homeostasis homeostasis in biology, homeostasis (british also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, chemical, and social conditions maintained by living systems. this is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance, being kept within certain pre-set limits (homeostatic range). other variables include the ph of extracellular fluid, the concentrations of sodium, potassium, and calcium ions, as well as the blood sugar level, and these need to be regulated despite changes in the environment, diet, or level of activity. each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life. not to be confused with hemostasis. part of a series onbiologyscience of life index outline glossary history (timeline) key components cell theory ecosystem evolution phylogeny properties of life adaptation energy processing growth order regulation reproduction response to environment domains and kingdoms of life archaea bacteria eukarya (animals, fungi, plants, protists) branches anatomy biotechnology botany cell biology ecology evolutionary biology genetics marine biology microbiology molecular biology mycology neuroscience paleontology phycology physiology protistology virology zoology research biologist (list) list of biology awards list of journals list of research methods list of unsolved problems applications agricultural science biomedical sciences health technology pharming biology portal category homeostasis is brought about by a natural resistance to change when already in optimal conditions, and equilibrium is maintained by many regulatory mechanisms; it is thought to be the central motivation for all organic action. all homeostatic control mechanisms have at least three interdependent components for the variable being regulated: a receptor, a control center, and an effector. the receptor is the sensing component that monitors and responds to changes in the environment, either external or internal. receptors include thermoreceptors and mechanoreceptors. control centers include the respiratory center and the renin-angiotensin system. an effector is the target acted on, to bring about the change back to the normal state. at the cellular level, effectors include nuclear receptors that bring about changes in gene expression through up-regulation or down-regulation and act in negative feedback mechanisms. an example of this is in the control of bile acids in the liver. some centers, such as the renin–angiotensin system, control more than one variable. when the receptor senses a stimulus, it reacts by sending action potentials to a control center. the control center sets the maintenance range—the acceptable upper and lower limits—for the particular variable, such as temperature. the control center responds to the signal by determining an appropriate response and sending signals to an effector, which can be one or more muscles, an organ, or a gland. when the signal is received and acted on, negative feedback is provided to the receptor that stops the need for further signaling. the cannabinoid receptor type 1 (cb1), located at the presynaptic neuron, is a receptor that can stop stressful neurotransmitter release to the postsynaptic neuron; it is activated by endocannabinoids (ecs) such as anandamide (n-arachidonoylethanolamide; aea) and 2-arachidonoylglycerol (2-ag) via a retrograde signaling process in which these compounds are synthesized by and released from postsynaptic neurons, and travel back to the presynaptic terminal to bind to the cb1 receptor for modulation of neurotransmitter release to obtain homeostasis. the polyunsaturated fatty acids (pufas) are lipid derivatives of omega-3 (docosahexaenoic acid, dha, and eicosapentaenoic acid, epa) or of omega-6 (arachidonic acid, ara) are synthesized from membrane phospholipids and used as a precursor for endocannabinoids (ecs) mediate significant effects in the fine-tuning adjustment of body homeostasis. etymology the word homeostasis (/ˌhoʊmioʊˈsteɪsɪs/) uses combining forms of homeo- and -stasis, neo-latin from greek: ὅμοιος homoios, "similar" and στάσις stasis, "standing still", yielding the idea of "staying the same". history the concept of the regulation of the internal environment was described by french physiologist claude bernard in 1849, and the word homeostasis was coined by walter bradford cannon in 1926. in 1932, joseph barcroft a british physiologist, was the first to say that higher brain function required the most stable internal environment. thus, to barcroft homeostasis was not only organized by the brain—homeostasis served the brain. homeostasis is an almost exclusively biological term, referring to the concepts described by bernard and cannon, concerning the constancy of the internal environment in which the cells of the body live and survive. the term cybernetics is applied to technological control systems such as thermostats, which function as homeostatic mechanisms but are often defined much more broadly than the biological term of homeostasis. overview the metabolic processes of all organisms can only take place in very specific physical and chemical environments. the conditions vary with each organism, and with whether the chemical processes take place inside the cell or in the interstitial fluid bathing the cells. the best-known homeostatic mechanisms in humans and other mammals are regulators that keep the composition of the extracellular fluid (or the "internal environment") constant, especially with regard to the temperature, ph, osmolality, and the concentrations of sodium, potassium, glucose, carbon dioxide, and oxygen. however, a great many other homeostatic mechanisms, encompassing many aspects of human physiology, control other entities in the body. where the levels of variables are higher or lower than those needed, they are often prefixed with hyper- and hypo-, respectively such as hyperthermia and hypothermia or hypertension and hypotension. circadian variation in body temperature, ranging from about 37.5 °c from 10 a.m. to 6 p.m., and falling to about 36.4 °c from 2 a.m. to 6 a.m. if an entity is homeostatically controlled it does not imply that its value is necessarily absolutely steady in health. core body temperature is, for instance, regulated by a homeostatic mechanism with temperature sensors in, amongst others, the hypothalamus of the brain. however, the set point of the regulator is regularly reset. for instance, core body temperature in humans varies during the course of the day (i.e. has a circadian rhythm), with the lowest temperatures occurring at night, and the highest in the afternoons. other normal temperature variations include those related to the menstrual