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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'. (Britannica, T. Editors of Encyclopaedia 2020)
Homeostasis provides ideal conditions for enzyme functioning and all cell activities all across the body. It is the preservation of a constant internal environment in the face of changing internal and external conditions.(BBC Bitesize) Homeostasis is in charge of controlling body temperature, heart rate, blood pressure, breathing rate, water levels and glucose levels.

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BBC Bitesize(no date). Homeostasis, Accessed December 2021, https://www.bbc.co.uk/bitesize/guides/zp29y4j/revision/

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The endocrine system uses hormones to allow the body to respond to stimuli. Stimuli are changes in the environment and can be from:

inside the body (internal) such as like an increase in blood glucose concentration,

outside of the body (external) like a drop in temperature

A is a chemical substance, produced by a and carried in the bloodstream, which alters the activity of specific target . Hormones bind to on the target organ. The binding of the hormone onto the receptors causes the target organ to respond. For example, when insulin is released into the blood from the pancreas, insulin binds to receptors on the surface of the liver, the target organ. This causes the liver to respond and take glucose out of the blood and store it.

Once a hormone has caused the required response, it is destroyed by the liver.

Different hormones

The body produces a range of different chemical that travel in the bloodstream and affect a number of different organs or cells in the body. The diagram and table below shows this in detail.

Comparing the endocrine and nervous system

The endocrine system causes slower responses than the nervous system, but the responses last for longer. There are important differences between hormonal control caused by the endocrine system and nervous control.

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Human endocrine system - Why do we need to maintain a constant internal environment? - OCR 21C - GCSE Combined Science Revision - OCR 21st Century - BBC Bitesize

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The endocrine system uses hormones to allow the body to respond to stimuli. Stimuli are changes in the environment and can be from: inside the body (internal) such as like an increase in blood glucose concentration, outside of the body (external) like a drop in temperature A is a chemical substance, produced by a and carried in the bloodstream, which alters the activity of specific target .

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Positive feedback amplifies rather than reverses a change in the body's physiological condition. When the system deviates from the typical range, it experiences greater change and goes further away from the usual range. Only when there is a clear end point to positive feedback in the body is considered normal.(Biga M. L et al) An example of a positive feedback mechanism is blood clotting. This is caused by a series of chemical interactions consisting of a number of blood clotting proteins contained in the plasma , which are stimulated in the blood. This causes fibrous protein net to develop, trapping red blood cells and causing a clot. The clotting process kicks in, causing additional platelets to subside on the wound, activating more clotting responses, and so on, until the wound is completely closed. (Wyatt 2017)

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Biga M. L et al( no date). Homeostasis, Accessed December 2021, https://open.oregonstate.education/aandp/chapter/1-3-homeostasis/

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Learning Objectives

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

List the components of a homeostatically controlled system

Discuss the role of homeostasis in the human body

Contrast negative and positive feedback, giving one physiologic example of each mechanism

Maintaining a stable system requires the body to continuously monitor its internal conditions. Though certain physiological systems operate within frequently larger ranges, certain body parameters are tightly controlled homeostatically. For example, body temperature and blood pressure are controlled within a very narrow range. A set point is the physiological value around which the normal range fluctuates. For example, the set point for typical human body temperature is approximately 37°C (98.6°F). Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a range of a few degrees above and below that point. Receptors located in the body's key places detect changes from this set point and relay information to the control centers located in the brain. The control centers monitor and send information to effector organs to control the body's response. If these effectors reverse the original condition, the system is said to be regulated through negative feedback.

Control centers in the brain and other parts of the body monitor and react to deviations from this set point using negative feedback. Negative feedback is a mechanism that reverses a deviation from the set point, and in turn, 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: a sensor, control center and an effector. ( Figure 1.3.2 a). A sensor, also referred to a receptor, monitors a physiological value, which is then reported to the control center. The control center 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 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 1.3.2 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, 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.

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. 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 1.3.3).

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.

Chapter Review

Homeostasis is the activity of cells throughout the body to maintain the physiological state within a narrow range that is compatible with life. Homeostasis is regulated by negative feedback loops and, much less frequently, by positive feedback loops. Both have the same components of a stimulus, sensor, control center, and effector; however, negative feedback loops work to prevent an excessive response to the stimulus, whereas positive feedback loops intensify the response until an end point is reached.

Interactive Link Questions

Critical Thinking Questions

Identify the four components of a negative feedback loop and explain what would happen if secretion of a body chemical controlled by a negative feedback system became too great.

The four components of a negative feedback loop are: stimulus, sensor, control center, and effector. If too great a quantity of the chemical were excreted, sensors would activate a control center, which would in turn activate an effector. In this case, the effector (the secreting cells) would be adjusted downward.

What regulatory processes would your body use if you were trapped by a blizzard in an unheated, uninsulated cabin in the woods?

Any prolonged exposure to extreme cold would activate the brain's heat-gain center. This would reduce blood flow to your skin, and shunt blood returning from your limbs away from the digits and into a network of deep veins. Your brain's heat-gain center would also increase your muscle contraction, causing you to shiver. This increases the energy consumption of skeletal muscle and generates more heat. Your body would also produce thyroid hormone and epinephrine, chemicals that promote increased metabolism and heat production.

