essay on excretion and homeostasis

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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

StatPearls [Internet].

Physiology, osmoregulation and excretion.

Jiatong (Steven) Chen ; Sarah Sabir ; Yasir Al Khalili .

Affiliations

Last Update: May 1, 2023 .

• Introduction

As living organisms, the maintenance of fluid balance is critical to sustaining many bodily functions, including metabolic and biochemical reactions, transport of nutrients and thermoregulation. The average adult has roughly 65% fluid mass, with this value being slightly lower in females than males. Our body fluids may subcategorize into intracellular and extracellular fluid compartments. [1] [2] [3]

Intracellular fluid, also considered as the cytosol, is all the fluid contained within cells. The intracellular fluid makes up roughly two-thirds of the total body volume.

Extracellular fluid constitutes the remaining one-third of fluid volume in the body and may further divide into its interstitial, intravascular and transcellular compartments. [1] [2] [3]

The maintenance of fluid homeostasis in each of these compartments is dependent on the excretion of fluids and the concentration of electrolytes that generate osmotic pressure. This process of passive regulation of osmotic pressure is known as osmoregulation. [1] [2] [3]

This article will provide an overview of osmoregulation and excretion, focusing on a discussion of the renal system involved in maintaining this intimate balance between fluid retention and excretion.

• Cellular Level

Osmosis occurs when two solutions containing different concentrations of solute are divided by a selectively permeable membrane. In the human body, this selectively permeable membrane may be the cellular membrane (in the case of intracellular fluid) or maybe a membrane lining your body cavity composed of cells (in the case of extravascular fluid). The solute concentration difference across the membrane gives rise to a gradient that facilitates the movement of a solvent (usually water in our body) until attaining equilibrium. The tendency of a solution to draw water in through the semipermeable membrane is the osmotic pressure. [1] [2] [3] [4]

The unit of osmoles is used to express the number of particles. One osmole refers to one mole of osmotically active solute particles. Though similar to molarity, osmolarity refers to the total number of active particles. For instance, one mole of glucose dissolved in one liter of solution would have molarity and osmolarity of 1 osm/L (or 1 mol/L). However, if a molecule dissociates into two ions (yielding two particles)—for instance, sodium chloride, then the 1 mol/L solution will yield an osmolarity of 2 osm/L. [5] [6]

 It is also noteworthy that there is a distinction between the terms osmolarity and osmolality. While osmolarity is the number of osmoles per liter, osmolality is the number of osmoles per kilogram. In terms of volume status, we are concerned more so with the plasma osmolality, since it is independent of temperature and pressure. However, clinically, it is much easier to express body fluid quantities in liters rather than kilograms. At low concentrations (as in the human body), these two terms are almost synonymous with each other. [5] [6]

The total osmolarity for each of the three fluid compartments (intracellular, interstitial, intravascular) is around 280 mOsm/L, with intravascular being slightly greater due to the osmotic effects of plasma proteins. The composition of the interstitial and intravascular fluid is similar, with sodium and chloride being the primary contributors to the osmolarity. For intracellular fluid, almost half the osmolarity is due to potassium ions, with the other half composed of various other substances (e.g., phosphate, phosphocreatine, magnesium ions). [1] [6]

• Organ Systems Involved

To maintain homeostasis, the excretion of water and electrolytes must match an individual’s intake. The kidneys play a substantial role in osmoregulation by controlling the quantity of fluid reabsorbed from the glomerular filtrate. This fluid is reabsorbed in the renal tubes and may be modulated by hormones such as antidiuretic hormone (ADH), aldosterone, as well as angiotensin II. The capacity of the kidneys to alter fluid excretion, as well as electrolyte excretion (e.g., sodium), is enormous. Studies have shown that sodium intake of 10 times the normal amount has relatively small changes in extracellular fluid volume and plasma sodium concentration as a result of renal compensation. [1] [3] [6] [7]

The glomerular filtration rate (GFR) of an average human is 180L/day. Given that the plasma volume of a person is only 3L, large amounts of body fluid and solutes are processed by the kidney each day. The advantage of this high GFR in terms of osmoregulation is that it enables the kidneys to rapidly and precisely regulate the volume and composition of body fluids. [8]

At the level of the hypothalamus, osmoreceptor response to extracellular fluid hypertonicity (increased osmolarity), will elicit ADH release from axons down to the posterior pituitary into the circulation. ADH serves a primary function to increase solute-free water reabsorption in the nephrons (less water excretion) to bring down body fluid hypertonicity. [9] [10]

Osmoregulation and the maintenance of body fluid levels are critical to our metabolic activities as organisms. As mentioned earlier, this is the result of ensuring adequate organ perfusion, proper thermoregulation, excretion of toxic waste and electrolyte balance. [1]

On a cellular level, the osmolarity of the extracellular fluid impacts the passage of water in and out of a cell. Isotonic fluids contain the same concentration as the intracellular milieu, whereas hypertonic fluids (higher concentration than inside the cell) lead to cell shrinkage and hypotonic fluids lead to cell swelling (lower concentration than inside the cell). Exaggerated amounts of solute changes result in osmotic stress, which is damaging to cells. [1] [2] [3] [4]

Several key mechanisms contributing to osmoregulation appear below:

Sympathetic regulation: Strong activation of the renal sympathetic nerves can constrict the renal arterioles and decrease renal blood flow and GFR, leading to increased fluid retention. [11]

Autoregulation: Renal autoregulation helps maintain a relatively consistent GFR and establish delicate control of the excretion of water and solutes. In particular, the tubuloglomerular feedback mechanism of the macula densa serves to ensure steady delivery of sodium chloride to the distal tubule, consequently reducing spurious fluctuations in renal salt excretion. [12] [13]

Hormonal regulation:

• Angiotensin II has numerous direct effects on tubular function, including decreased medullary blood flow in the vasa recta, tubule hypertrophy, relative efferent arteriolar constriction leading to the maintenance (or rise) in GFR, and compensatory sodium absorption to maintain fluid balance. Angiotensin II also stimulates the production and release of aldosterone and ADH, both important hormonal contributors to electrolyte and fluid balance. [14]
• Atrial natriuretic peptide gets released in response to elevated atrial pressure. It acts to increase GFR and sodium filtration as well as inhibit sodium uptake, leading to volume loss at the distal convoluted tubule. [15]
• Aldosterone has effects on the distal tubule and collecting duct by increasing sodium uptake and potassium excretion into the urine; this is mediated via upregulation of basolateral Na+/K+ pumps, epithelial sodium channels, amongst other mechanisms, resulting in net fluid retention. [14]
• ADH serves a primary function to increase solute-free water reabsorption in the nephrons (less water excretion) to bring down body fluid hypertonicity; this is induced by the insertion of water channels (aquaporin-2) on the apical membrane of the collecting duct. [9] [10]
• Pathophysiology

Fluid and electrolyte imbalance may manifest from numerous conditions or maybe the underlying etiology for some disease states.

Syndrome of inappropriate ADH secretion (SIADH) involves the excessive release of antidiuretic hormone. This excessive ADH release may stem from hypothalamic hyperactivity, or ectopic sources (e.g., small-cell carcinoma). Increased ADH promotes free-water reabsorption from the filtrate, leading to an inappropriately elevated urine osmolality (greater than 100 mOsmol/L) compared to blood plasma and by consequence, hyponatremia. [16]

Kidney disease, in both acute and chronic contexts, impairs glomerular function, thereby reducing the production of the filtrate. Typically, this translates to increased water retention, increased potassium retention and dilution of plasma sodium concentrations due to reduced water excretion. [17] [18]

Edema is the presence of excess fluid in either the intracellular fluid compartment or the extracellular fluid compartment. In the case of extracellular edema, more commonly considered clinically, osmotic causes may include acute or chronic kidney failure, mineralocorticoid excess, decreased plasma proteins (e.g., nephrotic syndrome) or decreased hepatic synthesis of proteins. Symptomatically, this may present as either generalized edema, also known as anasarca; or it may be more localized, as seen in sacral, pretibial, pulmonary edema. [19]

• Clinical Significance

Fluid maintenance in patients: When administering fluids to patients, an important consideration is the osmotic content of the solution. In the case that the patient does not have any underlying electrolyte abnormalities,  normal saline (0.9% NaCl) is a common choice for maintenance (particularly in pediatric populations) as it mimics the tonicity of blood. In cases of hypovolemic shock, hypertonic solutions are increasingly recognized as an alternative resuscitation fluid, albeit insufficient evidence, since they promote fluid movement into the intravascular space (by creating a high osmotic gradient across cellular membranes). [19] [20]

Osmolar gap: The osmolar gap is the difference between the measured osmolality and the calculated osmolarity. The calculated osmolarity is given as: 2[Na] + [Glucose] + [Urea] ( all in mmol/L). Clinically, the osmolar gap may be used to detect the presence of an osmotically active particle that is not normally present in plasma. Common examples include toxic alcohol such as methanol or butanol. [21]

Diuretics: The principles of osmoregulation apply nicely to explain the physiological effects of many diuretics. For instance, loop diuretics operate mechanistically by blocking the sodium potassium chloride pump (NKCC) in the ascending loop of Henle. This blockade prevents reabsorption of sodium back into the blood, and by osmotic pressure leads to increased water loss through the urine as well. [22] Another example is with osmotic diuretics, such as mannitol. Mannitol gets filtered through the glomerulus but cannot be reabsorbed in the nephron. Thus, increased osmotic pressure is exerted in the filtrate, causing water to be retained in the tubules to ensure urine osmolality. The result is increased water expulsion in the urine. [23]

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Disclosure: Jiatong (Steven) Chen declares no relevant financial relationships with ineligible companies.

