet is important for controlling pathological conditions in patients with metabolic disorders. In addition, intervention studies in humans have reported that several bioactive factors included in foods such as antioxidants [ 74 – 77 ] and n-3 unsaturated fatty acids [ 78 , 79 ] improve energy metabolism. Additional factors such as carotenoids, alpha lipoic acids, amino acids/peptides, and minerals may also offer preventive or therapeutic effects to combat hyperglycemia and several animal and culture studies have demonstrated their efficacy in improving insulin sensitivity [ 80 – 84 ]. The effects of these nutrients are only beneficial when administered in combination. In contrast to the successful application of dietary approaches or combined nutrients [ 85 – 87 ], various types of intervention studies using single nutrients have failed to clarify their beneficial action on cardiovascular risk and insulin resistance [ 88 , 89 ]. Therefore, administration of multiple nutrients is considered more effective when compared to administration of a single bioactive factor. Propolis, a natural product derived from the plant resins collected by honeybees, contains various types of compounds including polyphenols, phenolic aldehydes, sesquiterpene quinones, coumarins, amino acids, steroids, and inorganic compounds [ 90 ] and has been reported to reduce the metabolic defects caused by abnormal blood glucose and insulin in young (18 weeks of age) OLETF rats [ 42 ] characterized by hyperphagia, obesity, decreased glucose infusion rate in a euglycemic clamp at 16–18 weeks of age, hyperinsulinemia around 25 weeks of age responding to an intravenous glucose infusion, and developing type 2 diabetes [ 91 , 92 ]. Thus, our study indicates that propolis has a beneficial and preventive action on type 2 diabetes mellitus at early stages developing insulin resistance. Further, we have obtained evidence that intake of propolis elevates the pH of ascites and metabolic tissues compared with normal diet, indicating that dietary propolis diminishes production of organic acids or increases buffering capacity in those tissues. Therefore, propolis may be a useful compound to improve insulin sensitivity via prevention of metabolic acidosis. The molecular mechanism of how propolis improves interstitial pH is unclear, and we should strive to better understand the mechanism of this bioactive supplement.

6. Conclusion

Membrane transport of protons is required for preventing acidic states of body fluid, maintaining physical performance, and improving metabolic impairments. In contrast to the intracellular and blood pH, interstitial fluid pH can easily be reduced by acid stress. This can disturb homeostasis of the intracellular metabolism, leading to the development of metabolic diseases. However, detailed mechanisms including the involvement of membrane transport of protons responsible for the reduction of interstitial fluid pH are unknown. In addition, activity and expression of proton transporters such as MCT and NHE are easily altered by various changes in the cell environment. More studies are required to examine the detailed regulatory mechanisms of proton transporters, including gene expression, protein modification, and membrane trafficking, in addition to their contributions to metabolic homeostasis.

Acknowledgments

This work was supported by Grants-in-Aid from Japan Society of the Promotion of Science (25282199 and 25670111), Adaptable and Seamless Technology Transfer Program through Target-Driven R&D, Japan Science and Technology Agency (JST), Four-University Collaborative Research Grant, Salt Science Foundation (1235), and Cell Research Conference.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

26.1 Body Fluids and Fluid Compartments

Learning objectives.

By the end of this section, you will be able to:

• Explain the importance of water in the body
• Contrast the composition of the intracellular fluid with that of the extracellular fluid
• Explain the importance of protein channels in the movement of solutes
• Identify the causes and symptoms of edema

The chemical reactions of life take place in aqueous solutions. The dissolved substances in a solution are called solutes. In the human body, solutes vary in different parts of the body, but may include proteins—including those that transport lipids, carbohydrates, and, very importantly, electrolytes. Often in medicine, an electrolyte is referred to as a mineral dissociated from a salt that carries an electrical charge (an ion). For instance, sodium ions (Na + ) and chloride ions (Cl – ) are often referred to as electrolytes.

