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What Is Homeostasis in Biology? Definition and Examples

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

Simple Definition of Homeostasis

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

Origin and History of Discovery

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

Components of Homeostasis

Homeostasis involves three primary components:

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

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

1. Receptors: Detecting Blood Glucose Levels

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

2. Control Center: Pancreas as the Decision-Maker

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

3. Effectors: Actions to Lower Blood Glucose

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

Positive and Negative Feedback in Homeostasis

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

Negative Feedback

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

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

Positive Feedback

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

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

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

More Examples of Homeostasis

Examples in humans.

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

Examples in Other Organisms

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

Microbial Homeostasis

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

Importance of Homeostasis

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

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

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What is homeostasis?

What is an example of homeostasis in a living thing, what is an example of homeostasis in a mechanical system, are there examples of homeostasis in ecosystems.

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Homeostasis is any self-regulating process by which an organism tends to maintain stability while adjusting to conditions that are best for its survival. If homeostasis is successful, life continues; if it’s unsuccessful, it results in a disaster or death of the organism. The “stability” that the organism reaches is rarely around an exact point (such as the idealized human body temperature of 37 °C [98.6 °F]). Stability takes place as part of a dynamic equilibrium , which can be thought of as a cloud of values within a tight range in which continuous change occurs. The result is that relatively uniform conditions prevail.

Body temperature control in humans is one of the most familiar examples of homeostasis. Normal body temperature hovers around 37 °C (98.6 °F), but a number of factors can affect this value, including exposure to the elements, hormones , metabolic rate , and disease , leading to excessively high or low body temperatures. The hypothalamus in the brain regulates body temperature, and feedback about body temperature from the body is carried through the bloodstream to the brain, which results in adjustments in breathing rate, blood sugar levels, and metabolic rate. In contrast, reduced activity, perspiration , and heat-exchange processes that permit more blood to circulate near the skin surface contribute to heat loss. Heat loss is reduced by insulation, decreased circulation to the skin, clothing , shelter, and external heat sources.

A familiar example of homeostatic regulation in a mechanical system is the action of a thermostat , a machine that regulates room temperature . At the centre of a thermostat is a bimetallic strip that responds to temperature changes. The strip expands under warmer conditions and contracts under cooler conditions to either disrupt or complete an electric circuit . When the room cools, the circuit is completed, the furnace switches on, and the temperature rises. At a preset level, perhaps 20 °C (68 °F), the circuit breaks, the furnace stops, and no additional heat is released into the room. Over time, the temperature slowly drops until the room cools enough to trigger the process again.

The concept of homeostasis has also been used in studies of ecosystems. Canadian-born American ecologist Robert MacArthur first proposed in 1955 that homeostasis in ecosystems results from biodiversity (the variety of life in a given place) and the ecological interactions ( predation , competition , decomposition, etc.) that occur between the species living there. The term homeostasis has been used by many ecologists to describe the back-and-forth interaction that occurs between the different parts of an ecosystem to maintain the status quo. It was thought that this kind of homeostasis could help to explain why forests , grasslands , or other ecosystems persist (that is, remain in the same location for long periods of time). Since 1955 the concept has changed to incorporate the ecosystem’s nonliving parts, such as rocks , soil , and water .

homeostasis , any self-regulating process by which biological systems tend to maintain stability while adjusting to conditions that are optimal for survival . If homeostasis is successful, life continues; if unsuccessful, disaster or death ensues. The stability attained is actually a dynamic equilibrium , in which continuous change occurs yet relatively uniform conditions prevail. The general idea of this self-regulating process was explored by French physiologist Claude Bernard in 1849 and the word homeostasis coined by American neurologist and physiologist Walter Bradford Cannon in 1926.

Any system in dynamic equilibrium tends to reach a steady state, a balance that resists outside forces of change. When such a system is disturbed, built-in regulatory devices respond to the departures to establish a new balance; such a process is one of feedback control . All processes of integration and coordination of function, whether mediated by electrical circuits or by nervous and hormonal systems, are examples of homeostatic regulation.

A familiar example of homeostatic regulation in a mechanical system is the action of a room- temperature regulator, or thermostat . The heart of the thermostat is a bimetallic strip that responds to temperature changes by completing or disrupting an electric circuit . When the room cools, the circuit is completed, the furnace operates, and the temperature rises. At a preset level the circuit breaks, the furnace stops, and the temperature drops. Biological systems are more complex and have regulators only very roughly comparable to such mechanical devices. The two types of systems are alike, however, in their goal—to sustain activity within a prescribed range, whether to control the thickness of rolled steel or the pressure within the circulatory system .

The control of body temperature in humans is a good example of homeostasis in a biological system. In humans, normal body temperature fluctuates around the value of 37 °C (98.6 °F), but various factors can affect this value, including exposure, hormones , metabolic rate , and disease , leading to excessively high or low temperatures. The body’s temperature regulation is controlled by a region in the brain called the hypothalamus . Feedback about body temperature is carried through the nervous system to the brain and results in compensatory adjustments in the breathing rate, the level of blood sugar , and the metabolic rate. The circulatory system also plays important roles: its baroreceptors (pressure-sensitive receptors in the blood vessels that respond to stretching) relay blood pressure information back to the brain, and it transports hormones secreted by the hypothalamus and the thyroid gland to regulate the body’s metabolism . Heat loss in humans is aided by reduction of activity, by perspiration , and by heat-exchange mechanisms that permit larger amounts of blood to circulate near the skin surface. Heat loss is reduced by insulation, decreased circulation to the skin, and cultural modification such as the use of clothing, shelter, and external heat sources. The range between high and low body temperature levels constitutes the homeostatic plateau—the “normal” range that sustains life. As either of the two extremes is approached, corrective action (through negative feedback) returns the system to the normal range.

The concept of homeostasis has also been applied to ecological settings. First proposed by Canadian-born American ecologist Robert MacArthur in 1955, homeostasis in ecosystems is a product of the combination of biodiversity and large numbers of ecological interactions that occur between species. It was thought of as a concept that could help to explain an ecosystem’s stability—that is, its persistence as a particular ecosystem type over time ( see ecological resilience ). Since then, the concept has changed slightly to incorporate the ecosystem’s abiotic (nonliving) parts; the term has been used by many ecologists to describe the reciprocation that occurs between an ecosystem’s living and nonliving parts to maintain the status quo. The Gaia hypothesis —the model of Earth posited by English scientist James Lovelock that considers its various living and nonliving parts as components of a larger system or single organism—makes the assumption that the collective effort of individual organisms contributes to homeostasis at the planetary level. The single-organism aspect of the Gaia hypothesis is considered controversial because it posits that living things, at some level, are driven to work on behalf of the biosphere rather than toward the goal of their own survival.

January 3, 2000

What is Homeostasis?

Emeritus Professor Kelvin Rodolfo of the University of Illinois at Chicago's Department of Earth and Environmental Sciences provides this answer

Garry DeLong Getty Images

Homeostasis, from the Greek words for "same" and "steady," refers to any process that living things use to actively maintain fairly stable conditions necessary for survival. The term was coined in 1930 by the physician Walter Cannon. His book, The Wisdom of the Body , describes how the human body maintains steady levels of temperature and other vital conditions such as the water, salt, sugar, protein, fat, calcium and oxygen contents of the blood. Similar processes dynamically maintain steady-state conditions in the Earth's environment.

Homeostasis has found useful applications in the social sciences. It refers to how a person under conflicting stresses and motivations can maintain a stable psychological condition. A society homeostatically maintains its stability despite competing political, economic and cultural factors. A good example is the law of supply and demand, whereby the interaction of supply and demand keeps market prices reasonably stable.

Homeostatic ideas are shared by the science of cybernetics (from the Greek for "steersman"), defined in 1948 by the mathematician Norbert Wiener as "the entire field of control and communication theory, whether in the machine or in the animal." Cybernetic systems can "remember" disturbances and thus are used in computer science to store and transmit information. Negative feedback is a central homeostatic and cybernetic concept, referring to how an organism or system automatically opposes any change imposed upon it.