Disclosure: Sarah Sabir declares no relevant financial relationships with ineligible companies.

Disclosure: Yasir Al Khalili declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

• Cite this Page Chen JS, Sabir S, Al Khalili Y. Physiology, Osmoregulation and Excretion. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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Excretory System

Reviewed by: BD Editors

Excretory System Definition

The excretory system consists of organs which remove metabolic wastes and toxins from the body. In humans, this includes the removal of urea from the bloodstream and other wastes produced by the body. The removal of urea happens in the kidneys, while solid wastes are expelled from the large intestine.

The excretory system in humans consists mainly of the kidneys and bladder. The kidneys filter urea and other waste products from the blood, which are then added to the urine within the bladder. Other organs, such as the liver, process toxins but put their wastes back into the blood. It is up to the kidneys to filter the blood so that toxic substances do not accumulate. These organs can be seen in the image below.

The excretory system has other functions beyond removing waste products from the body. It is also crucial in maintaining internal homeostasis . Parts of the excretory system are also influenced by other body systems, such as the muscular system and skeletal system. For example, the kidneys secrete a hormone which tells the bones to produce more red blood cells.

When the excretory system is not functioning, bad things can happen. A build-up of urea within the blood can lead to a debilitating toxic shock. In other animals, the excretory system can include a number of other components. For example, sea turtles have excretory organs near their eyes which remove large amounts of salt from their bodies. This allows them to drink salt water to maintain their water balance.

Excretory System Function

The excretory system functions as the bulwark and balance to the digestive system. While we consume food and drink to nourish the body and provide energy, the excretory system ensures that homeostasis is maintained, irrespective of changes to the nutritive value of food.

It regulates the fluid balance of the body, maintaining adequate salt and water levels. When there is excess water, it is removed through the production of hypotonic urine. When we consume salty food or lose water through perspiration, the concentration of urine is increased to preserve the osmolarity of body fluids.

Excretory System Organs

The primary excretory organs in the human body are the kidneys, ureters and urinary bladder, involved with the creation and expulsion of urine. Through these organs, much of the nitrogenous waste of the body, especially urea, is expelled. Other organs such as the liver, large intestine and skin are also necessary for the excretion of specific metabolic wastes.

The kidneys are paired, bean-shaped organs located in the abdomen, on either side of the spine, under the diaphragm. T hey are made of a large number of structural and functional subunits called nephrons . These nephrons perform the primary task of filtering blood and removing waste products. Each nephron snakes between the outer cortex of the kidney and the inner medulla, with different activities occurring at each site.

The image above shows parts of two nephrons, with their relative positions within the kidney. Each nephron begins with a globular structure called the Bowman’s capsule located in the renal cortex. This structure receives blood from renal circulation through an afferent arteriole that further divides to form a tuft of capillaries called the glomerulus. The kidney is richly vascularized with capillary beds surrounding each nephron (intertubular capillaries) as well as blood vessels running between the lobes of the kidney (interlobular arteries and veins).

Kidney Function

A process of ultrafiltration creates the glomerular filtrate from blood, which is remarkably similar in composition to blood plasma. Water, small molecules, and proteins smaller than 30 kilodaltons in size can pass freely into the lumen of the Bowman’s capsule. The anatomy of each nephron is discussed below.

Bowman’s Capsule to the PCT

The Bowman’s capsule involutes and creates a neck, which then extends into the first elongated tubular structure called the proximal convoluted tubule or PCT. The PCT is the site for secreting some acids, and for reabsorbing nearly two-thirds of the glomerular filtrate. It also removes all glucose and amino acids. The presence of either glucose or other organic solutes in the urine is a sign of kidney damage, especially of the cortex. Some nitrogenous waste is also removed from the body as ammonia secreted from the cells forming the PCT. Many medications are also detoxified at this site.

PCT to the Loop of Henle

The PCT leads into a U-shaped structure called the Loop of Henle , extending into the medulla of the kidney. This has two functionally and anatomically distinct arms – the ascending and descending limbs. Between these two arms of the loop of Henle, through a set of electrolyte pumps, a high urea concentration is maintained in the medulla of the kidney.

The PCT initially leads into the descending loop, which is freely permeable to water and mostly impermeable to ions – especially urea. The high osmolarity of the medullary region of the kidney draws water out the descending loop, allowing the urine to become concentrated.

This is followed by the thin ascending loop, which has the opposite property of being permeable to ions and impermeable to water. Solutes such as sodium ions are actively reabsorbed, reducing the concentration of urine. However, by this time, the volume of fluid filtered at the glomerulus has been reduced to a fraction of its quantity.

Loop of Henle to the DCT

The ascending limb then leads into the distal convoluted tubule or DCT, also known as the second convoluted tubule. The DCT is the site for the activity of most hormones that regulate kidney function. This includes the antidiuretic hormone (ADH) and angiotensin II (AT II). This region regulates ion and pH balance. From the DCT, urine passes through collecting ducts that finally lead out of the kidney through ureters.

This image is a composite representation of the nephron , with details about the substances reabsorbed at each site, the osmolarity of the filtrate at different parts of the nephron, and the impact of different hormones or medications.

Urinary Bladder

The urinary bladder is a sac-like structure with muscular walls that holds urine until it is expelled from the body during micturition. The bladder receives urine through two ureters – one from each kidney –that enter through openings called ureteric orifices. These orifices are located at the convex fundus of the organ. Urine exits the bladder through the urethra.

The walls of the bladder are made of smooth muscle and the inner epithelial lining of this organ consists of a remarkable tissue called transitional epithelium. The cells of this stratified tissue change shape based on whether the bladder is empty or full, allowing it to remain elastic, accommodating up to half a liter of urine.

In men, the bladder lies on the pelvic floor in front of the rectum. In women, it is located near the uterus, leading to a number of changes to the patterns of micturition during the course of pregnancy. During the course of gestation, there are major changes to blood volume and increases in glomerular filtration rate. While the bladder itself increases in size, nearly doubling by the end of the third trimester, the enlarged uterus with the weight of the fetus, amniotic fluid, placenta, and other tissues can create stress incontinence.

The liver is the main detoxifying organ of the body, especially for nitrogenous wastes. The cells of the liver play host to biochemical processes that create ammonia from amino acids. Since ammonia is extremely toxic, it is quickly converted to urea before being transported in the blood towards the kidney.

Most animals make the choice between ammonia, urea, and uric acid as the preferred mode for nitrogenous waste excretion, based on the availability of water. While ammonia is toxic, it can be quickly diluted and removed from the body with ample water, and therefore remains the chemical used by aquatic animals. Terrestrial animals with regular access to water tend to use urea, which has lower toxicity. Birds and other animals that have minimal water intake expend energy to convert urea into uric acid, which needs a minimum amount of water to store safely until excretion.

Large Intestine

The liver is also necessary for the removal of the decomposed hemoglobin, some drugs, excess vitamins, sterols, and other lipophilic substances. These are secreted along with bile and finally removed from the body through feces. The large intestine, therefore, plays a role in excretion , especially for hydrophobic particles.

The skin is a secondary excretory organ since sweat glands in the dermis can remove salts and some excess water. The skin also has sebaceous glands that can secrete waxy lipids.

Lungs or Gills

A major product that must be excreted from all animals is carbon dioxide. Carbon dioxide is created in the cells, as they undergo aerobic respiration. This waste product is removed from the cells and transferred to the bloodstream. When the blood reaches the gills or lungs, it is exchanged for oxygen and released into the atmosphere. Fish also use their gills to expel a number of other waste products.

Excretory System Structure

The excretory system is necessary for preventing the toxic build up of nitrogenous wastes, such as ammonia or urea. However, the excretory system of animals has evolved in many different ways since the dawn of life on Earth.

In fish and aquatic animals, the excretory system is fairly simple. The gills are a major site of excretion, and some waste products are simply added to the blood to be excreted in the gills. These animals also rely on their skin and glands to excrete excess salts and other waste products. In fact, freshwater and saltwater fish have drastically different kidney functions, based on the concentration of salt in the surrounding water.