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By the end of this section, you will be able to: List the components of a homeostatically controlled system Discuss the role of homeostasis in the human body Contrast negative and positive feedback, giving one physiologic example of each mechanism Maintaining a stable system requires the body to continuously monitor its internal conditions.

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Wyatt, L et al(2017). Extended Diploma in Health and Social Care, Hodder education, London

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In homeostasis, a negative feedback mechanism is an important sort of dominance. When conditions deviate from the perfect or set point, it reacts and returns the situation to the perfect or set point. In negative feedback mechanisms, there is a never-ending cycle of occurrences. (BBC Bitesize) An example of a negative feedback mechanism is the control of blood sugar. The small intestine collects glucose from broken down food after a meal. The level of the glucose in the blood rises. Blood glucose levels which are increased promote the production of insulin by the beta cells in the pancreas. (Lumenlearning)

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Homeostasis of blood glucose (a negative feedback loop)

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Lumenlearning( no date). Homeostasis and Feedback Loops, Accessed December 2021, https://courses.lumenlearning.com/cuny-csi-ap-1/chapter/homeostasis-and-feedback-loops/

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Homeostasis and Feedback Loops | Anatomy and Physiology I

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Homeostasis relates to dynamic physiological processes that help us maintain an internal environment suitable for normal function. Homeostasis is not the same as chemical or physical equilibrium. Such equilibrium occurs when no net change is occurring: add milk to the coffee and eventually, when equ…

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Homeostasis and Feedback Loops

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The brain's hypothalamus functions similarly to a thermostat in a home heating method. As heated blood travels through the hypothalamus, an increase in body temperature is noticed, which triggers a feedback process involving the autonomic nervous system to operate vasodilation and encourage sweat output.

When the hypothalamus detects a dip in body temperature, it activates the autonomic nervous system, which causes vasoconstriction, hair arousal, shivering, and sweating suppression. As a result, body temperature remains constant while blood temperature recovers to normal. (Lenhardt, Kurz, Sessler 1996)

Hyperthermia is the outcome of a sustained increase in body temperature. This is an example of positive feedback that could be dangerous. Excess heat generated by metabolism will cause metabolism to grow even more, as temperature rises the rate of chemical reactions. (Lenhardt, Kurz, Sessler 1996)

Hypothermia is a condition due to prolonged exposure to extremely cold temperatures. The metabolic rate reduces as the rate of chemical reactions slows as the temperature drops. There comes a time when there isn't enough energy produced quickly enough to keep life going. (Wyatt 2017)

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To ensure that hypertonic and hypotonic does not occur the nervous, endocrine and renal systems work together.

We sweat to cool off when we exercise on a hot day. As a result of the water loss, our plasma becomes hypertonic to our cells. Special cells in the hypothalamus called osmoreceptors sense this increase in pressure. Nerve cells connect these structures to the pituitary gland in the brain, where nerve impulses cause the production of a hormone called ADH. ADH enters the bloodstream and travels throughout the body to the plasma.

Cells in the walls of the kidney nephrons' collecting ducts recognise ADH. It allows water to permeate the walls of these tubes. The henle loops of the kidney's nephrons are involved in a complex system that causes high salt concentrations in the tissues of the kidney medulla where they are found. While the salt finally returns to the blood, it remains in the medulla, causing a hypertonic state. (Wyatt 2017)

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Blood pressure is usually measured in various ways: the systolic pressure, which is the larger of the two and corresponds to the pressure applied by the heart when it contracts, and the diastolic pressure, which is the underlying pressure when the heart relaxes.

Certain blood vessel walls have pressure-sensitive cells called baroreceptors that directly monitor blood pressure. High blood pressure is detected by those located in the aorta and carotid arteries that connect to the brain. These provide messages to the medulla of the brain via the autonomic nervous system, causing vasodilation as well as a reduction in heart rate and contraction force. Vena cavae baroreceptors, on the other hand, detect poor blood pressure and respond by raising the secretion of ADH, aldosterone, and renin, all of which stimulate salt uptake in the kidney, leading to an increase in blood volume and therefore pressure. (Wyatt 2017)

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The pancreas produces insulin and glucagon in the islets of langerhans. Insulin’s function is to take out glucose from the blood and glucagon function is to put glucose into the blood. Insulin is produced by beta cells and glucagon is produced by alpha cells. Glucagon is released from the pancreas when blood sugar levels are too low. (BBC Bitesize)

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BBC Bitesize(no date). Control of blood glucose concentration by pancreas and insulin, Accessed December 2021,https://www.bbc.co.uk/bitesize/guides/z3gxb82/revision/5

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Control of blood glucose concentration by pancreas and insulin - Coordination and control - The human endocrine system - Edexcel - GCSE Biology (Single Science) Revision - Edexcel - BBC Bitesize

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Revise Coordination and control - The human endocrine system for Edexcel with BBC Bitesize.

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Lenhardt, R., Kurz, A., & Sessler, D. I. (1996). Thermoregulation and hyperthermia, Accessed December 2021

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Excellent definition of homeostasis with reference to positive and negative feedback mechanisms. Well done, Elisha.

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Excellent explanations of how homeostasis maintains healthy body function - well done, Elisha!

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You have clearly shown evidence of wider reading and used sources to support your points. Don't forget when you are citing in your writing, to include the year. E.G., (Barley, 2022).