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For example, the human body uses a number of processes to control its temperature, keeping it close to an average value or norm of 98.6 degrees Fahrenheit. One of the most obvious physical responses to overheating is sweating, which cools the body by making more moisture on the skin available for evaporation. On the other hand, the body reduces heat-loss in cold surroundings by sweating less and reducing blood circulation to the skin. Thus, any change that either raises or lowers the normal temperature automatically triggers a counteracting, opposite or negative feedback . Here, negative merely means opposite, not bad; in fact, it operates for our well being in this example. Positive feedback is a response to change from the normal condition that increases the departure even more.

For example, if a person's temperature is raised to about 107 degrees Fahrenheit, the negative feedback systems stop operating. A person with a high fever has hot, dry skin if they do sweat to help cool it. Not only have the negative feedback systems shut down in such a case; the increased temperature speeds up the body chemistry, which causes the temperature to rise even more, which in turn speeds up the body chemistry even more, and so forth. This vicious cycle of positive feedback, a "runaway" process, can only end in death if not stopped.

It is important to emphasize that homeostatic reactions are inevitable and automatic if the system is functioning properly, and that a steady state or homeostasis may be maintained by many systems operating together. For example, flushing is another of the body's automatic responses to heating: the skin reddens because its small blood vessels automatically expand to bring more heated blood close to the surface where it can cool. Shivering is another response to chilling: the involuntary movements burn body tissue to produce more body heat.

Negative feedback arises out of balances between forces and factors that mutually influence each other. To illustrate several of its important characteristics, we can regard a car and its driver as a unified, complex, homeostatic or "goal-seeking" system--a cyborg, or "cybernetic organism," in that it seeks to keep the car moving on track. The driver does not steer by holding the wheel in a fixed position but keeps turning the wheel slightly to the left and right, seeking the wheel positions that will bring the naturally meandering car back on track. Disturbance, or departure from equilibrium, is every bit as important as negative feedback: Systems cannot correct themselves if they do not stray.

Oscillation is a common and necessary behavior of many systems. If the car skids, the driver automatically responds by quickly steering in the opposite direction. Such abrupt negative feedback, however, usually over-corrects, causing the car to move toward the other side of the road. A negative feedback, if it is as large as the disturbance that triggered it, may become an impressed change in the direction opposite to that of the original disturbance. The car and driver recovers from the skid by weaving from side to side, swerving a little less each time. In other words, each feedback is less than the last departure from the goal, so the oscillations "damp out." Negative feedback takes time and such a time lag is an essential feature of many natural systems. This may set the system to oscillating above and below the equilibrium level.

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A physiologist's view of homeostasis

Harold modell.

1 Physiology Educational Research Consortium, Seattle, Washington;

William Cliff

2 Department of Biology, Niagara University, Niagara, New York;

Joel Michael

3 Department of Molecular Biophysics and Physiology, Rush Medical College, Chicago, Illinois;

Jenny McFarland

4 Department of Biology, Edmonds Community College, Lynnwood, Washington;

Mary Pat Wenderoth

5 Department of Biology, University of Washington, Seattle, Washington; and

6 Department of Biology, Canisius College, Buffalo, New York

Homeostasis is a core concept necessary for understanding the many regulatory mechanisms in physiology. Claude Bernard originally proposed the concept of the constancy of the “milieu interieur,” but his discussion was rather abstract. Walter Cannon introduced the term “homeostasis” and expanded Bernard's notion of “constancy” of the internal environment in an explicit and concrete way. In the 1960s, homeostatic regulatory mechanisms in physiology began to be described as discrete processes following the application of engineering control system analysis to physiological systems. Unfortunately, many undergraduate texts continue to highlight abstract aspects of the concept rather than emphasizing a general model that can be specifically and comprehensively applied to all homeostatic mechanisms. As a result, students and instructors alike often fail to develop a clear, concise model with which to think about such systems. In this article, we present a standard model for homeostatic mechanisms to be used at the undergraduate level. We discuss common sources of confusion (“sticky points”) that arise from inconsistencies in vocabulary and illustrations found in popular undergraduate texts. Finally, we propose a simplified model and vocabulary set for helping undergraduate students build effective mental models of homeostatic regulation in physiological systems.

in 2007, a group of 21 biologists from a wide range of disciplines agreed that “homeostasis” was one of eight core concepts in biology ( 14 ). Two years later, the American Association of Medical Colleges and Howard Hughes Medical Institute in its report ( 1 ) on the scientific foundations for future physicians similarly identified the ability to apply knowledge about “homeostasis” as one of the core competencies ( competency M1 ).

From our perspective as physiologists, it is clear that homeostasis is a core concept of our discipline. When we asked physiology instructors from a broad range of educational institutions what they thought the “big ideas” (concepts) of physiology were, we found that they too identified “homeostasis” and “cell membranes” as the two most important big ideas in physiology ( 15 ). In a subsequent survey ( 16 ), physiology instructors ranked homeostasis as one of the core concepts critical to understanding physiology.

If, as these surveys indicate, the concept of homeostasis is central to understanding physiological mechanisms, one would expect that instructors and textbooks would present a consistent model of the concept. However, an examination of 11 commonly used undergraduate physiology and biology textbooks revealed that this is not necessarily the case ( 17 ). Explanations of the concept of homeostasis and subsequent references to the concept suffer from a number of shortcomings. Although these texts define some terms related to homeostatic regulatory systems, many authors do not use these terms consistently. Moreover, they do not always use consistent visual representations of the concept. In addition, the explanation of the concept often conflicts with the current understanding of homeostatic regulatory mechanisms. These limitations of textbooks most likely carry over to classroom instruction, thereby weakening the power of the concept as a unifying idea for understanding physiology.

The goals of this article are to develop a correct description and visual representation of a general homeostatic mechanism that can serve as a learning tool for faculty members and students. We will limit our discussion to homeostatic mechanisms found in organismal systems that maintain a constant extracellular compartment and will not consider other types of homeostasis. Although this tool can be useful at any academic level, our primary focus is its application at the undergraduate level when students are first introduced to the concept. We will also briefly discuss the history of the concept and then address the “sticky points” that may lead to confusion for faculty members and students alike when attempting to apply the concept to mammalian, organismal physiology. We conclude with suggestions for improving instruction on homeostasis and its applications.

Claude Bernard asserted that complex organisms are able to maintain their internal environment [extracellular fluid (ECF)] fairly constant in the face of challenges from the external world ( 8 ). He went on to say that “a free and independent existence is possible only because of the stability of the internal milieu” ( 3 ). Walter Cannon coined the term “homeostasis” with the intent of providing a term that would convey the general idea proposed some 50 yr earlier by Bernard ( 8 ). Cannon's view focused on maintaining a steady state within an organism regardless of whether the mechanisms involved were passive (e.g., water movement between capillaries and the interstitium reflecting a balance between hydrostatic and osmotic forces) or active (e.g., storage and release of intracellular glucose) ( 6 ). While we recognize the validity of both passive and active mechanisms of homeostasis, our consideration will focus exclusively on the active regulatory processes involved in maintaining homeostasis.

Early physiology textbooks reflected this broad definition by briefly mentioning Bernard's concept of the constancy of the internal milieu, but the term “homeostasis” was not used in discussions of specific regulatory mechanisms ( 9 , 11 , 4 ).

This situation began to change in the mid-1960s, when a branch of biomedical engineering emerged that focused on applying engineering control systems analysis to physiological systems ( 18 , 19 , 2 , 20 ). Arthur Guyton was the first major physiology textbook author to include a control systems theory approach in his textbook, and his book included detailed attention to the body's many regulatory mechanisms ( 10 ). Hence, Guyton introduced many students to the concept of homeostasis as an active regulatory mechanism that tended to minimize disturbances to the internal environment.

Engineering control systems theory describes a variety of other mechanisms to maintain the stability of a system. Although many of these mechanisms may be found in biological systems ( 7 ), not all of them are components of homeostatic mechanisms. For instance, the ballistic system used by the nervous system for throwing a ball simply calculates in advance the pattern of commands needed to achieve some particular outcome based on previous experience ( 7 ). Here, there is no element involved that regulates the internal environment.