In terrestrial animals, such as humans, the excretory system is structured to retain as much water as possible. Birds and reptiles have even developed uric acid, which is a more concentrated and safer form of urea. As a whole system, every part and organ of the excretory system can be functioning at the same time to remove wastes from the body. However, if the structure of the excretory system gets damaged by disease, many bad consequences can ensue.

Excretory System Diseases

The excretory system, especially the kidneys, can be injured, damaged or have suboptimal functioning, either due to acute stress or through chronic conditions.

Renal Failure

Renal failure or renal insufficiency is the inability of the kidney to filter wastes from the blood and maintain fluid homeostasis. The causes of renal failure could be diseases such as diabetes mellitus and hypertension that can cause damage to glomerular capillaries. Diabetes insipidus arising from hormonal insufficiency, reduced blood flow from injury, infections in the body and bloodstream, medications, or kidney stones can also affect kidney efficiency.

Initial symptoms can be as mild as swelling in the legs, indicative of the inability of the kidney to maintain fluid homeostasis. The presence of toxins in the blood can cause a feeling of nausea and vomiting. Changes to the RBC metabolism and reduced erythropoietin secretion from the kidney can lead to anemia, weakness, sleepiness, and confusion. Excessive potassium ions can lead to cardiac arrhythmias, and changes to muscle tone and contractility.

Depending on the cause of renal insufficiency or failure, the injury can be reversed. In most cases, long term changes to diet and lifestyle are necessary to maintain health. When the kidney is functioning at extremely low efficiency, waste removal has to be done through an external apparatus, called the dialysis machine. Kidney transplant is also occasionally recommended.

Excretory System Facts

• The urinary bladder can hold up to 600 ml of liquid . During early pregnancy, the uterus presses on to the bladder, creating a greater frequency of urination.
• Most of the amniotic fluid surrounding the growing fetus is fetal urine, though its composition is very different from normal urine. The bladder of the fetus begins to empty around the 10th week of gestation.
• This fetal urine and the amniotic fluid are actually important for the development of fetal lungs.
• The white parts in bird excreta are composed mostly of uric acid.
• The brownish pigmentation of feces mostly derives from bile salts.

1. Which of these statements about the excretory system is true?

2. Which of these statements about a nephron is NOT true?

3. If albumin, with a molecular mass of 66.5 kilodaltons is found in urine, what might this indicate?

4. A single-celled organism does not have an excretory system, because it does not have tissues or organs. However, single-celled organisms still need to excrete certain substances. How do they complete this task?

5. What is the purpose of the bladder within the excretory system?

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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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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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• Introduction

Biological significance of elimination

• Nonmetabolic wastes
• Gaseous wastes
• Liquid wastes
• Solid wastes
• Alimentary canal
• Respiratory system
• The kidneys
• Nonspecific mechanisms of waste disposal
• Nemertine worms
• Other invertebrates
• Vertebrates
• Products of excretion
• Osmotic pressure
• Regulation of water and salt balance
• Principal excretory structures
• The contractile vacuoles of protozoans
• The nephridia of annelids, nemertines, flatworms, and rotifers
• The renal glands of mollusks
• The coxal glands of aquatic arthropods
• The malpighian tubules of insects
• Birds and reptiles
• Evolution of the vertebrate excretory system

• Why is biology important?

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• Case Western Reserve University - Environmental Health and Safety - Waste Disposal
• Chemistry LibreTexts - Excretion
• Thompson River University - Human Biology - Organs of Excretion
• National Center for Biotechnology Information - Physiology, Osmoregulation and Excretion
• Table Of Contents

excretion , the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism . Through excretion organisms control osmotic pressure—the balance between inorganic ions and water—and maintain acid-base balance. The process thus promotes homeostasis , the constancy of the organism’s internal environment .

Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination, which may be considered to encompass all of the various mechanisms and processes by which life forms dispose of or throw off waste products, toxic substances, and dead portions of the organism. The nature of the process and of the specialized structures developed for waste disposal vary greatly with the size and complexity of the organism.

Four terms are commonly associated with waste-disposal processes and are often used interchangeably, though not always correctly: excretion, secretion , egestion, and elimination.

Excretion is a general term referring to the separation and throwing off of waste materials or toxic substances from the cells and tissues of a plant or animal.

The separation, elaboration, and elimination of certain products arising from cellular functions in multicellular organisms is called secretion. Though these substances may be a waste product of the cell producing them, they are frequently useful to other cells of the organism. Examples of secretions are the digestive enzymes produced by intestinal and pancreatic tissue cells of vertebrate animals, the hormones synthesized by specialized glandular cells of plants and animals, and sweat secreted by glandular cells in the skins of some mammals. Secretion implies that the chemical compounds being secreted were synthesized by specialized cells and that they are of functional value to the organism. The disposal of common waste products should not, therefore, be considered to be of a secretory nature.

Egestion is the act of excreting unusable or undigested material from a cell, as in the case of single-celled organisms, or from the digestive tract of multicellular animals.

As defined above, elimination broadly defines the mechanisms of waste disposal by living systems at all levels of complexity. The term may be used interchangeably with excretion.

Elimination

Waste disposal by unicellular and multicellular organisms is vital to their health and to the continuance of life. Animals must take in (ingest) energy-containing chemical compounds, extract a portion of the energy to power their life processes, and dispose of the unusable material or by-products formed during the energy-extraction process. An analogous series of events occurs in an internal-combustion engine . Fuel, containing energy, is taken into the engine, where it is burned, and a portion of the energy released is used to move the pistons. As in living cells, a portion of the energy-containing material (fuel) not utilized in the engine is exhausted in the form of carbon monoxide , carbon dioxide , and other by-products of combustion. Blockage of the exhaust system in an engine results in loss of efficiency and eventual total breakdown. Similarly, the rate of waste disposal in biological systems can and does provide a means of controlling the metabolic rate. Complete blockage of waste-disposal mechanisms in living systems is as effective in destroying vital functions as the cutting off of food, oxygen , or water from the system. In addition, some substances produced as metabolic by-products are toxic in themselves and must be removed from living cells at a rate equal to that at which they are produced by those cells. Thus, the excretion of waste products from living cells must occur continually in order to ensure the normal progression of vital chemical events.

Waste and poisonous substances produced by the metabolic activities of plant and animal communities must, in a similar manner, be removed or detoxified if community health is to be preserved. Collective wastes of individual organisms constituting a community, if allowed to accumulate to any marked degree, will eventually destroy the lives of all the community members.

The biosphere , composed of all individuals and communities of life forms and their environments on the Earth, is equally sensitive to the effects of waste and poison accumulation. A continual buildup of substances harmful to life forms can only result in the eventual destruction of most or all of the presently existing species of plants and animals. Humans are unique among living things in that their activities result in the production of waste materials (pollutants) that, by virtue of their chemical structure, are poisonous to all living things, including themselves. (For information about waste disposal in the biosphere, see biosphere and conservation .)

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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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11.11: The Urinary System and Homeostasis

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Learning Objectives

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

• Describe the role of the kidneys in vitamin D activation
• Describe the role of the kidneys in regulating erythropoiesis
• Provide specific examples to demonstrate how the urinary system responds to maintain homeostasis in the body
• Explain how the urinary system relates to other body systems in maintaining homeostasis
• Predict factors or situations affecting the urinary system that could disrupt homeostasis
• Predict the types of problems that would occur in the body if the urinary system could not maintain homeostasis

All systems of the body are interrelated. A change in one system may affect all other systems in the body, with mild to devastating effects. A failure of urinary continence can be embarrassing and inconvenient, but is not life threatening. The loss of other urinary functions may prove fatal. A failure to synthesize vitamin D is one such example.

Vitamin D Synthesis

In order for vitamin D to become active, it must undergo a hydroxylation reaction in the kidney, that is, an –OH group must be added to calcidiol to make calcitriol (1,25-dihydroxycholecalciferol). Activated vitamin D is important for absorption of Ca ++ in the digestive tract, its reabsorption in the kidney, and the maintenance of normal serum concentrations of Ca ++ and phosphate. Calcium is vitally important in bone health, muscle contraction, hormone secretion, and neurotransmitter release. Inadequate Ca ++ leads to disorders like osteoporosis and osteomalacia in adults and rickets in children. Deficits may also result in problems with cell proliferation, neuromuscular function, blood clotting, and the inflammatory response. Recent research has confirmed that vitamin D receptors are present in most, if not all, cells of the body, reflecting the systemic importance of vitamin D. Many scientists have suggested it be referred to as a hormone rather than a vitamin.