cycle. the temperature regulator's set point is reset during infections to produce a fever. organisms are capable of adjusting somewhat to varied conditions such as temperature changes or oxygen levels at altitude, by a process of acclimatisation. homeostasis does not govern every activity in the body. for instance, the signal (be it via neurons or hormones) from the sensor to the effector is, of necessity, highly variable in order to convey information about the direction and magnitude of the error detected by the sensor. similarly, the effector's response needs to be highly adjustable to reverse the error – in fact it should be very nearly in proportion (but in the opposite direction) to the error that is threatening the internal environment. for instance, arterial blood pressure in mammals is homeostatically controlled and measured by stretch receptors in the walls of the aortic arch and carotid sinuses at the beginnings of the internal carotid arteries. the sensors send messages via sensory nerves to the medulla oblongata of the brain indicating whether the blood pressure has fallen or risen, and by how much. the medulla oblongata then distributes messages along motor or efferent nerves belonging to the autonomic nervous system to a wide variety of effector organs, whose activity is consequently changed to reverse the error in the blood pressure. one of the effector organs is the heart whose rate is stimulated to rise (tachycardia) when the arterial blood pressure falls, or to slow down (bradycardia) when the pressure rises above the set point. thus the heart rate (for which there is no sensor in the body) is not homeostatically controlled but is one of the effector responses to errors in arterial blood pressure. another example is the rate of sweating. this is one of the effectors in the homeostatic control of body temperature, and therefore highly variable in rough proportion to the heat load that threatens to destabilize the body's core temperature, for which there is a sensor in the hypothalamus of the brain. controls of variables core temperature main articles: thermoregulation and thermoregulation in humans further information: preoptic area birds huddling for warmth mammals regulate their core temperature using input from thermoreceptors in the hypothalamus, brain, spinal cord, internal organs, and great veins. apart from the internal regulation of temperature, a process called allostasis can come into play that adjusts behaviour to adapt to the challenge of very hot or cold extremes (and to other challenges). these adjustments may include seeking shade and reducing activity, seeking warmer conditions and increasing activity, or huddling. behavioral thermoregulation takes precedence over physiological thermoregulation since necessary changes can be affected more quickly and physiological thermoregulation is limited in its capacity to respond to extreme temperatures. when the core temperature falls, the blood supply to the skin is reduced by intense vasoconstriction. the blood flow to the limbs (which have a large surface area) is similarly reduced and returned to the trunk via the deep veins which lie alongside the arteries (forming venae comitantes). this acts as a counter-current exchange system that short-circuits the warmth from the arterial blood directly into the venous blood returning into the trunk, causing minimal heat loss from the extremities in cold weather. the subcutaneous limb veins are tightly constricted, not only reducing heat loss from this source but also forcing the venous blood into the counter-current system in the depths of the limbs. the metabolic rate is increased, initially by non-shivering thermogenesis, followed by shivering thermogenesis if the earlier reactions are insufficient to correct the hypothermia. when core temperature rises are detected by thermoreceptors, the sweat glands in the skin are stimulated via cholinergic sympathetic nerves to secrete sweat onto the skin, which, when it evaporates, cools the skin and the blood flowing through it. panting is an alternative effector in many vertebrates, which cools the body also by the evaporation of water, but this time from the mucous membranes of the throat and mouth. blood glucose main articles: blood sugar regulation and glycolysis § regulation of the rate limiting enzymes negative feedback at work in the regulation of blood sugar. flat line is the set-point of glucose level and sine wave the fluctuations of glucose. blood sugar levels are regulated within fairly narrow limits. in mammals, the primary sensors for this are the beta cells of the pancreatic islets. the beta cells respond to a rise in the blood sugar level by secreting insulin into the blood and simultaneously inhibiting their neighboring alpha cells from secreting glucagon into the blood. this combination (high blood insulin levels and low glucagon levels) act on effector tissues, the chief of which is the liver, fat cells, and muscle cells. the liver is inhibited from producing glucose, taking it up instead, and converting it to glycogen and triglycerides. the glycogen is stored in the liver, but the triglycerides are secreted into the blood as very low-density lipoprotein (vldl) particles which are taken up by adipose tissue, there to be stored as fats. the fat cells take up glucose through special glucose transporters (glut4), whose numbers in the cell wall are increased as a direct effect of insulin acting on these cells. the glucose that enters the fat cells in this manner is converted into triglycerides (via the same metabolic pathways as are used by the liver) and then stored in those fat cells together with the vldl-derived triglycerides that were made in the liver. muscle cells also take glucose up through insulin-sensitive glut4 glucose channels, and convert it into muscle glycogen. a fall in blood glucose, causes insulin secretion to be stopped, and glucagon to be secreted from the alpha cells into the blood. this inhibits the uptake of glucose from the blood by the liver, fats cells, and muscle. instead the liver is strongly stimulated to manufacture glucose from glycogen (through glycogenolysis) and from non-carbohydrate sources (such as lactate and de-aminated amino acids) using a process known as gluconeogenesis. the glucose thus produced is discharged into the blood correcting the detected error (hypoglycemia). the glycogen stored in muscles remains in the muscles, and is only broken down, during exercise, to glucose-6-phosphate and thence to pyruvate to be fed into the citric acid cycle or turned into lactate. it is only the lactate and the waste products of the citric acid cycle that are