Homeostasis is the state of steady internal conditions maintained by living things. This dynamic state of equilibrium is the condition of optimal functioning for the. The tendency to maintain a stable, relatively constant internal environment is called homeostasis. The body maintains homeostasis for many factors in addition . Homeostasis, any self-regulating process by which biological systems tend to maintain stability while adjusting to conditions that are optimal for survival.
When the room cools, the circuit is completed, the furnace switches on, and the temperature rises. 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 that homeostasis in ecosystems results from biodiversity the variety of life in a given place and the ecological interactions predation , competition , decomposition, etc.
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.
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, of greater complexity, however, have regulators only very roughly comparable to such mechanical devices. The two types of systems are alike, however, in their goals—to sustain activity within prescribed ranges, 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. Feedback about body temperature is carried through the bloodstream to the brain and results in compensatory adjustments in the breathing rate, the level of blood sugar , and the metabolic rate. 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. 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 , homeostasis in ecosystems is a product of the combination of biodiversity and large numbers of ecological interactions that occur between species.
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.
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Learn More in these related Britannica articles: As noted earlier, the term homeostasis refers to the maintenance of the internal environment of the body within narrow and rigidly controlled limits. The major functions important in the maintenance of homeostasis are fluid and electrolyte balance, acid-base regulation, thermoregulation, and metabolic control.
To maintain homeostasis, heat production and heat loss must be balanced. This is achieved by both the somatomotor and sympathetic systems. The obvious behavioral way of keeping warm or cool is by moving into a correct environment. The posture of the body is also used to balance…. Homeostasis major treatment In human disease: 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 glycogen stored in muscles remains in the muscles, and is only broken down, during exercise, to glucosephosphate 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 returned to the blood. The liver can take up only the lactate, and by the process of energy consuming gluconeogenesis convert it back to glucose. Changes in the levels of oxygen, carbon dioxide, and plasma pH are sent to the respiratory center , in the brainstem where they are regulated.
The partial pressure of oxygen and carbon dioxide in the arterial blood is monitored by the peripheral chemoreceptors PNS in the carotid artery and aortic arch. A change in the partial pressure of carbon dioxide is detected as altered pH in the cerebrospinal fluid by central chemoreceptors CNS in the medulla oblongata of the brainstem.
Information from these sets of sensors is sent to the respiratory center which activates the effector organs — the diaphragm and other muscles of respiration. An increased level of carbon dioxide in the blood, or a decreased level of oxygen, will result in a deeper breathing pattern and increased respiratory rate to bring the blood gases back to equilibrium. Too little carbon dioxide, and, to a lesser extent, too much oxygen in the blood can temporarily halt breathing, a condition known as apnea , which freedivers use to prolong the time they can stay underwater.
The partial pressure of carbon dioxide is more of a deciding factor in the monitoring of pH. With the lower level of carbon dioxide, to keep the pH at 7. The kidneys measure the oxygen content rather than the partial pressure of oxygen in the arterial blood.
When the oxygen content of the blood is chronically low, oxygen-sensitive cells secrete erythropoietin EPO into the blood. The increase in RBCs leads to an increased hematocrit in the blood, and subsequent increase in hemoglobin that increases the oxygen carrying capacity. This is the mechanism whereby high altitude dwellers have higher hematocrits than sea-level residents, and also why persons with pulmonary insufficiency or right-to-left shunts in the heart through which venous blood by-passes the lungs and goes directly into the systemic circulation have similarly high hematocrits.
Regardless of the partial pressure of oxygen in the blood, the amount of oxygen that can be carried, depends on the hemoglobin content. The partial pressure of oxygen may be sufficient for example in anemia , but the hemoglobin content will be insufficient and subsequently as will be the oxygen content. Given enough supply of iron, vitamin B12 and folic acid , EPO can stimulate RBC production, and hemoglobin and oxygen content restored to normal.
The brain can regulate blood flow over a range of blood pressure values by vasoconstriction and vasodilation of the arteries. High pressure receptors called baroreceptors in the walls of the aortic arch and carotid sinus at the beginning of the internal carotid artery monitor the arterial blood pressure. This causes heart muscle cells to secrete the hormone atrial natriuretic peptide ANP into the blood. This acts on the kidneys to inhibit the secretion of renin and aldosterone causing the release of sodium, and accompanying water into the urine, thereby reducing the blood volume.