Erythropoiesis

EPO is a 193-amino acid protein that stimulates the formation of red blood cells in the bone marrow. The kidney produces 85 percent of circulating EPO; the liver, the remainder. If you move to a higher altitude, the partial pressure of oxygen is lower, meaning there is less pressure to push oxygen across the alveolar membrane and into the red blood cell. One way the body compensates is to manufacture more red blood cells by increasing EPO production. If you start an aerobic exercise program, your tissues will need more oxygen to cope, and the kidney will respond with more EPO. If erythrocytes are lost due to severe or prolonged bleeding, or under produced due to disease or severe malnutrition, the kidneys come to the rescue by producing more EPO. Renal failure (loss of EPO production) is associated with anemia, which makes it difficult for the body to cope with increased oxygen demands or to supply oxygen adequately even under normal conditions. Anemia diminishes performance and can be life threatening.

Blood Pressure Regulation

Due to osmosis, water follows where Na + leads. Much of the water the kidneys recover from the forming urine follows the reabsorption of Na + . ADH stimulation of aquaporin channels allows for regulation of water recovery in the collecting ducts. Normally, all of the glucose is recovered, but loss of glucose control (diabetes mellitus) may result in an osmotic dieresis severe enough to produce severe dehydration and death. A loss of renal function means a loss of effective vascular volume control, leading to hypotension (low blood pressure) or hypertension (high blood pressure), which can lead to stroke, heart attack, and aneurysm formation.

The kidneys cooperate with the lungs, liver, and adrenal cortex through the renin–angiotensin–aldosterone system. The liver synthesizes and secretes the inactive precursor angiotensinogen. When the blood pressure is low, the kidney synthesizes and releases renin. Renin converts angiotensinogen into angiotensin I, and ACE produced in the lung converts angiotensin I into biologically active angiotensin II (Figure 1). The immediate and short-term effect of angiotensin II is to raise blood pressure by causing widespread vasoconstriction. angiotensin II also stimulates the adrenal cortex to release the steroid hormone aldosterone, which results in renal reabsorption of Na + and its associated osmotic recovery of water. The reabsorption of Na + helps to raise and maintain blood pressure over a longer term.

Regulation of Osmolarity

Blood pressure and osmolarity are regulated in a similar fashion. Severe hypo-osmolarity can cause problems like lysis (rupture) of blood cells or widespread edema, which is due to a solute imbalance. Inadequate solute concentration (such as protein) in the plasma results in water moving toward an area of greater solute concentration, in this case, the interstitial space and cell cytoplasm. If the kidney glomeruli are damaged by an autoimmune illness, large quantities of protein may be lost in the urine. The resultant drop in serum osmolarity leads to widespread edema that, if severe, may lead to damaging or fatal brain swelling. Severe hypertonic conditions may arise with severe dehydration from lack of water intake, severe vomiting, or uncontrolled diarrhea. When the kidney is unable to recover sufficient water from the forming urine, the consequences may be severe (lethargy, confusion, muscle cramps, and finally, death) .

Recovery of Electrolytes

Sodium, calcium, and potassium must be closely regulated. The role of Na + and Ca ++ homeostasis has been discussed at length. Failure of K + regulation can have serious consequences on nerve conduction, skeletal muscle function, and most significantly, on cardiac muscle contraction and rhythm.

pH Regulation

Recall that enzymes lose their three-dimensional conformation and, therefore, their function if the pH is too acidic or basic. This loss of conformation may be a consequence of the breaking of hydrogen bonds. Move the pH away from the optimum for a specific enzyme and you may severely hamper its function throughout the body, including hormone binding, central nervous system signaling, or myocardial contraction. Proper kidney function is essential for pH homeostasis.

Everyday Connections: Stem Cells and Repair of Kidney Damage

Stem cells are unspecialized cells that can reproduce themselves via cell division, sometimes after years of inactivity. Under certain conditions, they may differentiate into tissue-specific or organ-specific cells with special functions. In some cases, stem cells may continually divide to produce a mature cell and to replace themselves. Stem cell therapy has an enormous potential to improve the quality of life or save the lives of people suffering from debilitating or life-threatening diseases. There have been several studies in animals, but since stem cell therapy is still in its infancy, there have been limited experiments in humans.

Acute kidney injury can be caused by a number of factors, including transplants and other surgeries. It affects 7–10 percent of all hospitalized patients, resulting in the deaths of 35–40 percent of inpatients. In limited studies using mesenchymal stem cells, there have been fewer instances of kidney damage after surgery, the length of hospital stays has been reduced, and there have been fewer readmissions after release.

How do these stem cells work to protect or repair the kidney? Scientists are unsure at this point, but some evidence has shown that these stem cells release several growth factors in endocrine and paracrine ways. As further studies are conducted to assess the safety and effectiveness of stem cell therapy, we will move closer to a day when kidney injury is rare, and curative treatments are routine.

Chapter Review

The effects of failure of parts of the urinary system may range from inconvenient (incontinence) to fatal (loss of filtration and many others). The kidneys catalyze the final reaction in the synthesis of active vitamin D that in turn helps regulate Ca ++ . The kidney hormone EPO stimulates erythrocyte development and promotes adequate O 2 transport. The kidneys help regulate blood pressure through Na + and water retention and loss. The kidneys work with the adrenal cortex, lungs, and liver in the renin–angiotensin–aldosterone system to regulate blood pressure. They regulate osmolarity of the blood by regulating both solutes and water. Three electrolytes are more closely regulated than others: Na + , Ca ++ , and K + . The kidneys share pH regulation with the lungs and plasma buffers, so that proteins can preserve their three-dimensional conformation and thus their function.

Answer the question(s) below to see how well you understand the topics covered in the previous section.

https://oea.herokuapp.com/assessments/285

Critical Thinking Questions

• How does lack of protein in the blood cause edema?
• Which three electrolytes are most closely regulated by the kidney?

[reveal-answer q=”962202″]Show Answers[/reveal-answer] [hidden-answer a=”962202″]

• Protein has osmotic properties. If there is not enough protein in the blood, water will be attracted to the interstitial space and the cell cytoplasm resulting in tissue edema.
• The three electrolytes are most closely regulated by the kidney are calcium, sodium, and potassium.

[/hidden-answer]

osteomalacia: softening of bones due to a lack of mineralization with calcium and phosphate; most often due to lack of vitamin D; in children, osteomalacia is termed rickets; not to be confused with osteoporosis

Bagul A, Frost JH, Drage M. Stem cells and their role in renal ischaemia reperfusion injury. Am J Nephrol [Internet]. 2013 [cited 2013 Apr 15]; 37(1):16–29. Available from: http://www.karger.com/Article/FullText/345731

Contributors and Attributions

• Anatomy & Physiology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]

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Chapter 16 Answers: Excretory System

16.2 organs of excretion: review questions and answers.

• What is excretion, and what is its significance? Excretion is the process of removing wastes and excess water from the body. It is an essential process in all living things and a major way the human body maintains homeostasis.
• Self-marking
• Describe the excretory functions of the liver. The liver detoxifies and breaks down many substances in the blood including toxins. The liver also excretes bilirubin, a waste product of hemoglobin catabolism, in bile, which is eventually excreted in feces by the large intestine.
• What are the main excretory functions of the large intestine?  The main excretory function of the large intestine is to eliminate solid waste that remains after food is digested and water is extracted from the indigestible matter.
• List organs of the urinary system. Organs of the urinary system include the kidneys, ureters, urinary bladder, and urethra.
• Describe the physical states in which the wastes from the human body are excreted. Wastes excreted from the human body include solids, liquids, and gases.
• Give one example of why ridding the body of excess water is important. Answers may vary.  Sample answer:  One example of why it is important to rid the body of excess water is that the correct volume of extracellular fluid needs to be maintained, which is important for homeostasis throughout the body.
• What gives feces its brown colour? Why is that substance produced? Bilirubin, which is produced from the breakdown of hemoglobin from dead red blood cells.

16.3 Introduction to the Urinary System: Review Questions and Answers

• State the main function of the urinary system. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine.
• What are nephrons? Nephrons are the tiny structural and functional units of the kidneys that filter blood, reabsorb needed materials, and form urine. There are at least a million nephrons in each kidney.
• Other than the elimination of waste products, identify functions of the urinary system. Besides the elimination of waste products, functions of the urinary system include maintaining homeostasis of mineral ions in extracellular fluid, regulating acid-base balance in the blood, regulating the volume of extracellular fluids, and controlling blood pressure.
• How is the formation of urine regulated? The formation of urine is regulated by endocrine hormones, including antidiuretic hormone from the posterior pituitary gland, parathyroid hormone from the parathyroid glands, and aldosterone from the adrenal glands.
• Explain why it is important to have voluntary control over the sphincter at the end of the urethra. It is important to have voluntary control over the sphincter at the end of the urethra, because it allows us to control when and where we urinate. You know, so we don’t pee our pants.
• In terms of how they affect the kidneys, compare aldosterone to antidiuretic hormone. Both aldosterone and antidiuretic hormone cause the kidneys to excrete less water in urine. Aldosterone additionally causes less sodium to be excreted as well.
• If your body needed to retain more calcium, which of the hormones described in this concept is most likely to increase? Explain your reasoning. Parathyroid hormone, because it causes less calcium to be excreted in urine and therefore more is retained by the body.

