Homeostasis is the tendency of an organism or a cell to regulate its internal conditions, usually by a system of feedback controls, so as to stabilize health and functioning, regardless of the outside changing conditions. It aims to keep up a particular state of equilibrium, stability, or balance. In the biological context, homeostasis would entail several various physiological mechanisms in order to sustain and stabilize the functional, normal status of an organism. Thus, a homeostasis definition in biological precept is the ability or tendency of the body or a cell to seek and maintain a condition of equilibrium – a stable internal environment -- as it deals with external changes. It makes use of feedback controls and other regulatory mechanisms in order to maintain a constant internal environment. In the human body, homeostasis would include key players such as brain, kidney, liver, and skin.
The term homeostasis comes from the Ancient Greek ὅμοιος (hómoios, meaning "similar"), from στημι (hístēmi, “standing still”) and stasis, from στάσις (stásis, meaning "standing"). The concept of homeostasis was first described in 1865 by Claude Bernard, a French physiologist. However, the term was coined later in 1962 by the American physiologist Walter Bradford Cannon. Variant: homoeostasis.
An organism needs a system that effectively interconnects various biological processes and functions. In humans, for instance, the bodily organs are made up of cells functioning in unison. These organs, although distinct from one another, have to work alongside each other in order to sustain a set of internal conditions within the ideal range. There are various homeostatic processes in the human body and each of them works by regulating particular variables of the internal environment.
Homeostasis in the human body
The human body would not be able to function efficiently if there is a prolonged imbalance in the internal physical and chemical conditions. Just like any other living things, the human body employs various homeostatic mechanisms to sustain its optimal functioning. Variables such as body temperature, pH, sodium level, potassium level, calcium level, and blood sugar level have to be kept within the homeostatic range, i.e. the allowable upper and lower limits for a particular variable. Otherwise, the body would fail to carry out its tasks and hence become dysfunctional. In order for the body to keep these variables within efficacious limits, there are various regulatory mechanisms involved and each of them is comprised of three general components.
Components of homeostasis
The components of homeostasis are: (1) a receptor, (2) a control center, and (3) an effector. The receptor, as the name implies, is the part of a homeostatic system that receives information regarding the status of the body. It monitors and perceives the changes in its environment, both the internal and the external. It is in the form of a sensory nerve terminal that receives the information (i.e. stimulus) and then responds by producing a nerve impulse according to the type, presence/absence, or extent of stimulation. Examples of receptors in the human body are as follows:
- Photoreceptors, i.e. receptors that react to light stimuli
- Olfactory receptor cells, i.e. receptors in the olfactory epithelium at the roof of the nose that react to odors or smell
- gustation receptors, i.e. receptors for taste
- Auditory receptor cells, i.e. receptors in the epithelium of the organ of Corti that react to sound stimuli
- Thermoreceptors, i.e. receptors in a sensory cell sensitive to changes in temperature
- Mechanoreceptors, i.e. receptors in the skin that reacts to various mechanical stimuli
- Interoceptors, i.e. receptors that respond to stimuli inside the body
- Nociceptors, i.e. receptors responsible for detecting or responding to pain
- Peripheral chemoreceptors, i.e. receptors that respond to chemical changes in the blood, e.g. oxygen concentration
The control centers pertain to the homeostatic component that process impulses relayed by the receptors. Examples are the respiratory center and the renin-angiotensin system. The effectors are the target of the homeostatic response that would bring about the reversion of conditions to the optimal or normal range. At the tissue or organ level, they are exemplified by the muscle or the gland. At the cellular level, they are the receptors of a nerve, including the nuclear receptors.
These three components work by first detecting and then responding to the information (i.e. stimulus) by the receptors of sensory cells. They respond to the detected change in the environment by relaying the information to the control center for processing, or directly to a particular target effector. Processing in the control center entails deliberation and determination of the appropriate response to the relayed stimuli. Then, it sends this message to the effectors. The effectors upon receiving the message would bring about the supposed response that would revert to the normal homeostatic range. At the cellular level, the activated nuclear receptors will act upon by upregulating (or by downregulating) the expression of certain gene(s). The protein produced from the gene expression would then exert its effect on the target organ.
Thermoregulation pertains to the homeostatic regulation of body temperature. Humans maintain a constant internal temperature of 98.6˚F (or 37˚C) (also referred to as the set point). The core temperature is regulated chiefly by the nervous system, particularly the anterior hypothalamus and the preoptic area of the brain.