Homeostatic mechanisms originated to keep a regulated variable in the internal environment within a range of values compatible with life and, as has been more recently suggested, to reduce noise during information transfer in physiological systems ( 22 ). To emphasize the stabilizing process, we distinguish between a “regulated (sensed) variable” and a “nonregulated (controlled) variable” ( 5 , 23 ). A regulated (sensed) variable is one for which a sensor exists within the system and that is kept within a limited range by physiological mechanisms ( 5 ). For example, blood pressure and body temperature are sensed variables. Baroreceptors and thermoreceptors exist within the system and provide the value of the pressure or temperature to the regulatory mechanism. We call variables that can be changed by the system, but for which sensors do not exist within the system, nonregulated (controlled) variables. Nonregulated variables are manipulated or modulated to achieve regulation of the variable being held constant. For example, heart rate can be changed by the autonomic nervous system to regulate blood pressure, but there are no sensors in the system that measure heart rate directly. Hence, heart rate is a nonregulated variable.

A simple model illustrating the fundamental engineering control system concepts relevant to homeostatic regulatory mechanisms is shown in Fig. 1 .

Diagram of a generic homeostatic regulatory system. If the value of the regulated variable is disturbed, this system functions to restore it toward its set point value and, hence, is also referred to as a negative feedback system.

This model, some version of which appears in many current physiology texts, includes the following five critical components that a regulatory system must contain to maintain homeostasis:

• 1 . It must contain a sensor that measures the value of the regulated variable.
• 2 . It must contain a mechanism for establishing the “normal range” of values for the regulated variable. In the model shown in Fig. 1 , this mechanism is represented by the “set point,” although this term is not meant to imply that this normal range is actually a “point” or that it has a fixed value. In the next section, we further discuss the notion of a set point.
• 3 . It must contain an “error detector” that compares the signal being transmitted by the sensor (representing the actual value of the regulated variable) with the set point. The result of this comparison is an error signal that is interpreted by the controller.
• 4 . The controller interprets the error signal and determines the value of the outputs of the effectors.
• 5 . The effectors are those elements that determine the value of the regulated variable.

Such a system operates in way that causes any change to the regulated variable, a disturbance, to be countered by a change in the effector output to restore the regulated variable toward its set point value. Systems that behave in this way are said to be negative feedback systems.

While the model shown in Fig. 1 is a relatively simple one, there is a great deal of information that can be packed into each of the boxes that constitute the model. Homeostasis can also be described as a hierarchically arranged set of statements, a conceptual framework, that contains whatever breath and depth of information is appropriate for a particular set of students in a course. We have developed and described such an “unpacking” of the core concept of homeostasis ( 12 , 13 ). The model and the conceptual framework provide students with different tools for thinking about homeostasis.

Topics That Cause Confusion for Students and Instructors: Sticky Points

A sticky point is any conceptual difficulty that makes one's mental model of any phenomenon inaccurate and, hence, less useful. There are a number of factors that contribute to the generation of sticky points for both instructors and students:

• The phenomenon in question is a complex one.
• There are aspects of the phenomenon that are counterintuitive.
• The language or terminology used to describe the phenomenon or concept is inconsistent.
• The discipline's understanding of the phenomenon is uncertain or incomplete.

In this section, we will describe some sticky points about homeostatic regulatory mechanisms that we have uncovered as we have interacted with instructors and students about their understanding of homeostasis. We will address these sticky points in the form of a series of questions and answers.

What environment is regulated by organismal homeostasis?

Organismal homeostasis, as originally defined by Cannon ( 6 ), refers to physiological mechanisms that maintain relatively constant the variables related to the internal milieu of the organism. This includes variables related to the entire ECF compartment or to its subcompartments (e.g., the plasma). We will not be discussing intracellular homeostatic mechanisms.

Are all negative feedback systems homeostatic?

Although negative feedback is an essential element of homeostatic regulatory mechanisms, the presence of negative feedback in a system does not mean that the system is homeostatic in function. Negative feedback exists in many systems that do not involve homeostatic regulation. For example, negative feedback plays a role in the muscle stretch reflex, but this reflex is not involved with maintaining the constancy of the internal environment. In other cases, the presence of negative feedback may minimize oscillation of a variable, even though that variable itself is not maintained relatively constant (i.e., it is not a regulated variable). Control of the blood levels of cortisol is an example of the oscillating damping effects of negative feedback (see further discussion below).

Can other types of control mechanisms (e.g., feedforward) maintain homeostasis?

Feedforward or anticipatory control mechanisms permit the body to predict a change in the physiology of the organism and initiate a response that can reduce the movement of a regulated variable out of its normal range ( 7 , 23 ). Thus, feedforward mechanisms may help minimize the effects of a perturbation and can help maintain homeostasis. For example, anticipatory increases in breathing frequency will reduce the time course of the response to exercise-induced hypoxia. Because of this, attempts have been made to broaden the definition of homeostasis to include a range of anticipatory mechanisms ( 23 ).

However, we have decided to limit our generic model of a homeostatic regulatory system ( Fig. 1 ) to one that illustrates negative feedback and demonstrates the minimization of an error signal. We have done this because our model is intended to help faculty members teach and students learn the core concept of homeostasis in introductory physiology ( 12 , 13 ). There are additional complex features found in feedback systems that are not included here because our intention is to first help students make sense of the foundational concept of homeostatic regulation. As situations are encountered where this basic model is no longer adequate to predict system behavior ( 7 , 23 ), additional elements like feedforward mechanisms can be added to the model.

What is a set point?

Understanding the concept of a set point is central to understanding the function of a homeostatic mechanism. The set point in an engineering control system is easily defined and understood; it is the value of the regulated variable that the designer or operator of the system wants as the output of the system. The cruise control mechanism in an automobile is an example of a system with an easy to understand set point. The driver determines the desired speed for the car (the set point). The regulatory mechanism uses available effectors (the throttle actuators) and a negative feedback system to hold the speed constant in the face of changes in terrain and wind conditions. In such a system, we can envision an electronic circuit located in the engine control module that compares the actual ground speed with the set speed programmed by the driver and uses the error signal to control the throttle actuator appropriately.

In physiological systems, the set point is conceptually similar. However, one source of difficulty is that, in most cases, we do not know the molecular or cellular mechanisms that generate a signal of a particular magnitude. What is clear is that certain physiological systems behave as though there is a set point signal that is used to regulate a physiological variable ( 23 ).

Another challenge to our understanding of set points arises from the fact that set points are clearly changeable, either physiologically or as the result of a pathological change in the system ( 23 ). The mechanisms that cause variations in a set point can operate temporarily, permanently, or cyclically. Physiologically, this can occur as a result of discrete physiological phenomena (e.g., fever), the operation of hierarchical homeostats (e.g., regulation of ECF P co 2 ) (see Ref. 7 ), or through the influence of biological clocks (e.g., circadian or diurnal rhythms of body temperature). The observation that set points can be changed adds complexity to our understanding of homeostatic regulation and can lead to confusion about whether the measured change in a regulated variable results from a change in the physiological stimulus or from a changing set point ( 23 ). In these cases, it is important to make such distinctions between a change in the stimulus and the modulation of the set point to arrive at an accurate picture of how a particular homeostatically regulated system operates.

Do homeostatic mechanisms operate like an on/off switch?

Control signals are ALWAYS present, and they continuously determine the output of the effectors. Changes in the control signals alter effector outputs and therefore change the regulated variable. The amplitude of these control signals vary when there is an error signal (i.e., when the regulated variable is not the same as the set point). Thus, homeostatic regulation is a constant, continuous process and does not ordinarily operate as an on/off switch that results in an all-or-none response.

What is the difference between an effector and a physiological response?

Textbook diagrams and narratives can blur the distinction between the effector and a response generated by the effector, making it difficult for students to build a correct mental model. This problem can occur if, when a visual representation of a homeostatic mechanism is presented (see Fig. 1 ), a physiological response is placed in the same “concept” box as the effector. For example, “increased secretion by sweat glands” and “vasodilation of blood vessels in the skin” might be identified as effectors in the control system for thermoregulation. However, only “sweat glands” and “blood vessels” are effectors, whereas “increased secretion” and “vasodilation” are the responses of the effectors. Comprehensive understanding of homeostatic mechanisms requires that we, and students, make clear distinctions between effectors and responses. The term “effector” should only be applied to a physical entity such as a cell, tissue, or organ, whereas responses such as secretion and vasodilation are actions, not physical entities.