In the body, water moves through semi-permeable membranes of cells and from one compartment of the body to another by a process called osmosis. Osmosis is basically the diffusion of water from regions of higher concentration to regions of lower concentration, along an osmotic gradient across a semi-permeable membrane. As a result, water will move into and out of cells and tissues, depending on the relative concentrations of the water and solutes found there. An appropriate balance of solutes inside and outside of cells must be maintained to ensure normal function.

Body Water Content

Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 50–60 percent in adult men and women, to as low as 45 percent in old age. The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone, and other tissues change from infancy to adulthood ( Figure 26.1.1 ). Your brain and kidneys have the highest proportions of water, which composes 80–85 percent of their masses. In contrast, teeth have the lowest proportion of water, at 8–10 percent.

Fluid Compartments

Body fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some form of a physical barrier. The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. Extracellular fluid (ECF) surrounds all cells in the body. Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood ( Figure 26.1.2 ).

Intracellular Fluid

The ICF lies within cells and is the principal component of the cytosol/cytoplasm. The ICF makes up about 60 percent of the total water in the human body, and in an average-size adult male, the ICF accounts for about 25 liters (seven gallons) of fluid ( Figure 26.1.3 ). This fluid volume tends to be very stable, because the amount of water in living cells is closely regulated. If the amount of water inside a cell falls to a value that is too low, the cytosol becomes too concentrated with solutes to carry on normal cellular activities; if too much water enters a cell, the cell may burst and be destroyed.

Extracellular Fluid

The ECF accounts for the other one-third of the body’s water content. Approximately 20 percent of the ECF is found in plasma. Plasma travels through the body in blood vessels and transports a range of materials, including blood cells, proteins (including clotting factors and antibodies), electrolytes, nutrients, gases, and wastes. Gases, nutrients, and waste materials travel between capillaries and cells through the IF. Cells are separated from the IF by a selectively permeable cell membrane that helps regulate the passage of materials between the IF and the interior of the cell.

The body has other water-based ECF. These include the cerebrospinal fluid that bathes the brain and spinal cord, lymph, the synovial fluid in joints, the pleural fluid in the pleural cavities, the pericardial fluid in the cardiac sac, the peritoneal fluid in the peritoneal cavity, and the aqueous humor of the eye. Because these fluids are outside of cells, these fluids are also considered components of the ECF compartment.

Composition of Body Fluids

The compositions of the two components of the ECF—plasma and IF—are more similar to each other than either is to the ICF ( Figure 26.1.4 ). Blood plasma has high concentrations of sodium, chloride, bicarbonate, and protein. The IF has high concentrations of sodium, chloride, and bicarbonate, but a relatively lower concentration of protein. In contrast, the ICF has elevated amounts of potassium, phosphate, magnesium, and protein. Overall, the ICF contains high concentrations of potassium and phosphate (HPO42−HPO42−), whereas both plasma and the ECF contain high concentrations of sodium and chloride.

External Website

Watch this video to learn more about body fluids, fluid compartments, and electrolytes. When blood volume decreases due to sweating, from what source is water taken in by the blood?

Most body fluids are neutral in charge. Thus, cations, or positively charged ions, and anions, or negatively charged ions, are balanced in fluids. As seen in the previous graph, sodium (Na + ) ions and chloride (Cl – ) ions are concentrated in the ECF of the body, whereas potassium (K + ) ions are concentrated inside cells. Although sodium and potassium can “leak” through “pores” into and out of cells, respectively, the high levels of potassium and low levels of sodium in the ICF are maintained by sodium-potassium pumps in the cell membranes. These pumps use the energy supplied by ATP to pump sodium out of the cell and potassium into the cell ( Figure 26.1.5 ).