When the ambient temperature is less than the skin temperature, heat loss occurs. This means that in colder surroundings (e.g. during the cold winter season) the body loses heat mainly from the hands and the feet. As a result, the core temperature drops. This is picked up by the thermoregulatory center of the brain and initiates control mechanisms to return the core temperature to the set point. One of the homeostatic mechanisms is by shivering to generate heat. The thermoregulatory center in the brain sends signals to the muscles to shiver. Since the body remains still while shivering, less heat will be dissipated to the environment.
In the other way around, when the ambient temperature is higher than the skin temperature, the body gains heat and consequently, the core temperature rises. This occurs during the hot summer days. The thermoregulatory control center in the brain responds, for example by stimulating the eccrine sweat glands to secrete sweat to cool the body off (by evaporative cooling).
Thermoregulation is an important homeostatic mechanism not just in humans but also in mammals. Mammals maintain a constant body temperature that makes them characteristically warm-blooded. The body maintains an optimal core temperature through internal regulation by a bodily system comprised of thermoreceptors in the hypothalamus, the brain, the spinal cord, the internal organs, and the great veins. Another way is allostasis, which is a behavioural form of homeostatic regulation. For instance, during hot weather, they tend to seek for shady, cooler places and/or they do not move around a lot. During the cold season, they look for warm spots and they tend to increase their activity. Some species, such as birds, huddle or nestle together for warmth.
Human blood is comprised of cellular elements and plasma. While the cellular elements include the blood cells and the platelets, the plasma consists chiefly of water, about 95% by volume and the remaining percentage includes dissolved proteins (e.g. serum albumins, globulins, fibrinogen), glucose, clotting factors, electrolytes, hormones, carbon dioxide, and oxygen. The levels of these components in the blood plasma go through homeostatic regulation. For example, blood sugar level is regulated to set the blood glucose concentration within the tolerable limit. The body maintains homeostasis in this regard largely through the pancreas. The pancreas is a glandular structure made up of two major types of cells: the alpha and beta cells. The alpha cells produce and secrete glucagon whereas the beta cells, insulin. Glucagon and insulin are hormones from the pancreas that regulate glucose concentration in the blood. Insulin, in particular, lowers blood sugar level by inciting the skeletal muscles, and the fat tissues to take up glucose from the bloodstream. It also incites the liver cells to take glucose in and store it into glycogen. Conversely, glucagon raises blood sugar level by stimulating the liver to convert its stored glycogen into glucose by glycogenolysis or produce glucose by gluconeogenesis and release it into the bloodstream. Thus, when the glucose level is high in the blood circulation (e.g. when consuming a carbohydrate-rich food), the beta cells of the pancreas secrete insulin and inhibits the alpha cells from secreting glucagon. But when glucose level drops (e.g. during an energy-demanding workout), the alpha cells secrete glucagon and insulin secretion is stopped.
Blood pressure homeostasis
Another instance of homeostasis is the homeostatic regulation of blood pressure. The blood pressure is the force exerted by the circulating blood as it hits the arterial walls. The pressure comes from the heart when it creates a pulsing act. This blood pressure is regulated within the homeostatic range through the cardiovascular center. This control center has three distinct activities related to blood pressure regulation: (1) The cardiac center sending nerve impulses to the sympathetic cardiac nerves to increase cardiac output (by increasing heart rate). (2) The cardiac center sending nerve impulses to the parasympathetic vagus nerves to decrease cardiac output (by decreasing heart rate). (3) The vasomotor center that regulates the diameter of blood vessels.
The cardiovascular center receives blood pressure changes information from receptors, e.g. baroreceptors. The baroreceptors are the receptors that are mostly found in the carotid sinus. They are sensitive to blood pressure changes. For example, when the arterial wall stretches from an increased blood volume, the baroreceptors detect the consequential rise in blood pressure. They send signals to the atrial heart muscle cells to secrete atrial natriuretic peptide (ANP) into the bloodstream. ANP is a potent vasodilator whose actions include lowering blood pressure. In this regard, its target organ is the kidney that apart from the major function of excreting wastes out of the body as urine it also plays an important role in managing blood volume through the renin-angiotensin-aldosterone system. In particular, ANP stimulates the kidney to stop secreting renin.