Students may also be confused if only the change in the regulated variable is thought of as being the response of the effector. The change in the regulated variable is typically a consequence of changes in function caused by effectors that determine the value of the regulated variable. By applying the term “response” to only the change in the regulated variable, the intermediary steps between the action of the effector and the change in the regulated variable are not acknowledged explicitly. Under these circumstances, it would reasonable for students to conclude that the intermediary steps are, in some way, aspects of the effector rather than the effect of actions of the effectors. This practice may also reflect a lack of understanding of the difference between the regulated variable, e.g., body temperature, and all of the nonregulated variables that are modified (e.g., arteriole diameter and rate of sweat production) in the steps between the action of the effector and the change in the regulated variable.

What does “relatively constant over time” mean?

In the above sections, we emphasized that homeostatic mechanisms operate to keep a regulated variable in the internal environment “relatively constant.” This is a common phrase used to describe what normally happens to the value of the regulated variable over time. A potential sticky point arises from the use of this phrase. How much change can occur to a regulated variable that is held relatively constant? Three points of clarification need to be made. By saying relatively constant, we mean that:

• 1 . Regulated variables are held within a narrower range of values than if they were not regulated.
• 2 . The regulated value is maintained within a range that is consistent with the viability of the organism.
• 3 . There are differences in the range of values permitted for different regulated variables.

The second point is key to understanding the range over which regulated variables can change; homeostatic mechanisms operate to prevent a potentially lethal change in the internal environment. Indeed, as it is often used, relatively constant essentially serves as a surrogate phrase for within the range compatible with an organism's viability. For some regulated variables, the range is quite narrow (e.g., extracellular H + concentration or extracellular osmolarity). For other variables, the range can be broad under some circumstances (e.g., blood glucose concentration during the fed state) and narrow in other situations (e.g., blood glucose during the fasting state). The factors that contribute to the normal range or, in our model, the set point, of a particular variable are undoubtedly complex and, in most cases, have not been elucidated.

What physiological variables are homeostatically regulated?

To identify specific variables that may be homeostatically regulated, the five critical components illustrated in the model shown in Fig. 1 must be present. That is, a regulatory system for that variable must exist that contains the five critical components described in Fig. 1 . Based on this test, we have generated a partial list of the physiological variables that are homeostatically regulated ( Table 1 ). The list of widely recognized and clearly established regulated variables in humans includes a number of inorganic ions (e.g., H + , Ca 2+ , K + , and Na + ), blood-borne nutrients (e.g., glucose), blood pressure, blood volume, blood osmolarity, and core body temperature.

Homeostatically regulated variables typically found in undergraduate human physiology textbooks

This table includes commonly found components of control systems involved in physiological regulation (i.e., homeostasis). This is not meant to be an exhaustive list but rather reflect the current understanding of homeostatically regulated variables that undergraduate physiology students should understand and be able to apply to problems (e.g., making predictions about responses to perturbations or explaining symptoms of disease).

A potential sticky point occurs when textbooks identify variables as homeostatically regulated even though the system involved does not have all of the required components. The proposition that certain metabolic waste products (e.g., nitrogenous wastes, bilirubin, and creatinine) are homeostatically regulated illustrates such a failure. We are not suggesting that the levels of these substances are not kept relatively constant by steady-state processes in the body. Rather, the concentrations of these substances are not maintained by a system that meets the definition of a homeostatic mechanism listed above. The body does not possess a physiological sensor for detecting these substances in the ECF and therefore cannot homeostatically regulate the ECF concentration of these substances.

Conversely, some mechanisms for controlling the level of a physiological variable include one component of the model (e.g., negative feedback) and may give the appearance of homeostatic regulation but, in the final analysis, do not meet all criteria and should not be considered homeostatic. For example, textbook diagrams illustrating control of blood cortisol levels show several negative feedback loops. This can cause students to think that cortisol is a regulated variable. However, the sensed variable(s) in this system is(are) the variables (e.g., blood glucose or “stress”) whose values are processed by the higher brain centers or hypothalamus and result in the release of corticotropin-releasing hormone. The result of the negative feedback loops involving adrenocorticotropic hormone and cortisol is a modulation of the release rate of the respective hormones. Therefore, corticotropin-releasing hormone, adrenocorticotropic hormone, and cortisol should not be considered homeostatically regulated variables. They are signaling elements controlling the effectors that determine the value of the regulated variable(s).

Another possible source of confusion about the identification of regulated variables arises when a physiological variable is regulated under one set of circumstances but behaves as a controlled variable under other circumstances. This can happen if a regulated variable is under the control of two different homeostatic systems or if a regulated variable can be “coopted” by another homeostatic system. This often happens if a physiological variable plays a role in more than one function in the body.

It is here that the concept of nested homeostasis or hierarchies of homeostats can be helpful. Carpenter ( 7 ) has pointed out that there are circumstances in which the maintenance of one regulated variable at its set point value is more important for continued viability of the organism than the simultaneous regulation of another variable.

One example of this is provided by the value of P co 2 in the ECF. As a variable in the internal environment that affects cell viability, P co 2 meets all of the criteria for a homeostatically regulated variable. P co 2 in the ECF depends on the action of respiratory muscles that alter the rate and depth of ventilation. As such, P co 2 in the ECF is maintained within defined limits by a regulatory system that senses P co 2 and operates by negative feedback. However, as any student of acid-base physiology knows, P co 2 in the ECF is not maintained relatively constant during compensatory adjustments in the acid-base balance of the body. From the perspective of H + homeostasis, P co 2 functions as a controlled variable.

At this point, some of our students might ask “Which is it? Is P co 2 a regulated variable or is it a controlled variable?” Our answer is that P co 2 is “both,” and we can explain this using the idea of nested homeostatic mechanisms. There are circumstance in which it is more important to maintain arterial H + concentration (pH) in the normal range that maintaining a constant P co 2 , perhaps because of the particular impact of the H + concentration on cell survival. Therefore, effective regulation of the H + concentration of the ECF can only be achieved by allowing P co 2 to dramatically vary from its normal range during acid-base disturbances. By introducing the concept of nested homeostatic mechanisms, we have refined how we view P co 2 as a homeostatically regulated variable, and we have offered another way to resolve other, “sticky” situations where the authenticity of a homeostatically regulated variable might be called into question.

Given the centrality of the concept of homeostasis ( 15 , 16 ), one would expect that both instructional resources and instructors would provide a consistent model of the concept and apply this model to appropriate systems in which variables are sensed and maintained relatively constant.

However, examination of undergraduate textbooks revealed that this is not the case ( 17 ). The problems found include, but were not limited to, inconsistent language used to describe the phenomenon and incomplete or inadequate pictorial representations of the model. In addition, texts often define homeostasis early in the narrative but fail to reinforce application of the model when specific regulatory mechanisms are discussed ( 17 ).

Furthermore, our work focusing on developing a concept inventory for homeostatic regulation ( 12 , 13 ) revealed considerable confusion among faculty members regarding the concept. We think this confusion may stem, in part, from the level of faculty uncertainty about the concept and degree of complexity of homeostatic regulatory mechanisms. Our discussion of the sticky points associated with homeostasis is an attempt to suggest potential sources of this confusion and to indicate ways that instructors can work through these difficulties.

How do we ameliorate this situation? We propose five strategies that will help in approaching the problem.

1 . Faculty members members should adopt a standard set of terms associated with the model. There is inconsistency within and among textbooks with respect to the names for critical components of the model. We propose the terminology shown in Table 2 to be used when discussing homeostatic regulatory mechanisms.

Definitions of terms for homeostasis paper

A glossary of terms used in discussing the core concept of homeostasis. The components of a homeostatically regulated system ( Fig. 1 ) are defined here as are some other terms that occur in teaching this concept.

2 . A standard standard pictorial representation of the model should be adopted when initially explaining homeostasis, and it should be used to frame the discussion of the specific system being considered. Figure 1 shows such a diagram.

The argument could be made that this diagram may be difficult for undergraduate students to understand. This may be the rationale for presenting the much-simplified diagrams found in most undergraduate texts ( 17 ). However, because these simple diagrams do not explicitly include all components of a homeostatic regulatory system (e.g., a set point), they may be a source of the misconceptions discussed as sticky points. As a result, students may not recognize that an essential feature of homeostatic regulatory systems is minimizing an error signal. A simplified representation of the model that includes the critical components of the regulatory system is shown in Fig. 2 . Depending on the course content and level of the student, this model can be expanded to add more levels of complexity as are required.