Fluid Movement between Compartments

Hydrostatic pressure, the force exerted by a fluid against a wall, causes movement of fluid between compartments. The hydrostatic pressure of blood is the pressure exerted by blood against the walls of the blood vessels by the pumping action of the heart. In capillaries, hydrostatic pressure (also known as capillary blood pressure) is higher than the opposing “colloid osmotic pressure” in blood—a “constant” pressure primarily produced by circulating albumin—at the arteriolar end of the capillary ( Figure 26.1.6 ). This pressure forces plasma and nutrients out of the capillaries and into surrounding tissues. Fluid and the cellular wastes in the tissues enter the capillaries at the venule end, where the hydrostatic pressure is less than the osmotic pressure in the vessel. Filtration pressure squeezes fluid from the plasma in the blood to the IF surrounding the tissue cells. The surplus fluid in the interstitial space that is not returned directly back to the capillaries is drained from tissues by the lymphatic system, and then re-enters the vascular system at the subclavian veins.

Watch this video to see an explanation of the dynamics of fluid in the body’s compartments. What happens in the tissue when capillary blood pressure is less than osmotic pressure?

Hydrostatic pressure is especially important in governing the movement of water in the nephrons of the kidneys to ensure proper filtering of the blood to form urine. As hydrostatic pressure in the kidneys increases, the amount of water leaving the capillaries also increases, and more urine filtrate is formed. If hydrostatic pressure in the kidneys drops too low, as can happen in dehydration, the functions of the kidneys will be impaired, and less nitrogenous wastes will be removed from the bloodstream. Extreme dehydration can result in kidney failure.

Fluid also moves between compartments along an osmotic gradient. Recall that an osmotic gradient is produced by the difference in concentration of all solutes on either side of a semi-permeable membrane. The magnitude of the osmotic gradient is proportional to the difference in the concentration of solutes on one side of the cell membrane to that on the other side. Water will move by osmosis from the side where its concentration is high (and the concentration of solute is low) to the side of the membrane where its concentration is low (and the concentration of solute is high). In the body, water moves by osmosis from plasma to the IF (and the reverse) and from the IF to the ICF (and the reverse). In the body, water moves constantly into and out of fluid compartments as conditions change in different parts of the body.

For example, if you are sweating, you will lose water through your skin. Sweating depletes your tissues of water and increases the solute concentration in those tissues. As this happens, water diffuses from your blood into sweat glands and surrounding skin tissues that have become dehydrated because of the osmotic gradient. Additionally, as water leaves the blood, it is replaced by the water in other tissues throughout your body that are not dehydrated. If this continues, dehydration spreads throughout the body. When a dehydrated person drinks water and rehydrates, the water is redistributed by the same gradient, but in the opposite direction, replenishing water in all of the tissues.

Solute Movement between Compartments

The movement of some solutes between compartments is active, which consumes energy and is an active transport process, whereas the movement of other solutes is passive, which does not require energy. Active transport allows cells to move a specific substance against its concentration gradient through a membrane protein, requiring energy in the form of ATP. For example, the sodium-potassium pump employs active transport to pump sodium out of cells and potassium into cells, with both substances moving against their concentration gradients.

Passive transport of a molecule or ion depends on its ability to pass through the membrane, as well as the existence of a concentration gradient that allows the molecules to diffuse from an area of higher concentration to an area of lower concentration. Some molecules, like gases, lipids, and water itself (which also utilizes water channels in the membrane called aquaporins), slip fairly easily through the cell membrane; others, including polar molecules like glucose, amino acids, and ions do not. Some of these molecules enter and leave cells using facilitated transport, whereby the molecules move down a concentration gradient through specific protein channels in the membrane. This process does not require energy. For example, glucose is transferred into cells by glucose transporters that use facilitated transport ( Figure 26.1.7 ).

Disorders of the… Fluid Balance: Edema

Edema is the accumulation of excess water in the tissues. It is most common in the soft tissues of the extremities. The physiological causes of edema include water leakage from blood capillaries. Edema is almost always caused by an underlying medical condition, by the use of certain therapeutic drugs, by pregnancy, by localized injury, or by an allergic reaction. In the limbs, the symptoms of edema include swelling of the subcutaneous tissues, an increase in the normal size of the limb, and stretched, tight skin. One quick way to check for subcutaneous edema localized in a limb is to press a finger into the suspected area. Edema is likely if the depression persists for several seconds after the finger is removed (which is called “pitting”).