16.4 Kidneys: Review Questions and Answers

• Contrast the renal artery and renal vein. A renal artery connects each kidney with the aorta and transports unfiltered blood to the kidney. A renal vein connects each kidney with the inferior vena cava and transports filtered blood back to the circulation.
• Identify the functions of a nephron. Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.  The functions of a nephron are filtering materials out of the blood, allowing needed materials to be absorbed back into the blood, and secreting additional materials from the blood to form urine. As blood passes through capillaries in the glomerulus, substances are filtered out of blood and pass into Bowman’s capsule and then the renal tubule. The filtered substances form a fluid called filtrate. As filtrate passes through the renal tubule, some substances are reabsorbed into the blood from the filtrate, and other substances are secreted from the blood into the filtrate, forming urine.
• Identify two endocrine hormones secreted by the kidneys, along with the functions they control. The kidneys secrete the endocrine hormones calcitriol, which helps control the blood calcium level; and erythropoietin, which stimulates the bone marrow to produce of red blood cells.
• Name two regions in the kidney where water is reabsorbed. Two regions in the kidney where water is reabsorbed are: from the renal tubule into the peritubular capillaries and from the collecting ducts.
• Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer. Blood in the glomerular capillaries is less filtered than blood in the peritubular capillaries because it is just starting to be filtered in the glomerular capsule. Blood in the peritubular capillaries comes in large part from filtered blood that is reabsorbed from the renal tubule.
• What do you think would happen if blood flow to the kidneys is blocked?  Answers will vary. Sample answer:  If blood flow to the kidneys is blocked, I think that wastes would build up in the blood, as well as excess water and ions. This would be very dangerous and potentially deadly.

16.5 Ureters, Urinary Bladder, and Urethra: Review Questions and Answers

• What are ureters?  Describe the location of the ureters relative to other urinary tract organs. Ureters are tube-like structures that are part of the urinary system.
• Identify layers in the walls of a ureter. How do they contribute to the ureter’s function? The walls of the ureter contain smooth muscle that can contract to push urine through the ureter by peristalsis. They are lined with transitional epithelium that can expand and stretch to allow urine to pass through.
• Describe the urinary bladder. What is the function of the urinary bladder? The urinary bladder is a hollow, muscular organ that rests on the pelvic floor. It is lined with transitional epithelium. The function of the urinary bladder is to collect and store urine from the kidneys before the urine is eliminated through urination.
• How does the nervous system control the urinary bladder? As the urinary bladder fills with urine, the autonomic nervous system causes the detrusor muscle in the bladder wall to relax so the bladder can hold more urine. Once the bladder is about half full, it triggers the sensation of needing to urinate. When the individual is ready to void, conscious control by the somatic nervous system causes the detrusor muscle to relax and the bladder sphincter to contract and open. This forces urine out of the bladder and allows it to flow into the urethra.
• What is the urethra? The urethra is a tube that connects the urinary bladder to the external urethral orifice.
• How does the nervous system control urination? Somatic nerves control the sphincter at the distal end of the urethra. This allows the opening of the sphincter for urination to be under voluntary control.
• Identify the sphincters that are located along the pathway from the ureters to the external urethral orifice. The first sphincter is at the entrance of the bladder. Next, the internal urethral sphincter is at the base of the bladder and allows urine to flow into the urethra when open. Finally the external urethral sphincter controls the flow of urine out of the body.
• What are two differences between the male and female urethra? The male urethra is longer than the female urethra because it travels through the penis. The male urethra also carries semen in addition to urine.
• When the bladder muscle contracts, the smooth muscle in the walls of the urethra relax .

16.6 Disorders of the Urinary System: Review Questions and Answers

• Define kidney failure. Kidney failure is a condition that may be caused by diabetic nephropathy, PKD, or chronic hypertension in which the kidneys are no longer able to adequately filter metabolic wastes from the blood.
• When kidney function drops below the level needed to sustain life, what are potential treatments for kidney failure? Potential treatments for kidney failure when kidney function drops below the level needed to sustain life include kidney transplantation or repeated, frequent hemodialysis.
• Describe hemodialysis. Hemodialysis is a medical procedure in which a patient’s blood is filtered artificially through a machine and then returned to the patient’s circulation.
• How may a large kidney stone be removed from the body? A large kidney stone may be shattered with high-intensity ultrasound into pieces small enough to pass through the urinary tract, or it may be removed surgically.
• How are bladder infections usually treated? A bladder infection is generally caused by bacteria, so treatment usually includes antibiotic drugs.
• Why are bladder infections much more common in females than in males? Bladder infections are much more common in females than in males because the female urethra is much shorter and closer to the anus.
• Compare and contrast stress incontinence and urge incontinence. Stress incontinence is caused by stretching of pelvic floor muscles during childbirth. It involves leakage of small amounts of urine when coughing, sneezing, or lifting. Urge incontinence is caused by an “overactive bladder” that empties without warning. It involves leakage of large amounts of urine while experiencing a sudden urge to urinate.
• Why is the presence of a protein(such as albumin) in the urine a cause for concern? Proteins such as albumin are not usually filtered out of the blood in the glomeruli of the kidneys. When the glomerular capillaries are damaged, it allows albumin to leak into the filtrate from the blood. As a result, albumin ends up being excreted in the urine. Therefore, the presence of proteins such as albumin in the urine is a cause for concern because it may indicate that there is a problem with kidney function.
• Patients undergoing hemodialysis usually have to do this procedure a few times a week. Why does it need to be done so frequently?  Answers may vary. Sample answer:  Hemodialysis artificially filters the blood when kidney function is significantly impaired. It needs to be done frequently because wastes build up continually in the blood as the body carries out its functions. Therefore, these wastes need to be removed frequently to avoid health problems or even death.

16.7 Case Study Conclusion and Chapter Summary: Review Questions and Answers

• In what ways can the alveoli of the lungs be considered analogous to the nephrons of the kidney? Answers may vary. Sample answer:  Both the alveoli and the nephrons are tiny functional units within a larger organ that take wastes from the blood and excrete them. The alveoli are in the lungs and excrete waste gases, while the nephrons are in the kidneys and excrete wastes in urine.
• What is urea? Where is urea produced, and what is it produced from? How is urea excreted from the body?   Urea is a waste product produced by the body as a result of protein catabolism. Urea is produced in the liver from ammonia, which is a by-product of protein catabolism. Urea is mainly excreted in the urine after being filtered out from the blood by the kidney, but small amounts are also excreted in sweat.
• If a person has a large kidney stone preventing urine that has left the kidney from reaching the bladder, where do you think this kidney stone is located? Explain your answer.  The kidney stone is located in a ureter because the ureters connect the kidney to the bladder.
• As it relates to urine production, explain what is meant by “Excretion = Filtration – Reabsorption + Secretion. “Excretion = Filtration – Reabsorption + Secretion” means that what is excreted from the kidney in the form of urine is the product of what is filtered out by the nephron (filtrate), minus what is reabsorbed back into the body from the filtrate, plus what is secreted from the blood into the filtrate.
• Which disease discussed in the chapter specifically affects the glomerular capillaries of the kidneys? Where are the glomerular capillaries located within the kidneys, and what is their function?  Diabetic nephropathy. The glomerular capillaries are located in the nephrons of the kidney. Their function is to filter substances out of the blood and into the Bowman’s capsule.
• Describe one way in which the excretory system helps maintain homeostasis in the body. Answers will vary. Sample answer:  One way in which the excretory system helps maintain homeostasis in the body is by regulating water and salt balance through the function of the kidneys.
• High blood pressure can both contribute to the development of kidney disorders and be a symptom of kidney disorders. What is a kidney disorder that can be caused by high blood pressure? Kidney failure. What is a kidney disorder that has high blood pressure as a symptom? Polycystic kidney disease. How does blood pressure generally relate to the function of the kidney? The kidneys help regulate blood pressure by regulating the amount of water and salts in the blood. Because blood flows into the kidney to be filtered, changes in blood pressure (such as hypertension) can affect the functioning of the kidney itself.
• If the body is dehydrated, what do the kidneys do? What does this do to the appearance of the urine produced?  If the body is dehydrated, the kidneys retain more water, releasing less into the urine. This causes the urine to appear darker and more concentrated.
• Identify three risk factors for the development of kidney stones.   Answers will vary. Sample answer:  Three risk factors for the development of kidney stones are: high consumption of cola soft drinks, not drinking enough fluids, and being overweight.

Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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Homeostatic and Excretory Functions of the Kidney

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The kidneys have a central role in the homeostasis of water and electrolytes, i.e., in the maintenance of volume and ionic composition of body fluids. This function is accomplished by appropriate changes in the rate of renal excretion of water and electrolytes, controlled by feedback mechanisms which involve participation of the nervous system, the endocrine system, or both. The homeostatic functions of the kidney include the control of the balance of water, sodium, chloride, potassium, calcium, magnesium, hydrogen ions, and phosphate. The adaptability of the kidney to the requirements of homeostasis is demonstrated by the large changes in urine volume and composition which occur in response to alterations in the diet. There is no fixed normal composition of the urine. Normal homeostatic renal function is defined by the capacity of the organ to vary the volume and composition of the urine over a wide range, according to requirements imposed by intake, extrarenal losses, and other factors.

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Bello-Reuss, E., Reuss, L. (1983). Homeostatic and Excretory Functions of the Kidney. In: Klahr, S. (eds) The Kidney and Body Fluids in Health and Disease. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-3524-5_2

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26.2 Water Balance

Learning objectives.

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

• Explain how water levels in the body influence the thirst cycle
• Identify the main route by which water leaves the body
• Describe the role of ADH and aldosterone and their effect on body water levels
• Define dehydration and identify common causes of dehydration

On a typical day, the average adult will take in about 2500 mL (almost 3 quarts) of aqueous fluids. Although most of the intake comes through the digestive tract, about 230 mL (8 ounces) per day is generated metabolically, in the last steps of aerobic respiration. Additionally, each day about the same volume (2500 mL) of water leaves the body by different routes; most of this lost water is removed as urine. The kidneys also can adjust blood volume though mechanisms that draw water out of the filtrate and urine. The kidneys can regulate water levels in the body; they conserve water if you are dehydrated, and they can make urine more dilute to expel excess water if necessary. Water is lost through the skin through evaporation from the skin surface without overt sweating and from air expelled from the lungs. This type of water loss is called insensible water loss because a person is usually unaware of it.

Regulation of Water Intake

Osmolality is the ratio of solutes in a solution to a volume of solvent in a solution. Plasma osmolality is thus the ratio of solutes to water in blood plasma. A person’s plasma osmolality value reflects his or her state of hydration. A healthy body maintains plasma osmolality within a narrow range, by employing several mechanisms that regulate both water intake and output.

Drinking water is considered voluntary. So how is water intake regulated by the body? Consider someone who is experiencing dehydration , a net loss of water that results in insufficient water in blood and other tissues. The water that leaves the body, as exhaled air, sweat, or urine, is ultimately extracted from blood plasma. As the blood becomes more concentrated, the thirst response—a sequence of physiological processes—is triggered ( Figure 26.2.1 ). Osmoreceptors are sensory receptors in the thirst center in the hypothalamus that monitor the concentration of solutes (osmolality) of the blood. If blood osmolality increases above its ideal value, the hypothalamus transmits signals that result in a conscious awareness of thirst. The person should (and normally does) respond by drinking water. The hypothalamus of a dehydrated person also releases antidiuretic hormone (ADH) through the posterior pituitary gland. ADH signals the kidneys to recover water from urine, effectively diluting the blood plasma. To conserve water, the hypothalamus of a dehydrated person also sends signals via the sympathetic nervous system to the salivary glands in the mouth. The signals result in a decrease in watery, serous output (and an increase in stickier, thicker mucus output). These changes in secretions result in a “dry mouth” and the sensation of thirst.

Decreased blood volume resulting from water loss has two additional effects. First, baroreceptors, blood-pressure receptors in the arch of the aorta and the carotid arteries in the neck, detect a decrease in blood pressure that results from decreased blood volume. The heart is ultimately signaled to increase its rate and/or strength of contractions to compensate for the lowered blood pressure.

Second, the kidneys have a renin-angiotensin hormonal system that increases the production of the active form of the hormone angiotensin II, which helps stimulate thirst, but also stimulates the release of the hormone aldosterone from the adrenal glands. Aldosterone increases the reabsorption of sodium in the distal tubules of the nephrons in the kidneys, and water follows this reabsorbed sodium back into the blood. Circulating angiotensin II can also stimulate the hypothalamus to release ADH.

If adequate fluids are not consumed, dehydration results and a person’s body contains too little water to function correctly. A person who repeatedly vomits or who has diarrhea may become dehydrated, and infants, because their body mass is so low, can become dangerously dehydrated very quickly. Endurance athletes such as distance runners often become dehydrated during long races. Dehydration can be a medical emergency, and a dehydrated person may lose consciousness, become comatose, or die, if his or her body is not rehydrated quickly.

Regulation of Water Output

Water loss from the body occurs predominantly through the renal system. A person produces an average of 1.5 liters (1.6 quarts) of urine per day. Although the volume of urine varies in response to hydration levels, there is a minimum volume of urine production required for proper bodily functions. The kidney excretes 100 to 1200 milliosmoles of solutes per day to rid the body of a variety of excess salts and other water-soluble chemical wastes, most notably creatinine, urea, and uric acid. Failure to produce the minimum volume of urine means that metabolic wastes cannot be effectively removed from the body, a situation that can impair organ function. The minimum level of urine production necessary to maintain normal function is about 0.47 liters (0.5 quarts) per day.

The kidneys also must make adjustments in the event of ingestion of too much fluid. Diuresis , which is the production of urine in excess of normal levels, begins about 30 minutes after drinking a large quantity of fluid. Diuresis reaches a peak after about 1 hour, and normal urine production is reestablished after about 3 hours.

Role of ADH

Antidiuretic hormone (ADH) , also known as vasopressin, controls the amount of water reabsorbed from the collecting ducts and tubules in the kidney. This hormone is produced in the hypothalamus and is delivered to the posterior pituitary for storage and release ( Figure 26.2.2 ). When the osmoreceptors in the hypothalamus detect an increase in the concentration of blood plasma, the hypothalamus signals the release of ADH from the posterior pituitary into the blood.

ADH has two major effects. It constricts the arterioles in the peripheral circulation, which reduces the flow of blood to the extremities and thereby increases the blood supply to the core of the body. ADH also causes the epithelial cells that line the renal collecting tubules to move water channel proteins, called aquaporins, from the interior of the cells to the apical surface, where these proteins are inserted into the cell membrane ( Figure 26.2.3 ). The result is an increase in the water permeability of these cells and, thus, a large increase in water passage from the urine through the walls of the collecting tubules, leading to more reabsorption of water into the bloodstream. When the blood plasma becomes less concentrated and the level of ADH decreases, aquaporins are removed from collecting tubule cell membranes, and the passage of water out of urine and into the blood decreases.

A diuretic is a compound that increases urine output and therefore decreases water conservation by the body. Diuretics are used to treat hypertension, congestive heart failure, and fluid retention associated with menstruation. Alcohol acts as a diuretic by inhibiting the release of ADH. Additionally, caffeine, when consumed in high concentrations, acts as a diuretic.

Chapter Review

Homeostasis requires that water intake and output be balanced. Most water intake comes through the digestive tract via liquids and food, but roughly 10 percent of water available to the body is generated at the end of aerobic respiration during cellular metabolism. Urine produced by the kidneys accounts for the largest amount of water leaving the body. The kidneys can adjust the concentration of the urine to reflect the body’s water needs, conserving water if the body is dehydrated or making urine more dilute to expel excess water when necessary. ADH is a hormone that helps the body to retain water by increasing water reabsorption by the kidneys.

Review Questions

Critical thinking questions.

1. Describe the effect of ADH on renal collecting tubules.

2. Why is it important for the amount of water intake to equal the amount of water output?

Answers for Critical Thinking Questions

• ADH constricts the arterioles in the peripheral circulation, limiting blood to the extremities and increasing the blood supply to the core of the body. ADH also causes the epithelial cells lining the renal collecting tubules to move water channel proteins called aquaporins from the sides of the cells to the apical surface. This greatly increases the passage of water from the renal filtrate through the wall of the collecting tubule as well as the reabsorption of water into the bloodstream.
• Any imbalance of water entering or leaving the body will create an osmotic imbalance that will adversely affect cell and tissue function.

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BIOLOGY TOPICAL QUESTIONS AND ANSWERS-Excretion and Homeostasis

CHECK THE ANSWERS HERE

1.         Explain the following:-          i) Fresh water fish excrete ammonia                                                 

                                                            ii) Glucose is absent in urine yet present in glomerular filtrate        

2.         (a) State two functions of the kidney                                                                                    

            (b) Name two substances that are not found in urine of a healthy person                            

            (c) Name two diseases that affect the kidney                                                                        

3.         (a) State two structural modification of the kidneys of deserts animals like kangaroo rat.