Simplified representation of a homeostatic regulatory system. Several components shown in Fig. 1 are combined in this representation. The reader should refer to Table 1 to find correspondence between components of physiologically significant homeostatic regulatory systems and this simplified representation. For example, chemosensors in the carotid bodies and aortic body are “sensors,” the brain stem is the “control center,” and the diaphragm and other respiratory muscles are “effectors” in the homeostatic regulatory system for arterial P o 2 .

3 . Faculty members should introduce the concept of homeostatic regulation early in the course and continue to apply and hence reinforce the model as each new homeostatic system is encountered. It is important to continue to use the standard terminology and visual representation as recommended in the first and second points above. Students tend to neither spontaneously or readily generalize their use of core concepts. It is therefore incumbent on the instructor to create a learning environment where this kind of transfer behavior is promoted. Faculty members can facilitate this by providing multiple opportunities for students to test and refine their understanding of the core concept of homeostatic regulation.

One way to reinforce the broad application of the model of homeostasis and help students demonstrate that they understand any particular homeostatic mechanism is to have them ask (and answer) a series of questions about each of homeostatically regulated systems they encounter (see Table 3 ). In doing so, they demonstrate that they can determine the essential components of the mental model needed to define the homeostatic system. The effort to thoroughly and accurately answer these questions will help students uncover gaps in their understanding and will reveal uncertainties in the resource information that they are using.

Questions students should ask about any homeostatically regulated system

[Adapted/modified from Ref. 21 .]

4 . Faculty members should use care when they select and explain the physiological examples or analogical models they chose to introduce and illustrate homeostasis in the classroom. In particular, instructors should ensure that the representative examples they use do not introduce additional misconceptions into student thinking. This is especially so when thermoregulation may be considered as an example of homeostatic regulation.

An informal survey of physiology textbooks indicated that thermoregulation is almost universally used as an example of a homeostatic mechanism. The most likely reasons for this selection are that 1 ) there is an everyday, seemingly easy to understand process involving the regulation of air temperature in room or building (i.e., the operation of a furnace and an air conditioner) and 2 ) the body's physiological responses are commonly and obviously observable and/or experienced by the learner (sweating, shivering, and changes in skin coloration). However, based on our description of the typical homeostatic regulatory system, there are compelling reasons to recommend that caution be taken if thermoregulation is used as the initial and representative example of homeostasis.

Most concerning, the typical home heating and cooling system operates in a manner that is distinctly different from mechanisms of human thermoregulation. The effectors in most houses, the furnace and air conditioner, operate in a full-on/full-off manner. For example, when the temperature at the thermostat falls below the value that has been dialed in (the set point temperature), the furnace turns on and stays on at maximum output until the temperature returns to the set point value. However, this is not how the human thermoregulatory system functions or how other homeostatic mechanisms operate. One potential consequence of using this model system to illustrate a homeostatic system is the creation of a common student misconception that homeostatic mechanisms operate in an on/off manner ( 12 , 24 ), a sticky point we have addressed above. Faculty members need to help students overcome this problem area if they chose to use thermoregulation as a representative example of homeostasis.

What alternatives might be recommended? We suggest the automobile cruise control as a helpful nonbiological analog for homeostasis. The use of cruise control is not an uncommon activity for students, and, as we have described previously, the operation of a cruise control is theoretically easy to understand. What about a physiological example to represent homeostasis? A review of Table 1 would suggest the insulin-mediated system for blood glucose regulation during the fed state has much to recommend it. Students are generally familiar with the particulars of the system from either previous coursework or from personal experience. Other systems are likely to be less accessible to the beginning student of physiology.

However, faculty members should be aware that blood glucose regulation is not without its downsides as a representative example of homeostatic regulation. It is not easy to identify or explain the operation of the glucose sensor, the set point, and the controller involved in glucose homeostasis. Furthermore, there is probably no widely understood analog to glucose regulation that can be easily drawn from everyday life. Neither cruise controls, navigation systems on airplanes, autofocuses on cameras or other common, nor everyday examples of servomechanisms fully correspond to the operation of the feedback system involved in regulating blood glucose during the fed state. This points out the tradeoffs that must be made when any particular example or model is adopted to represent homeostatic regulation. Recognizing this, the use of a physiological control system such as glucose regulation during the fed state, where the effectors operate continuously, seems preferable to thermoregulation as a representative example for teaching the concept of homeostatic regulation.

5 . When discussing discussing organismal physiology, restrict the use of the term “homeostatic regulation” to mechanisms related to maintaining consistency of the internal environment (i.e., the ECF).

Adopting these five strategies will provide students with a consistent framework for building their own mental models of specific homeostatic mechanisms and will help them recognize the functional similarities among different homeostatic regulatory systems at the organismal level. Because of its widespread application to different systems in organismal biology, homeostasis is one of the most important unifying ideas in physiology ( 15 , 16 ). To construct a robust and enduring understanding of this concept, students need the proper tools. By giving them a precise and consistent terminology and encouraging them to use a standardized pictorial representation of the homeostatic model, we enable them to build a proper foundation for comprehending homeostatic systems. By making students aware of the potential sources of confusion surrounding the concept of homeostasis, i.e., the sticky points, we help prevent their thinking from becoming misguided or out of square. By doing so, we set the stage for our students to develop an accurate understanding of a wide range of physiological phenomena and to arrive at an integrated sense of the “wisdom of the body.”

This work was supported by National Science Foundation Grant DUE-1043443 .

Any results, conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the NSF.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

Author contributions: H.I.M., W.H.C., and J. Michael conception and design of research; H.I.M. and W.H.C. analyzed data; H.I.M., W.H.C., and J. Michael drafted manuscript; H.I.M., W.H.C., J. Michael, J. McFarland, M.P.W., and A.W. edited and revised manuscript; H.I.M., W.H.C., J. Michael, J. McFarland, M.P.W., and A.W. approved final version of manuscript; J. Michael prepared figures.

1.5 Homeostasis

Learning objectives.

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

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

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

Negative Feedback

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

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

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

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

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

Interactive Link

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

Positive Feedback

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

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

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

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

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Homeostasis and Regulation in the Human Body Essay

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

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

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

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

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

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

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

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

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

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

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

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

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What Is Homeostasis?

The body's need to maintain a state of equilibrium

AleksandarNakic / Getty Images

• Maintaining Homeostasis

Homeostasis and Mental Health

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

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

How Is Homeostasis Maintained?

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

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

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

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

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

Types of Homeostatic Regulation

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

Thermoregulation

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

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

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

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

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

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

Osmoregulation

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

Chemical Regulation

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

Impact of Homeostasis

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

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

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

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

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

APA Dictionary of Psychology. Osmoregulation .

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

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

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

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

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

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

Essay on Homeostasis

Homeostasis is the tendency to maintain a stable and relatively constant internal environment. It is crucial for any living thing to maintain a stable internal condition since it must always remain constant. Despite the external environment’s dynamicity, the body employs different physiological strategies that support the system’s proper function. This capability is one of the crucial aspects that enable the human body to stay alive. The body acts upon and resists the effects of external factors to prevent its deviation from the state of balance, equilibrium, and stability it favors rather than doing nothing. According to Modell et al. (2015), three general components enable the human body to maintain homeostasis. They include the receptor, control, and effector centers.

Maintaining homeostasis

The hemostasis mechanism is in the form of a loop that can either be positive or negative. Positive feedback propels the situation and results in more stimulation, whereas negative feedback decelerates the process and inhibits the stimulus (Castanho & Dos Anjos Garnes, 2019). For example, a high body temperature triggers the negative loop, which returns it towards the set point. The sensors, which are primary nerve cells with endings in the brain, will detect the high temperature and relay it to the temperature-regulatory control center. Processing this information will take part in the control center, and effectors such as sweat glands will be activated (Modell et al., 2015). The function of these effectors is to lower the body temperature by opposing the stimulus.

The body temperature does not always go above the setpoint. In some situations, it can go below the set point. In general, there are at least two negative feedback loops that are usually involved in the homeostatic circuit. The first negative feedback loop is designed to lower a parameter after it has gone above the setpoint (Modell et al., 2015). The second negative feedback loop is intended to return the parameter up when it is below the set point.

For example, when the external body temperature is either too cold or too hot, the hypothalamus, the temperature regulatory center in the brain, is notified by the sensors in the periphery that the temperature has strayed from the setpoint (Tansey et al., 2015). For instance, when an individual has been exercising too hard, the internal body temperature rises above the setpoints, and cooling it down will require activating the necessary mechanisms (Library, 2019). The increase in blood flow in the skin seeks to increase heat loss to the surrounding. Sweating is also a mechanism that allows the body to cool off when the sweat evaporates.