The arterioles are the main resistance vessels in the arterial tree , and small changes in diameter cause large changes in the resistance to flow through them. When the arterial blood pressure rises the arterioles are stimulated to dilate making it easier for blood to leave the arteries, thus deflating them, and bringing the blood pressure down, back to normal.
At the same time the heart is stimulated via cholinergic parasympathetic nerves to beat more slowly called bradycardia , ensuring that the inflow of blood into the arteries is reduced, thus adding to the reduction in pressure, and correction of the original error.
Low pressure in the arteries, causes the opposite reflex of constriction of the arterioles, and a speeding up of the heart rate called tachycardia. If the drop in blood pressure is very rapid or excessive, the medulla oblongata stimulates the adrenal medulla , via "preganglionic" sympathetic nerves , to secrete epinephrine adrenaline into the blood. This hormone enhances the tachycardia and causes severe vasoconstriction of the arterioles to all but the essential organ in the body especially the heart, lungs, and brain.
These reactions usually correct the low arterial blood pressure hypotension very effectively. The sensors for the second are the parafollicular cells in the thyroid gland. The parathyroid chief cells secrete parathyroid hormone PTH in response to a fall in the plasma ionized calcium level; the parafollicular cells of the thyroid gland secrete calcitonin in response to a rise in the plasma ionized calcium level.
The effector organs of the first homeostatic mechanism are the bones , the kidney , and, via a hormone released into the blood by the kidney in response to high PTH levels in the blood, the duodenum and jejunum.
Parathyroid hormone in high concentrations in the blood causes bone resorption , releasing calcium into the plasma. This is a very rapid action which can correct a threatening hypocalcemia within minutes. High PTH concentrations cause the excretion of phosphate ions via the urine. Since phosphates combine with calcium ions to form insoluble salts, a decrease in the level of phosphates in the blood, releases free calcium ions into the plasma ionized calcium pool.
PTH has a second action on the kidneys. It stimulates the manufacture and release, by the kidneys, of calcitriol into the blood. This steroid hormone acts on the epithelial cells of the upper small intestine, increasing their capacity to absorb calcium from the gut contents into the blood. The second homeostatic mechanism, with its sensors in the thyroid gland, releases calcitonin into the blood when the blood ionized calcium rises.
This hormone acts primarily on bone, causing the rapid removal of calcium from the blood and depositing it, in insoluble form, in the bones. The two homeostatic mechanisms working through PTH on the one hand, and calcitonin on the other can very rapidly correct any impending error in the plasma ionized calcium level by either removing calcium from the blood and depositing it in the skeleton, or by removing calcium from it.
Longer term regulation occurs through calcium absorption or loss from the gut. The homeostatic mechanism which controls the plasma sodium concentration is rather more complex than most of the other homeostatic mechanisms described on this page.
The sensor is situated in the juxtaglomerular apparatus of kidneys, which senses the plasma sodium concentration in a surprisingly indirect manner. Instead of measuring it directly in the blood flowing past the juxtaglomerular cells , these cells respond to the sodium concentration in the renal tubular fluid after it has already undergone a certain amount of modification in the proximal convoluted tubule and loop of Henle.
In response to a lowering of the plasma sodium concentration, or to a fall in the arterial blood pressure, the juxtaglomerular cells release renin into the blood. This decapeptide is known as angiotensin I.
However, when the blood circulates through the lungs a pulmonary capillary endothelial enzyme called angiotensin-converting enzyme ACE cleaves a further two amino acids from angiotensin I to form an octapeptide known as angiotensin II. Angiotensin II is a hormone which acts on the adrenal cortex , causing the release into the blood of the steroid hormone , aldosterone. Angiotensin II also acts on the smooth muscle in the walls of the arterioles causing these small diameter vessels to constrict, thereby restricting the outflow of blood from the arterial tree, causing the arterial blood pressure to rise.
This, therefore, reinforces the measures described above under the heading of "Arterial blood pressure" , which defend the arterial blood pressure against changes, especially hypotension. The angiotensin II-stimulated aldosterone released from the zona glomerulosa of the adrenal glands has an effect on particularly the epithelial cells of the distal convoluted tubules and collecting ducts of the kidneys. Here it causes the reabsorption of sodium ions from the renal tubular fluid , in exchange for potassium ions which are secreted from the blood plasma into the tubular fluid to exit the body via the urine.
The hyponatremia can only be corrected by the consumption of salt in the diet. However, it is not certain whether a "salt hunger" can be initiated by hyponatremia, or by what mechanism this might come about.