            (b) Describe how ingestion of very salty food may reduce the amount of water excreted in urine.

4.         A student mixed a sample of urine from a person with Benedict’s solution and heated, the colour

changed to orange.

(a) What was present in the urine sample?                                                                            

(b) What did the student conclude on the health status of the person?                                 

(c) Which organ in the person may not be functioning properly?                                         

5.         (a) If the human pancrease is not functional:-                                                                       

               (i) Name the hormone which will be deficient                                                                   

               (ii) Name the disease the human is likely to suffer from                                                   

            (b) What is diuresis?                                                                                                              

6.         State one structural adaptation of nephron in the kidney of a desert mammal                      

7.         Name the nitrogenous wastes excreted by the following organisms:-                                   

            Animal                                    Nitrogenous Waste

            (i) Desert mole                                   

            (ii) Marine fish                                   

            (iii) Tilapia                             

8.         The table below shows description of sizes of glomeruli renal tubules of two animals which are

            living in different environments                                                                                            

            (a) Name the likely environment in which each animal lives            :    (i)Animal X          

                                                                                                     (ii) Animal Y        

            (b) What role does vasoconstriction play in thermoregulation?                                

9.         The table below shows the approximate percentage concentration of various components in blood

plasma entering the kidney, glomerular filtrate and urine of a healthy human being           

            (a) Name the process responsible for the formation of glomerular filtrate                           

            (b) What process is responsible for the absence of glucose and amino acids in urine?        

(c) Explain why there are no plasma proteins in the glomerular filtrate?                             

10.       What is the importance of sebaceous glands in the human skin?                                          

11.       Explain why sweat accumulates on a person’s skin in a hot humid environment                 

12.       Distinguish between diabetes mellitus and diabetes inspidus                                                           

13.       State two processes through which plants excrete their metabolic wastes.                          

14.       The figure below shows a vertical section through a mammalian kidney.

(a) Label the parts A and B                                                                                                               

            (b) Which part is the Bowman’s capsule found?                                          

15.       (a) Explain the effects of the production of large amounts of Antidiuretic hormone in the human

                  body                                                                                                                     

            (b) State two functions of the loop of Henle             

16.       Study the homeostatic scheme below:

(a) Identify the hormone labeled A                                                                            

            (b) Name the site of action of hormone A                                                                  

            (c) Identify the feedback labeled D                                                                            

17.       State three importance of Osmosis in plants                                                                        

18.       A patient was complaining of thirst most of the times. A sample of the patient’s urine was found

not to contain a lot of sugar but was dilute:-                                                                         

            (a) Name the hormone the person’s body was deficient of                                                   

            (b) Which gland produces the above hormone                                                                      

            (c) Name the disease that the patient was most likely suffering from                                              

19.       State two features in the nephron that facilitate ultra filtration                                             

20.       The table  below  shows  a  description  of  size of  glomeruli and  renal  tubules  of  two animals 

which are  adapted  to living in different environment:-                                                       

            a) Name the likely environment in which animal A lives                                                                              b) Suggest the main nitrogenous waste produced by animal B                                                                      c) Name the organelle of osmoregulation in each of the following animal:   i) Paramecium    

                                                                                                                                    ii) Insects  

21.       What role is played by the liver in excretion?                                                                      

22.       The equation below represents a metabolic process that occurs in the mammalian liver:               Amino acids                        organic compound + urea                                                          

            a) Name the process

            (b) What is the importance of the process to the mammals?                                                 

23.       A person was found to pass out large volume of dilute urine frequently. Name the:-          

            (a) disease the person was suffering from?                                                                           

            (b) hormone that was deficient                                                                                                          

24.       Explain the effects of the following on the quantity and composition of urine                    

            (a) Drinking large amount of clean water                                                                                         

            (b) Drinking very salty soup                                                                                                  

            (c) Removal of pancreas                                                                                                                    

25.       (a) Distinguish between excretion and egestion                                                                  

(b) State the importance of excretion in the bodies of living organisms.                              

26.       The diagram below shows simplified structures of kidneys from two different animals.

(a) Suggest possible habitat in which animal N is found.                                                     

 (b) Give two reasons for your answer in (a) above.                                                             

27.       (a) What is poikilotherm?                                                                                                      

 (b) State two classes of phylum chordata where all members are poikilothermic .             

28.       The diagram below represents a mammalian nephron

 (i) Name the structure labelled Q       …………………………………………………………………………           

             (ii) State two adaptations of part labeled R                                                                           

29.       Distinguish between internal environment and external environment as used in

30.       The diagram below represents a nephron of a mammal:

(a) Name the parts labeled A, B and D                                                                                              

            (b) Name a major substance in glomerular filtrate whose concentration remains the same

                  between A and C                                                                                                              

31.       Name the parts of the flower that are responsible for the production of gametes                 

32.       The equation below represents a metabolic process that occurs in a certain organ in the m    

             mammalian body:-                                                                                                                

          Ammonia   enzymes        Organic compound Q + water

                        Carbon (IV) oxide

            a) Name the process represented in the equation.                                                                 

            b) Name the organ in which the process occurs.                                                                   

            c) Why is the process important to the mammal?                                                                  

            d) Identify the organic compound Q.                                                                                    

            e) Explain the source of ammonia in the organ named in (b) above.                                                             f) What happens to organic compound Q ?                                                                            

33.       Kosgei and Onyancha collided during a football match and each got bruised. Kosgei’s bruise             stopped bleeding after ten minutes while Onyancha’s bruise continued bleeding and he had

            to be taken to hospital for treatment.                                                                                    

            (a) Explain the process which brought about stoppage of Kosgei’s bleeding

            (b) Distinguish between blood clotting and haemagglutination.                                                    

            (c) Name the disease, that Onyancha could be suffering from.

34.       The table below shows the percentage of some substances in the glomerular filtrate and urine

            of a certain mammal:-                                                                                                            

(a) From the above table, account for ;  (i) The absence of glucose in urine                                                                                                 (ii) The absence of protein in both glomerular filtrate and urine                                                      

            (b) Explain the significance of the flow system in the nephron where the glomerular filtrate

               flows in opposite direction to that of blood in the surrounding capillaries                                     

            (c) Name the hormone that controls the percentage of water in urine and that which control the

                 amount of  salts                                                                                                                                    Percentage of water

            Amount of salts

            (d) List any two diseases /disorders of the kidney                                                                             35.       Study the diagram below and answer the questions that follow       

(a) Name the structure represented by the diagram

            (b) (i) Name the parts labelled D and M                                                                                           

           (ii) Name the hormones whose sites of action are Q and G                                                                (c) Name one substance that is present in part N but absent in part Z                                   

            (d) The contents of part V were boiled with Benedicts’ solution and an orange precipitate was

      formed. Account for the results

36.       In an investigation, two persons A and B drunk the same amount of glucose solution.  Their blood

            sugar levels were determined immediately and thereafter at intervals of one hour for the next six

hours.                                                                                                                                     

            The results were as shown in the following table:-

            (a) Draw a graph of blood sugar levels of persons A and B against time on the same axis  

            (b) Explain each of the following observations;-

    (i) Blood sugar level increased in person A between 0 and 1 hour                                               

    (ii) The blood sugar level dropped in person A between 1 and 4 hours                                        

(c) From the graph, what is the normal blood glucose sugar level for human beings                       

            (d) Suggest a reason for the high sugar level in person B                                                                  

(e) How can the high blood sugar level in person B controlled?                                                      

            (f) What is the biological significance of maintaining a relatively constant sugar level in a human

    being                                                                                                                                              

(g) Account for the decrease in the blood glucose level of person B after 4 hours                           

37.       An experiment was carried out to determine the effect of drinking on excess amount of water on

the flow of urine. A person drinks one litre of water and urine was collected at intervals of 15minutes.                                                                                                                                    

            The results were as shown below:

            (a) Plot a suitable graph to represent urine output with time.                                               

            (b) Explain the rate of flow of urine between the following times;

                (i) 15 and 60minutes.                                                                                                                     

                (ii) 60 and 75minutes.                                                                                                                    

    (iii) 75 and 135 minutes.                                                                                                    

(c) Name two hormones responsible for regulation of relative amount of salts and water in man. 