The temperature center in the brain will also trigger responses to keep warm when an individual is sitting in a cold room and is not dressed warmly. Tansey et al. (2015) note that a person may begin shivering since the blood flow in the skin decreases. This action allows the body to generate more heat. The skin may also develop goosebumps, allowing the body hair to stand up and trap air near the skin.

The negative feedback loops play a fundamental factor in homeostasis, and any interference with this feedback mechanism disrupts homeostasis and may eventually result in disease. For example, a broken feedback loop involving insulin hormone results in diabetes disease (Röder et al., 2016). The human body finds it challenging to lower high blood sugar levels when the feedback loop is broken. When an individual consumes a meal, the blood glucose levels increase, triggering the β cells in the pancreas to secrete insulin (Library, 2019). It then activates body cells to absorb this glucose for energy. Insulin is also responsible for the conversion of glucose to glycogen in the liver. These processes are responsible for reducing glucose levels in the blood, which returns the system to homeostasis.

On the other hand, glucagon increases glucose concentration in the blood. When the blood glucose levels are low, the pancreatic α cells release glucagon, which initiates the breakdown of glycogen to glucose in the liver (Röder et al., 2016). This increases the glucose level in the body. The system is brought back to homeostasis when glucagon secretion is reduced. Therefore, diabetes occurs when the human body stops responding to insulin or when the pancreas fails to produce enough insulin. Therefore, blood sugar remains high under these conditions since the body cells cannot absorb glucose readily.

Ecosystem homeostasis

Ecosystems comprise a network of animals from the tiniest insects to the largest mammals, alongside various microorganisms, fungi, and plants, making the ecosystem complex. There is an interaction between all these lifeforms since caterpillars will feed on leaves, bears prey on fish, while shrews eat insects. A delicate balance is maintained by everything that exists in nature, and scientists refer to the balance of organisms in an ecosystem as ecosystem homeostasis (Zakharov et al., 2018). The fundamental goal of ecosystem homeostasis is equilibrium. But nothing is ever perfectly balanced in the real-world ecosystem. Various animal species have their population at a similar range, resulting in a relatively stable state of an ecosystem in equilibrium (Ecological Center, 2021). Therefore, as long as there is no general downward or upward trend, populations can go up and down in cycles.

Negative feedback in ecosystem homeostasis operates more diffusely than physiology due to the decentralized nature of ecological systems such as communities, populations, and ecosystems. The interactions among species, individuals, and their environment result in negative feedback since a central processing unit that implements and coordinates negative feedback is unavailable (Ecological Center, 2021). A classic example of how negative feedback could stabilize the system due to consumer or resource dynamics is the interaction between predators and prey. There is an increase in resource availability for the predators when the prey population increases. This increase results in increased survival and reproduction rates for the predator due to the consumption of the prey. Hence, the predator population increases. However, an increase in predator population increases the demand for prey, increasing the predators’ death rates. This is due to the decline in the prey population, which cannot support the high predator abundance. Eventually, the predator population also reduces. Therefore, the limiting resource of the prey induces negative feedback, which counteracts the initial increase in predator abundance.

Many systems experience such a case, and there is a strong stabilizing constraint on the community’s dynamic when food resources are limiting. An increase in resource consumption results in a decrease in other components since the resources are limited. The Ecological Center (2021) defines compensatory dynamics or species compensation as the balance between increased and decreased species. The negative feedback that counteracts the increased consumption rates by various communities emerges from the finite nature of these resources. Resources are responsible for reproduction, growth, maintenance, and survival and as a result, limiting them affects net production, birth, and death rates. Therefore, resource constraints are essential for stabilizing the overall consumption and stabilizing critical ecosystem properties such as biomass production, total abundance, and standing biomass.

In conclusion, people have been steadily growing, particularly since the industrial revolution. It took several years for the human population to reach one billion, and the world is now swiftly approaching eight billion, more than three hundred years later. This sounds like the world is on its way to a catastrophe due to overpopulation. However, due to decreased birth rates resulting from increasing access to contraception and women’s education, the world’s population is not predicted to grow exponentially. The average family size in countries where women are empowered is tiny, with very few children. An increase in natural and economic resource demand and competitiveness has led to reduced birth rates, leading to a drop in the world population.

Castanho, F. L., & Dos Anjos Garnes, S. (2019).  Homeostasis: An Integrated Vision . IntechOpen. https://books.google.co.ke/books?id=y7-QDwAAQBAJ

Ecological Center. (2021).  Maintenance of Homeostasis in Ecological Systems – Population Dynamics . https://www.ecologycenter.us/population-dynamics-2/maintenance-of-homeostasis-in-ecological-systems.html

Library, T. O. T. O. C. (2019).  The Animal Body – Basic Form and Function: Biology . Independently Published. https://books.google.co.ke/books?id=IPNMzQEACAAJ

Modell, H., Cliff, W., Michael, J., McFarland, J., Pat Wenderoth, M., & Wright, A. (2015). A Personal View A physiologist’s view of homeostasis.  Adv Physiol Educ ,  39 , 259–266. https://doi.org/10.1152/advan.00107.2015.-Homeostasis

Röder, P. V., Wu, B., Liu, Y., & Han, W. (2016). Pancreatic regulation of glucose homeostasis.  Experimental & Molecular Medicine ,  48 (November 2015), e219. https://doi.org/10.1038/emm.2016.6

Tansey, E. A., Johnson, C. D., & Johnson, C. D. (2015). Staying Current Recent advances in thermoregulation.  Adv Physiol Educ ,  39 , 139–148. https://doi.org/10.1152/advan.00126.2014.-Ther

Zakharov, V., Minin, A., & Trofimov, I. (2018). Study of developmental homeostasis: From population developmental biology and the health of environment concept to the sustainable development concept.  Russian Journal of Developmental Biology ,  49 (1). https://doi.org/10.1134/S1062360418010071

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What is homeostasis?

Homeostasis is a core tenet of the life sciences.

• How it works

Modern models of homeostasis

• An information hypothesis

Homeostasis is the ability to maintain a relatively stable internal state that persists despite changes in the world outside. All living organisms, from plants to puppies to people, must regulate their internal environment to process energy and ultimately survive. If your blood pressure skyrockets or body temperature plummets, for example, your organ systems may struggle to do their jobs and eventually fail.

Why is homeostasis important?

Physiologist Walter Cannon coined the term "homeostasis" in the 1920s, expanding on previous work by late physiologist Claude Bernard. In the 1870s, Bernard described how complex organisms must maintain balance in their internal environment, or "milieu intérieur , " in order to lead a "free and independent life" in the world beyond. Cannon honed the concept, and introduced homeostasis to popular audiences through his book, "The Wisdom of the Body" (The British Medical Journal, 1932).

Hailed as a core tenet of physiology, Cannon's basic definition of homeostasis remains in use today. The term derives from Greek roots meaning "similar" and "a state of stability." The prefix "homeo" stresses that homeostasis doesn't work like a thermostat or cruise control in a car, fixed at one precise temperature or speed. Instead, homeostasis holds important physiological factors within an acceptable range of values, according to a review in the journal Appetite .

Related: Has the average human body temperature always been the same?

The human body , for example, regulates its internal concentrations of hydrogen, calcium, potassium and sodium, charged particles that cells rely on for normal function. Homeostatic processes also maintain water, oxygen, pH and blood sugar levels, as well as core body temperature, according to a 2015 review in Advances in Physiology Education .

In healthy organisms, homeostatic processes unfold constantly and automatically, according to Scientific American . Multiple systems often work in tandem to hold steady a single physiological factor, like body temperature. If these measures falter or fail, an organism may succumb to disease, or even death.

How homeostasis is maintained

Many homeostatic systems listen for distress signals from the body to know when key variables fall out of their appropriate range. The nervous system detects these deviations and reports back to a control center, often based in the brain. The control center then directs muscles, organs and glands to correct for the disturbance. The continual loop of disturbance and adjustment is known as "negative feedback," according to the online textbook Anatomy and Physiology .