When the plasma sodium ion concentration is higher than normal hypernatremia , the release of renin from the juxtaglomerular apparatus is halted, ceasing the production of angiotensin II, and its consequent aldosterone-release into the blood. The kidneys respond by excreting sodium ions into the urine, thereby normalizing the plasma sodium ion concentration. The low angiotensin II levels in the blood lower the arterial blood pressure as an inevitable concomitant response.
The reabsorption of sodium ions from the tubular fluid as a result of high aldosterone levels in the blood does not, of itself, cause renal tubular water to be returned to the blood from the distal convoluted tubules or collecting ducts. This is because sodium is reabsorbed in exchange for potassium and therefore causes only a modest change in the osmotic gradient between the blood and the tubular fluid. Furthermore, the epithelium of the distal convoluted tubules and collecting ducts is impermeable to water in the absence of antidiuretic hormone ADH in the blood.
ADH is part of the control of fluid balance. Its levels in the blood vary with the osmolality of the plasma, which is measured in the hypothalamus of the brain. Aldosterone's action on the kidney tubules prevents sodium loss to the extracellular fluid ECF. However, low aldosterone levels cause a loss of sodium ions from the ECF, which could potentially cause a change in extracellular osmolality and therefore of ADH levels in the blood.
High potassium concentrations in the plasma cause depolarization of the zona glomerulosa cells' membranes in the outer layer of the adrenal cortex. Aldosterone acts primarily on the distal convoluted tubules and collecting ducts of the kidneys, stimulating the excretion of potassium ions into the urine. The total amount of water in the body needs to be kept in balance. Fluid balance involves keeping the fluid volume stabilized, and also keeping the levels of electrolytes in the extracellular fluid stable.
Fluid balance is maintained by the process of osmoregulation and by behavior. Osmotic pressure is detected by osmoreceptors in the median preoptic nucleus in the hypothalamus. Measurement of the plasma osmolality to give an indication of the water content of the body, relies on the fact that water losses from the body, through unavoidable water loss through the skin which is not entirely waterproof and therefore always slightly moist, water vapor in the exhaled air , sweating , vomiting , normal feces and especially diarrhea are all hypotonic , meaning that they are less salty than the body fluids compare, for instance, the taste of saliva with that of tears.
The latter has almost the same salt content as the extracellular fluid, whereas the former is hypotonic with respect to the plasma. Saliva does not taste salty, whereas tears are decidedly salty. Nearly all normal and abnormal losses of body water therefore cause the extracellular fluid to become hypertonic. Conversely, excessive fluid intake dilutes the extracellular fluid causing the hypothalamus to register hypotonic hyponatremia conditions.
When the hypothalamus detects a hypertonic extracellular environment, it causes the secretion of an antidiuretic hormone ADH called vasopressin which acts on the effector organ, which in this case is the kidney. The effect of vasopressin on the kidney tubules is to reabsorb water from the distal convoluted tubules and collecting ducts , thus preventing aggravation of the water loss via the urine. The hypothalamus simultaneously stimulates the nearby thirst center causing an almost irresistible if the hypertonicity is severe enough urge to drink water.
The cessation of urine flow prevents the hypovolemia and hypertonicity from getting worse; the drinking of water corrects the defect. Hypo-osmolality results in very low plasma ADH levels.
This results in the inhibition of water reabsorption from the kidney tubules, causing high volumes of very dilute urine to be excreted, thus getting rid of the excess water in the body. Urinary water loss, when the body water homeostat is intact, is a compensatory water loss, correcting any water excess in the body. However, since the kidneys cannot generate water, the thirst reflex is the all-important second effector mechanism of the body water homeostat, correcting any water deficit in the body.
The plasma pH can be altered by respiratory changes in the partial pressure of carbon dioxide; or altered by metabolic changes in the carbonic acid to bicarbonate ion ratio. The bicarbonate buffer system regulates the ratio of carbonic acid to bicarbonate to be equal to 1: A change in the plasma pH gives an acid—base imbalance. In acid—base homeostasis there are two mechanisms that can help regulate the pH. Respiratory compensation a mechanism of the respiratory center , adjusts the partial pressure of carbon dioxide by changing the rate and depth of breathing, to bring the pH back to normal.
The partial pressure of carbon dioxide also determines the concentration of carbonic acid, and the bicarbonate buffer system can also come into play. Renal compensation can help the bicarbonate buffer system.