Pulmonary edema is excess fluid in the air sacs of the lungs, a common symptom of heart and/or kidney failure. People with pulmonary edema likely will experience difficulty breathing, and they may experience chest pain. Pulmonary edema can be life threatening, because it compromises gas exchange in the lungs, and anyone having symptoms should immediately seek medical care.

In pulmonary edema resulting from heart failure, excessive leakage of water occurs because fluids get “backed up” in the pulmonary capillaries of the lungs, when the left ventricle of the heart is unable to pump sufficient blood into the systemic circulation. Because the left side of the heart is unable to pump out its normal volume of blood, the blood in the pulmonary circulation gets “backed up,” starting with the left atrium, then into the pulmonary veins, and then into pulmonary capillaries. The resulting increased hydrostatic pressure within pulmonary capillaries, as blood is still coming in from the pulmonary arteries, causes fluid to be pushed out of them and into lung tissues.

Other causes of edema include damage to blood vessels and/or lymphatic vessels, or a decrease in osmotic pressure in chronic and severe liver disease, where the liver is unable to manufacture plasma proteins ( Figure 28.1.8 ). A decrease in the normal levels of plasma proteins results in a decrease of colloid osmotic pressure (which counterbalances the hydrostatic pressure) in the capillaries. This process causes loss of water from the blood to the surrounding tissues, resulting in edema.

Mild, transient edema of the feet and legs may be caused by sitting or standing in the same position for long periods of time, as in the work of a toll collector or a supermarket cashier. This is because deep veins in the lower limbs rely on skeletal muscle contractions to push on the veins and thus “pump” blood back to the heart. Otherwise, the venous blood pools in the lower limbs and can leak into surrounding tissues.

Medications that can result in edema include vasodilators, calcium channel blockers used to treat hypertension, non-steroidal anti-inflammatory drugs, estrogen therapies, and some diabetes medications. Underlying medical conditions that can contribute to edema include congestive heart failure, kidney damage and kidney disease, disorders that affect the veins of the legs, and cirrhosis and other liver disorders.

Therapy for edema usually focuses on elimination of the cause. Activities that can reduce the effects of the condition include appropriate exercises to keep the blood and lymph flowing through the affected areas. Other therapies include elevation of the affected part to assist drainage, massage and compression of the areas to move the fluid out of the tissues, and decreased salt intake to decrease sodium and water retention.

Chapter Review

Your body is mostly water. Body fluids are aqueous solutions with differing concentrations of materials, called solutes. An appropriate balance of water and solute concentrations must be maintained to ensure cellular functions. If the cytosol becomes too concentrated due to water loss, cell functions deteriorate. If the cytosol becomes too dilute due to water intake by cells, cell membranes can be damaged, and the cell can burst. Hydrostatic pressure is the force exerted by a fluid against a wall and causes movement of fluid between compartments. Fluid can also move between compartments along an osmotic gradient. Active transport processes require ATP to move some solutes against their concentration gradients between compartments. Passive transport of a molecule or ion depends on its ability to pass easily through the membrane, as well as the existence of a high to low concentration gradient.

Interactive Link Questions

The interstitial fluid (IF).

Watch this video to see an explanation of the dynamics of fluid in the body’s compartments. What happens in tissues when capillary blood pressure is less than osmotic pressure?

Fluid enters the capillaries from interstitial spaces.

Review Questions

Critical thinking questions.

1. Plasma contains more sodium than chloride. How can this be if individual ions of sodium and chloride exactly balance each other out, and plasma is electrically neutral?

2. How is fluid moved from compartment to compartment?

Answers for Critical Thinking Questions

• There are additional negatively charged molecules in plasma besides chloride. The additional sodium balances the total negative charges.
• Fluid is moved by a combination of osmotic and hydrostatic pressures. The osmotic pressure results from differences in solute concentrations across cell membranes. Hydrostatic pressure results from the pressure of blood as it enters a capillary system, forcing some fluid out of the vessel into the surrounding tissues.

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Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.