For example, the human body maintains a core temperature of about 98.6 degrees Fahrenheit (37 degrees Celsius). When overheated, thermosensors in the skin and brain sound an alarm, initiating a chain reaction that directs the body to sweat and flush. When chilled, the body responds by shivering, and reducing blood circulation to the skin. Similarly, when sodium levels spike, the body signals the kidneys to conserve water and expel excess salt in concentrated urine, according to two NIH-funded studies .

Animals will also adjust their behavior in response to negative feedback. For example, when overheated , we may shed a layer of clothing, move into the shade, or drink a cold glass of water.

The concept of negative feedback dates back to Cannon's description of homeostasis in the 1920s, and was the first explanation of how homeostasis works. But in recent decades, many scientists have argued that organisms are able to anticipate potential disruptions to homeostasis, rather than only reacting to them after the fact.

This alternate model of homeostasis, known as allostasis, implies that the ideal set point for a particular variable can shift in response to transient environmental changes, according to a 2015 article in Psychological Review . The point may shift under the influence of circadian rhythms , menstrual cycles or daily fluctuations in body temperature. Set points may also change in response to physiological phenomena, like fever, or to compensate for multiple homeostatic processes taking place at the same time, according to a 2015 review in Advances in Physiology Education .

"The set points themselves aren't fixed but can show adaptive plasticity," said Art Woods, a biologist at the University of Montana in Missoula. "This model allows for anticipatory responses to upcoming potential disturbances to set points."

Woods is a physiological ecologist working at the University of Montana. His primary interests are in plant-insect ecology and the physiological and biophysical details of their interaction. He studied biology during his undergraduate years at Stanford, which later led him to the Zoology (now Biology) PhD program at the University of Washington. 

For example, in anticipation of a meal, the body secretes extra insulin, ghrelin and other hormones, according to a 2007 review in Appetite . This preemptive measure readies the body for the incoming flood of calories , rather than wrestling to control blood sugar and energy stores in its wake.

The ability to shift set points allows animals to adapt to short-term stressors, but they may fail in the face of long-term challenges, such as climate change .

"Activating homeostatic response systems can be fine for short periods of time," Woods said. But they're not designed to last for long. "Homeostatic systems can fail catastrophically if they are pushed too far; so, although systems may be able to handle near-term novel climates, they may not be able to handle larger changes over longer periods of time."

An "information hypothesis" for homeostasis

Homeostatic systems may have primarily evolved to help organisms maintain optimal function in different environments and situations. But, according to a 2013 essay in the journal Trends in Ecology & Evolution , some scientists theorize that homeostasis primarily provides a "quiet background" for cells, tissues and organs to communicate with each other. The theory posits that homeostasis makes it easier for organisms to extract important information from the environment and shuttle signals between body parts.

— Why do we shiver when we're cold?

— How did people wake up before alarm clocks?

— What are the most common elements in the human body?

Regardless of its evolutionary purpose, homeostasis has shaped research in the life sciences for nearly a century. Though mostly discussed in the context of animal physiology, homeostatic processes also enable plants to manage energy stores, nourish cells and respond to environmental challenges. Beyond biology, the social sciences, cybernetics, computer science and engineering all use homeostasis as a framework to understand how people and machines maintain stability despite disruptions.

Additional resources

• Check out helpful graphics about homeostasis from the Khan Academy
• Learn how homeostasis impacts human physiology with Crash Course
• Watch this video from the Amoeba Sisters to learn more about negative feedback.

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Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She holds a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Her work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains heavily involved in dance and performs in local choreographers' work.

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Homeostasis

Reviewed by: BD Editors

Homeostasis Definition

Homeostasis is an organism’s process of maintaining a stable internal environment suitable for sustaining life. The word homeostasis derives from Greek, with home meaning “similar,” and stasis , meaning “stable.” When used as an adjective, it is homeostatic.

We normally think about homeostasis in terms of the whole body, but individual systems – that is, groups of organs – also maintain homeostatic conditions. Nonetheless, prolonged imbalance in just one system can negatively impact the homeostasis of the entire organism.

Examples of Homeostasis

Homeostasis is a regulatory procedure. In the human body, homeostatic processes regulate:

• Ratios of water and minerals
• Body temperature
• Chemical levels

The Formation of a Kidney Stone

Vitamins and minerals provide our bodies with the nutrients essential to thrive. While our large intestine and salivary glands absorb most of these nutrients, excess quantities leave our bodies through sweat and urination.

Of course, minerals vary in size. Calcium, phosphorus, and sodium are considered stone-promoting compounds , because they form crystals in the urinary tract that pass through the bladder. Technically, most humans always have kidney stones; not all of them are painful.

This is where homeostasis comes in. Under homeostatic conditions, our kidney stones (or crystals, in technical terms) are so small, we urinate them without a second thought. On the other hand, an overabundance of stone-promoting compounds or a lack of fluids in the urinary system can cause crystals to build and combine in the urinary tract, forming a stone. These stones, while excruciatingly painful, usually pass naturally. Sometimes, however, due to location or size, they require surgery.

Running a Fever

You are exposed to over a million germs and bacteria cells per day – more if you work in a school, barn, doctor’s office or other high-contact area. Thankfully, the human immune system – lymph nodes, enzymes, t-cells, and b-cells – protects your body from the diseases these organisms may cause.

But some germs are tougher than the rest. Whether as mild as the common cold or as severe as tuberculosis, some strains , or varieties of disease, overcome your first line of defense and make you their host.

Microscopic invasions definitely disrupt homeostasis, often enough that the body knows exactly how to restore normal conditions. The hypothalamus raises the body’s temperature, making your insides both unwelcome to and uninhabitable for any uninvited guests. Furthermore, your immune system records these diseases in its “memory,” making it more difficult for you to catch the same bug twice.

Producing Insulin in Response to High Blood Sugar

In homeostatic conditions, our bodies keep our blood sugar within a tight range – between 70 and 100 mg/dL, to be exact. However, this is a delicate balance. Our weight, diet, age, and activity levels can easily push us out of these normal levels.

Of the blood-glucose-affecting factors listed above, diet plays the largest role. Whether old or young, underweight or overweight, diabetic or non-diabetic, we use food to manage our blood glucose. We typically recognize its ability to raise levels, but even this benefit can be taken too far.

Especially since the dawn of processed foods, our diets have become increasingly sugary. While we have consumed complex sugars – like the ones that come from fruits and grains – for centuries, simple sugars – like those in candy and cereal – only hit our systems a few decades ago.

Simple sugars reach our bloodstream fast, and can therefore cause blood glucose levels to surge in as little as half an hour. To balance our blood sugar and maintain homeostasis, our pancreas produces insulin, a hormone that converts glucose to energy or stores it for future use. People with diabetes , a condition characterized by chronic high blood sugar, inject insulin after eating in order to maintain this same state of homeostasis.

Related Biology Terms

• Osmoregulation – Also called excretion, the maintenance by an organism of an internal balance between water and dissolved minerals regardless of environmental conditions.
• Thermoregulation – Maintaining an optimal internal temperature.
• Glucoregulation – The regulation of blood sugar.
• Filtration – The mass movement of water and solutes into the kidney, where they are processed into urine.

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How Homeostasis Helps Us Balance the ‘Sweet Spot’ for Life

Being able to regulate our body functions can enable a freer and more independent life, scientists say.

• Homeostasis is the process through which organisms regulate biological functions like heart rate, blood pressure, and temperature .
• Like a home heating system, homeostasis turns body functions on and off in response to feedback about an organism .
• This process allows an organism to operate freely and independently in the world.

Living things have a remarkable ability to function and thrive in a variety of environments that are constantly changing. Despite exposure to wildly different temperatures during heat waves and cold snaps, or even changes in rainfall and food availability, our bodies are able to keep a constant temperature, blood pressure, and electrolyte balance. How do animals keep doing this in the face of all the changing conditions around them? The answer lies in homeostasis.

Homeostasis is one of the big ideas in biology . The term, first coined in 1929 by American physiologist Walter Cannon, was meant to describe the regulation of our internal environment through feedback and feedforward systems.

🔬 Science explains the world around us. We’ll help you make sense of it all—join Pop Mech Pro.

“Homeostasis is the sweet spot,” Muzlifah Haniffa , an immunologist at Newcastle University in England, tells Popular Mechanics . “And the process is to maintain it actively in that sweet spot and to return any deviation that occurs back to the sweet spot.”