38.       a) Explain how urea  is formed in  the  human body                                                            

b) Describe the path taken by urea from the organ where it is formed until it is released from

             the human    body    

39.       The diagram below represents a mammalian nephron. 

 (a) Name the structures labeled B,C and D                                                              

            (c) Name the process by which substances are reabsorbed from structure C into blood capillaries             (d) How is the pressure in structure A achieved?      

a) Identify substance X

            (c) Give the end products of the process labelled H                                                              

            (d) Give three other functions of the liver                                                                             41.       The flow diagram below represents blood clotting process              

a) Name the proteins represented by the letters; V, Y, Z                                                                               b) State the importance of blood clotting                                                                                              

            c) Why doesn’t the physiological process above occur in undamaged blood vessels                       

42.       How does an Endotherm respond to both heat gain and heat loss?                                       

43.       The diagram below represents a mammalian nephron

(a) Name the:    (i) Structure labelled P                                                                                                       

            (b) State the structural modifications of the part label led Q for

                  (i) Desert mammals                                                                                             

                  (ii) Fresh water mammals                                                                                                

            (c ) (i) Name one substance present at point R but absent at point S in a healthy mammal.

                  (ii) The appearance of the substance you have named in (c)(i) above is a symptom of a

                         certain   disease.  Name the disease                                                                                      

44.     Describe how the mammalian skin regulates body temperature                                 

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Excretion in Humans ( CIE IGCSE Biology )

Topic questions.

Fig. 1 shows the urinary system in humans.

Identify structure A , B and D in Fig. 1.

How did you do?

Give the name of organ C and describe its function in the urinary system.

The urinary system is responsible for the process of excretion.

Define the term excretion .

Carbon dioxide is one of the substances that is excreted by the body.

Explain the importance of excreting carbon dioxide.

Did this page help you?

Extended only

Fig. 1 shows the structure of a nephron in the kidneys.

Identify structure 1 in Fig. 1.

Describe the process taking place at structure 1 .

The fluid moving along structure 2 of the nephron in Fig. 1 is known as filtrate and it consists of various substances.

State two substances that will form part of the filtrate in Structure 2 .

Water is reabsorbed from the filtrate back into the blood at various points along the nephron, such as structure 3 in Fig. 1.

Identify structure 3 in Fig. 1.

Describe how the structure identified at part (i) allows water to be reabsorbed into the blood.

Structure 5 represents the collecting duct, which will adjust the amount of water reabsorbed into the blood depending on the needs of the body.

Complete the following sentences on reabsorption and excretion of water in the collecting duct.

On a hot, sunny day, the body will _________________ more water due to sweating. In order to conserve water, the collecting duct will reabsorb ____________________ water into the blood. This results in the excretion of _______________ urine that is more concentrated.

Fig. 1 shows the general structure of an amino acid.

During the process of deamination, the liver removes part of the amino acid.

Identify the part of the amino acid that is removed during deamination by circling it on Fig. 1.

One of the products of deamination is urea.

Describe the fate of urea once it is produced by the liver.

Urea is a toxic substance that should not be allowed to accumulate in the body.

State two consequences of high urea levels in the body.

Amino acids are the building blocks of proteins.

Give two examples of proteins found in the body.

The nephron consists of several different structures such as the Bowman's capsule and glomerulus.

Describe and explain how the structure of the glomerulus and Bowman's capsule help to remove urea from the bloodstream.

Table 1 shows the results of an investigation of the effects of a high protein diet on the synthesis of urea and the production of urine.

Calculate the percentage increase in the mass of urea produced when switching from a normal diet to a high protein diet.

Show your working and give your answer to the nearest percent.

Explain why a high protein diet would result in a greater mass of urea being produced.

One of the symptoms of consuming too much protein for extended periods of time is dehydration.

Use the information in Table 1 to suggest an explanation for this.

The kidneys are responsible for osmoregulation, which involves maintaining the balance of water and mineral salts in the body.

Explain the importance of osmoregulation to cells.

Table 1 shows the concentration of different substances in blood plasma, filtrate entering the proximal convoluted tubule and urine.

Explain the difference in concentration of protein in the blood plasma and filtrate.

Under normal circumstances there should be no glucose present in urine.

Explain the reason for this.

Suggest an explanation for the diagnosis that a doctor could make if glucose was present in the urine.

Fig. 1 shows one of the cells found in the proximal convoluted tubule of the nephron.

Explain the presence of structure A in Fig. 1.

The concentration of substance X in different parts of the urinary system is shown in Table 1 at part (a).

Identify whether substance X can be considered a waste product or a useful substance to the body.

Use the information in Table 1 to explain your answer at part (i).

Suggest an explanation for the effect that this may have on the process of ultrafiltration.

Explain this symptom.

Patients suffering from kidney failure will have to undergo kidney dialysis on a regular basis while waiting for a kidney transplant.

During the process of dialysis, blood from the patient flows through a dialysis machine which contains a series of convoluted tubes. These tubes are made from a selectively permeable substance called a dialysis membrane.

The tubes are surrounded by a solution called dialysate, which ensures that the dialysis machine performs the same function as normal kidneys. Once the blood has flowed through the entire dialysis machine, it returns back to the body.

Fig. 1 shows the process of kidney dialysis.

Describe and explain the possible composition of the dialysate.

In order for kidney dialysis to be effective, the dialysate must be constantly replaced with a fresh solution.

Suggest a reason for this.

The kidneys excrete excess water in urine.

The main component of urine is water.

State two other substances that are excreted by healthy kidneys.

A scientist investigated the effect of drinking sugar solutions, of different concentrations, on the volume of urine produced.

• 1.5 dm 3 of sugar solution A was consumed by a healthy adult.
• Urine was collected at thirty minute intervals for 150 minutes.
• The volume of urine produced every thirty minutes was added to the previous total volume.

This procedure was repeated with sugar solutions B and C .

The results are shown in Fig. 1.

Complete Table 1 using the information in Fig. 1.

Suggest which of the three solutions, A , B or C , contained the most sugar.

Give a reason for your suggestion.

List two factors that will affect the volume and concentration of urine produced.

The body loses water in the urine.

State two other ways in which the body loses water.

The liver is an important organ in many processes.

The liver responds to changes in insulin concentration.

Insulin is a hormone.

Extended Only

The liver is also involved in the processing of amino acids.

The liver is also involved in the processing of toxins.

Lactic acid is an example of a toxin that is produced during vigorous exercise and processed in the liver.

Describe how lactic acid is processed.

Alcohol is another toxin that is processed in the liver.

The effect of alcohol consumption on the risk of dying from liver disease was investigated in men and women.

Describe the results shown in Fig. 1.

Excretion is the removal of toxic substances or substances in excess, from the body. 

Excess water is excreted from the lungs and the kidneys.

State the name of one other substance that is excreted from

• The kidneys. 

The volume and concentration of urine varies with changing conditions.

Table 1 shows three changing conditions.

Write increase or decrease in each of the boxes in Table 1 to show how each change affects the volume and the concentration of urine.

Excretion is a characteristic of living organisms.

Growth is another characteristic of living organisms.

Define the term growth .

State three characteristics of living organisms other than excretion and growth.

Complete the sentences using words from the list.

You may use the words once, more than once or not at all.

Urea passes through the kidney and forms part of the urine.

Urine leaves the kidney in the ............................................. . This tube takes the urine to the .............................................. where the urine is stored until it leaves the body. It leaves the body through the ................................................... . 

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JSmol Viewer

Marginal zinc deficiency promotes pancreatic islet enlargement while zinc supplementation improves the pancreatic insulin response in zucker diabetic fatty rats.

1. Introduction

2.1. animals and diets, 2.2. glycemic control and glucose tolerance, 2.3. body composition, 2.4. tissue collection, 2.5. pancreatic islet insulin immunostaining and liver lipid concentration, 2.6. serum and urine biochemistry, 2.7. mineral status, 2.8. western immunoblotting, 2.9. statistical analyses, 3.1. feed intake, body weight and body composition, 3.2. zinc status, 3.3. pancreatic islets and zinc, 3.4. glycemic control, 3.5. hepatic steatosis and circulating lipids, 3.6. renal parameters, 4. discussion, 5. conclusions, supplementary materials, author contributions, institutional review board statement, informed consent statement, data availability statement, acknowledgments, conflicts of interest.

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Rech, L.; Zahradka, P.; Taylor, C.G. Marginal Zinc Deficiency Promotes Pancreatic Islet Enlargement While Zinc Supplementation Improves the Pancreatic Insulin Response in Zucker Diabetic Fatty Rats. Nutrients 2024 , 16 , 1819. https://doi.org/10.3390/nu16121819

Rech L, Zahradka P, Taylor CG. Marginal Zinc Deficiency Promotes Pancreatic Islet Enlargement While Zinc Supplementation Improves the Pancreatic Insulin Response in Zucker Diabetic Fatty Rats. Nutrients . 2024; 16(12):1819. https://doi.org/10.3390/nu16121819

Rech, Leslie, Peter Zahradka, and Carla G. Taylor. 2024. "Marginal Zinc Deficiency Promotes Pancreatic Islet Enlargement While Zinc Supplementation Improves the Pancreatic Insulin Response in Zucker Diabetic Fatty Rats" Nutrients 16, no. 12: 1819. https://doi.org/10.3390/nu16121819

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