Renin is an enzyme that converts angiotensinogen from the liver to angiotensin I. The angiotensin I is converted next by the angiotensin-converting enzyme in the lungs into a potent vasoconstrictor peptide, angiotensin II. The latter causes the target blood vessel to constrict thereby raising peripheral resistance. An increase in peripheral resistance leads to the rise in blood pressure. Angiotensin II also acts on the adrenal glands by stimulating them to secrete aldosterone. Aldosterone reduces urine output. It does so by entering the principal cells of the distal tubule and the collecting duct of the kidney nephron to bind to the nuclear mineralocorticoid receptor. This activates the cell to release sodium (Na+) ions via the basolateral Na+/K+ pumps. Three Na+ ions are released out of the cell into the interstitial fluid. Concurrently, 2 K+ ions are taken into the cell from the interstitial fluid. As a result, a concentration gradient causes Na+ ions and water to enter the bloodstream (as for K+ ions, they are secreted from the lumen of the collecting duct into the urine). The re-absorption of both Na+ ions and water into the blood raises blood volume.
By inhibiting the kidney to secrete renin, its effects and the ensuing events would be inhibited as well. As a result, the blood volume decreases and the blood pressure drops.
The body is comprised largely of water and the total amount of water needs to be regulated and stabilized. The body does so by osmoregulation. The homeostatic mechanism is initiated by the osmoreceptors in the hypothalamus. These receptors are sensitive to osmotic pressure changes. When these receptors detect hypertonicity (more solute) or hyper-osmolality in the extracellular environment, vasopressin is released into the circulation. In the case of osmoregulation, vasopressin targets the kidney to exert an antidiuretic response, particularly by promoting water reabsorption, thereby inhibiting further water loss. Apart from the vasopressin release, the hypothalamus also stimulates the thirst center of the brain to increase the urge to drink water. In the case of hypo-osmolality in the external environment, there is a low plasma vasopressin level. In consequence, water is not reabsorbed from the kidney tubules and therefore excreted into the urine.
Biological importance of homeostasis
Homeostasis is important to maintain and sustain life. Without these homeostatic mechanisms to ensure that the innate variables are kept within the optimal or suitable values, there would be instability in the body. The system would not be able to function properly and efficaciously. In the long run, the individual would get ill, or worse, face death from the failure of the body to rectify rogue variables that impede the system to function as it should.
- Tansey, Etain A.; Johnson, Christopher D (2015). "Recent advances in thermoregulation". Advances in Physiology Education. 39 (3): 139–148.
- Campbell, Neil A. (1990). Biology (Second ed.). Redwood City, California: The Benjamin/Cummings Publishing Company.
- Control of Blood Pressure. (2015). Retrieved from Cliffsnotes.com website: https://www.cliffsnotes.com/study-guides/anatomy-and-physiology/the-cardiovascular-system/control-of-blood-pressure
- Modell, H., Cliff, W., Michael, J., McFarland, J., Wenderoth, M. P., & Wright, A. (2015). A physiologist’s view of homeostasis. Advances in Physiology Education, 39(4), 259–266. https://doi.org/10.1152/advan.00107.2015
- Homeostasis, Steady States, and Equilibria. (2019). Retrieved from Rice.edu website: https://www.ruf.rice.edu/~bioslabs/studies/invertebrates/steadystate.html
- Homeostasis. (2019). Retrieved from Bellarmine.edu website: https://www.bellarmine.edu/faculty/dobbins/microbial/MBernard.htm
- 1.3 Homeostasis | Anatomy & Physiology. (2019). Retrieved from Oregonstate.edu website: http://library.open.oregonstate.edu/aandp/chapter/1-5-homeostasis/
- HOMEOSTASIS: POSITIVE AND NEGATIVE FEEDBACK MECHANISM. (n.d.). Retrieved from https://www.michigan.gov/documents/explorelabscience/Presentation_on_Homeostasis_560162_7.pdf
- Lecture 21. (2019). Retrieved from Columbia.edu website: http://www.columbia.edu/cu/biology/courses/c2006/lectures08/lect21.08.html
- ANIMAL ORGAN SYSTEMS. (2019). Retrieved from Estrellamountain.edu website: https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookANIMORGSYS.html
© Biology Online. Content provided and moderated by Biology Online Editors