It is often likened to a home’s heating system that kicks on the heater when the room temperature falls too low, and turns it off when the temperature gets high enough; in other words, the system turns on and off in response to feedback about the temperature of the room. Your body, it turns out, has similar systems in place that keep its biological functions maintained at set points. Levels of sodium and potassium levels in the blood, for example, are examples of nutrients that are homeostatically controlled. Some biological functions, like body temperature, are strictly maintained at a constant number. Others, like blood cortisol levels, may fluctuate between different set points based on stress levels or sleep cycles.

“Homeostasis describes how variables in the body are maintained in a relatively constant range, despite widely changing environmental conditions,” George Billman tells Popular Mechanics . Billman, a physiologist at Ohio State University, explains that homeostasis is what allows animals to be independent and interact in the world without disrupting their internal function.

An often-cited example is blood pressure. Throughout the day, blood pressure varies within a narrow range, which is important for keeping the blood supplied with a fresh batch of oxygen and nutrients. If the pressure is too low, then it cannot supply oxygen to the body’s tissues; if it is too high, it will damage the blood vessels. There are sensors in the blood vessels that monitor blood pressure and adjust the vessels accordingly. If an animal starts running, for example, its blood pressure will increase and deliver more oxygen and nutrients to the muscles, which need an extra boost. When the creature comes to a rest, the pressure comes back down.

“Something that’s alive tries to keep itself going, so we eat when we’re hungry, we drink when we’re thirsty,” Dr. Marc Basson , a physician and biological scientist at the University of North Dakota, tells Popular Mechanics.

“Homeostasis is not a condition for life. There are organisms that only regulate a certain few things, and they’re very successful.”

Dr. Basson describes a scenario in which a person is busy during the workday and they forget to drink water and become dehydrated. The blood becomes more concentrated and the heart rate will jump up to make sure the smaller volume of blood gets distributed evenly throughout the body. Eventually, the brain will get the message and send a signal to drink. The person will become thirsty and get a glass of water, and eventually the blood volume will go up and the heart rate will come down. “When everything goes back to normal, that’s homeostasis,” he says.

“The best descriptor of homeostasis is stability,” William Cliff , a biologist at Niagara University in New York, tells Popular Mechanics . “It’s the idea that something’s in a relatively stable state, even though the environment around it may be changing.”

Cliff points out that the purpose of homeostasis is to allow organisms to operate freely and independently in the world. He says that organisms with more evolved homeostatic mechanisms generally have more independence than those with more primitive systems. Turtles , for instance, have a more pared-down version. In order to tolerate cold temperatures in the winter, they must bury themselves in the muddy bottoms of ponds. They can slow down their metabolism and live without food for months, surviving with very low levels of oxygen. But in their low-energy state, they cannot move about in the pond, reproduce, or carry out other basic life tasks.

“Homeostasis is not a condition for life. There are organisms that only regulate a certain few things, and they’re very successful,” Cliff says. But being better able to regulate our body functions can enable a freer and more independent life. “A polar bear living in the Arctic is independent of the thermal temperature around it. It can go on day after day doing the same thing whether it’s hot or cold,” he says. “The snake under those situations is not independent.”

Monique Brouillette is a freelance contributor who writes about biology. 

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Module 3: Biological Macromolecules

Homeostasis, learning objectives.

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

• Define homeostasis
• Describe the factors affecting homeostasis
• Discuss positive and negative feedback mechanisms used in homeostasis
• Describe thermoregulation of endothermic and ectothermic animals

Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium.

Homeostatic Process

The goal of homeostasis is the maintenance of equilibrium around a point or value 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 deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use.

Control of Homeostasis

When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine system of mammals.

Negative Feedback Mechanisms

Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure 1. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used in homeostasis.

Figure 1. Blood sugar levels are controlled by a negative feedback loop. (credit: modification of work by Jon Sullivan)

Positive Feedback Loop

A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure 2. The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.

Art Connection

State whether each of the following processes is regulated by a positive feedback loop or a negative feedback loop.

• A person feels satiated after eating a large meal.
• The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney.

It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop.

Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than it is accustomed to. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels.

Link to Learning

Feedback mechanisms can be understood in terms of driving a race car along a track: watch a short video lesson on positive and negative feedback loops.

Homeostasis: Thermoregulation

Body temperature affects body activities. Generally, as body temperature rises, enzyme activity rises as well. For every ten degree centigrade rise in temperature, enzyme activity doubles, up to a point. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50 o C for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions. Some fish can withstand freezing solid and return to normal with thawing.

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

Endotherms and Ectotherms

Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that do not control their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, which rely on external temperatures to set their body temperatures, poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Endotherms are animals that rely on internal sources for body temperature but which can exhibit extremes in temperature. These animals are able to maintain a level of activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity.

Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

Figure 3. Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction. (credit b: modification of work by “Kullez”/Flickr; credit c: modification of work by Chad Rosenthal; credit d: modification of work by “stacey.d”/Flickr)

Heat Conservation and Dissipation

Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. 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 have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.

Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, 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. This adaption can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaption is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears.

Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may 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 to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter.

Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.

Neural Control of Thermoregulation

The nervous system is important to thermoregulation, as illustrated in Figure 4. The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.

Figure 4. The body is able to regulate temperature in response to signals from the nervous system.

When bacteria are destroyed by leuckocytes, 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?

The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism.

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 in equilibrium because body functions are kept within a normal range, with some fluctuations around a set point for the processes.

Additional Self Check Questions

2. When bacteria are destroyed by leuckocytes, 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?

3. Why are negative feedback loops used to control body homeostasis?

4. Why is a fever a “good thing” during a bacterial infection?

5. How is a condition such as diabetes a good example of the failure of a set point in humans?

1. Both processes are the result of negative feedback loops. Negative feedback loops, which tend to keep a system at equilibrium, are more common than positive feedback loops.

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

3. An adjustment to a change in the internal or external environment requires a change in the direction of the stimulus. A negative feedback loop accomplishes this, while a positive feedback loop would continue the stimulus and result in harm to the animal.4. Mammalian enzymes increase activity to the point of denaturation, increasing the chemical activity of the cells involved. Bacterial enzymes have a specific temperature for their most efficient activity and are inhibited at either higher or lower temperatures. Fever results in an increase in the destruction of the invading bacteria by increasing the effectiveness of body defenses and an inhibiting bacterial metabolism.

5. Diabetes is often associated with a lack in production of insulin. Without insulin, blood glucose levels go up after a meal, but never go back down to normal levels.

acclimatization:  alteration in a body system in response to environmental change

alteration:  change of the set point in a homeostatic system

homeostasis:  dynamic equilibrium maintaining appropriate body functions

negative feedback loop:  feedback to a control mechanism that increases or decreases a stimulus instead of maintaining it

positive feedback loop:  feedback to a control mechanism that continues the direction of a stimulus

set point:  midpoint or target point in homeostasis

thermoregulation:  regulation of body temperature

• Biology. Authored by : Open Stax. Located at : http://cnx.org/contents/[email protected]:1/Biology . License : CC BY: Attribution
• Discovery Channel: Thermoregulation in Animals . Authored by : Nisar Ahmad Shah. Provided by : Discovery Channel. Located at : http://youtu.be/NJEBfl_LKno . License : All Rights Reserved . License Terms : Standard YouTube license
• Feedback Loops. Authored by : Johnny Clore. Located at : https://youtu.be/_QbD92p_EVs . License : All Rights Reserved . License Terms : Standard YouTube License

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

The Importance of Homeostasis Within The Human Body

• Categories: Homeostasis

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Published: Jan 4, 2019

Words: 632 | Page: 1 | 4 min read

Works Cited

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• Silverthorn, D. U. (2010). Human Physiology: An Integrated Approach (5th ed.). Pearson.
• Saladin, K. S. (2018). Anatomy & Physiology: The Unity of Form and Function (8th ed.). McGraw-Hill Education.
• Sherwood, L. (2015). Human Physiology: From Cells to Systems (9th ed.). Cengage Learning.
• Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). Wiley.
• Pocock, G., Richards, C., & Richards, D. (2006). Human Physiology: The Basis of Medicine (3rd ed.). Oxford University Press.
• Johnson, L. R. (Ed.). (2000). Essential Medical Physiology. Academic Press.
• Berne, R. M., Levy, M. N., Koeppen, B. M., & Stanton, B. A. (2017). Physiology (7th ed.). Elsevier.

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