The sensor for the plasma bicarbonate concentration is not known for certain. It is very probable that the renal tubular cells of the distal convoluted tubules are themselves sensitive to the pH of the plasma. Bicarbonate ions are simultaneously secreted into the blood that decreases the carbonic acid, and consequently raises the plasma pH. When hydrogen ions are excreted into the urine, and bicarbonate into the blood, the latter combines with the excess hydrogen ions in the plasma that stimulated the kidneys to perform this operation.
The resulting reaction in the plasma is the formation of carbonic acid which is in equilibrium with the plasma partial pressure of carbon dioxide. This is tightly regulated to ensure that there is no excessive build-up of carbonic acid or bicarbonate. The overall effect is therefore that hydrogen ions are lost in the urine when the pH of the plasma falls. The concomitant rise in the plasma bicarbonate mops up the increased hydrogen ions caused by the fall in plasma pH and the resulting excess carbonic acid is disposed of in the lungs as carbon dioxide.
This restores the normal ratio between bicarbonate and the partial pressure of carbon dioxide and therefore the plasma pH. The converse happens when a high plasma pH stimulates the kidneys to secrete hydrogen ions into the blood and to excrete bicarbonate into the urine. The hydrogen ions combine with the excess bicarbonate ions in the plasma, once again forming an excess of carbonic acid which can be exhaled, as carbon dioxide, in the lungs, keeping the plasma bicarbonate ion concentration, the partial pressure of carbon dioxide and, therefore, the plasma pH, constant.
Cerebrospinal fluid CSF allows for regulation of the distribution of substances between cells of the brain,  and neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system. For example, high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and syncope. Inhibitory neurons in the central nervous system play a homeostatic role in the balance of neuronal activity between excitation and inhibition.
Inhibitory neurons using GABA , make compensating changes in the neuronal networks preventing runaway levels of excitation. The neuroendocrine system is the mechanism by which the hypothalamus maintains homeostasis, regulating metabolism , reproduction, eating and drinking behaviour, energy utilization, osmolarity and blood pressure.
The regulation of metabolism, is carried out by hypothalamic interconnections to other glands. Two other regulatory endocrine axes are the hypothalamic—pituitary—adrenal axis HPA axis and the hypothalamic—pituitary—thyroid axis HPT axis. The liver also has many regulatory functions of the metabolism. An important function is the production and control of bile acids. Too much bile acid can be toxic to cells and its synthesis can be inhibited by activation of FXR a nuclear receptor.
At the cellular level, homeostasis is carried out by several mechanisms including transcriptional regulation that can alter the activity of genes in response to changes. The amount of energy taken in through nutrition needs to match the amount of energy used. To achieve energy homeostasis appetite is regulated by two hormones, grehlin and leptin. Grehlin stimulates hunger and the intake of food and leptin acts to signal satiety fullness.
Many diseases are the result of a homeostatic failure. Almost any homeostatic component can malfunction either as a result of an inherited defect , an inborn error of metabolism , or an acquired disease. Some homeostatic mechanisms have inbuilt redundancies, which ensures that life is not immediately threatened if a component malfunctions; but sometimes a homeostatic malfunction can result in serious disease, which can be fatal if not treated. A well-known example of a homeostatic failure is shown in type 1 diabetes mellitus.
Here blood sugar regulation is unable to function because the beta cells of the pancreatic islets are destroyed and cannot produce the necessary insulin. The blood sugar rises in a condition known as hyperglycemia. The abnormally high plasma ionized calcium concentrations cause conformational changes in many cell-surface proteins especially ion channels and hormone or neurotransmitter receptors  giving rise to lethargy, muscle weakness, anorexia, constipation and labile emotions.
The body water homeostat can be compromised by the inability to secrete ADH in response to even the normal daily water losses via the exhaled air, the feces , and insensible sweating. On receiving a zero blood ADH signal, the kidneys produce huge unchanging volumes of very dilute urine, causing dehydration and death if not treated. As organisms age, the efficiency of their control systems becomes reduced.
The inefficiencies gradually result in an unstable internal environment that increases the risk of illness, and leads to the physical changes associated with aging.
Learn about homeostasis, the regulation of conditions in the body such as temperature, water content and carbon dioxide levels. Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain. Homeostasis: A property of cells, tissues, and organisms that allows the maintenance and regulation of the stability and constancy needed to function properly.