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Homeostasis

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Homeostasis

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Not to be confused with hemostasis.

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In biology, homeostasis is the state of steady internal, physical, and chemical conditions maintained by living systems.[1] 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 that of 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.

Homeostasis is brought about by a natural resistance to change when already in the optimal conditions,[2] and equilibrium is maintained by many regulatory mechanisms. All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: a receptor, a control centre, and an effector.[3] 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 centres include the respiratory centre, 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, receptors 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.[4]

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.[5]

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.[6]

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-tune adjustment of body homeostasis.[7]

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.[8][9] 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.[10] 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.[8][9][11] The term cybernetics is applied to technological control systems such as thermostats, which function as homeostatic mechanisms, but is often defined much more broadly than the biological term of homeostasis.[5][12][13][14]

Etymology

The word homeostasis (/ˌhoʊmioʊˈsteɪsɪs/[15][16]) uses combining forms of homeo- and -stasis, New Latin from Greek: ὅμοιος homoios, “similar” and στάσις stasis, “standing still”, yielding the idea of “staying the same”.

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.[17] However, the set point of the regulator is regularly reset.[18] 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.[19][20] The temperature regulator’s set point is reset during infections to produce a fever.[17][21][22] 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.[23][24] 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.[25][26][27] 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.[13][14] For instance, the arterial blood pressure in mammals is homeostatically controlled, and measured by stretch receptors in the walls of the aortic arch and carotid sinuses at beginnings of the internal carotid arteries.[17] 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 set point.[17] Thus the heart rate (for which there is no sensor in the body) is not homeostatically controlled, but is one of effector responses to errors in the 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,[17][28]spinal cord, internal organs, and great veins.[29][30] 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).[31] These adjustments may include seeking shade and reducing activity, or seeking warmer conditions and increasing activity, or huddling.[32]
Behavioural 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.[33]

When core temperature falls, the blood supply to the skin is reduced by intense vasoconstriction.[17] 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).[28][32][34] This acts as a counter-current exchange system which 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.[28][32][35] The subcutaneous limb veins are tightly constricted,[17] 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,[36] 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.[37] In mammals the primary sensors for this are the beta cells of the pancreatic islets.[38][39] 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.[38] This combination (high blood insulin levels and low glucagon levels) act on effector tissues, chief of which are 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.[40] 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 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.

Iron levels
Main article: Human iron metabolism
Copper regulation
Main article: Copper in health § Homeostasis
Levels of blood gases
Main articles: Respiratory center and Gas exchange
Further information: Blood gas tension
The respiratory center

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.[41] However, at high altitude (above 2500 m) the monitoring of the partial pressure of oxygen takes priority, and hyperventilation keeps the oxygen level constant. With the lower level of carbon dioxide, to keep the pH at 7.4 the kidneys secrete hydrogen ions into the blood, and excrete bicarbonate into the urine.[42][43] This is important in the acclimatization to high altitude.[44]

Blood oxygen content

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.[45] The effector tissue is the red bone marrow which produces red blood cells (RBCs)(erythrocytes). 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.[46][47]

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.[46][48]

Arterial blood pressure
Main articles: Baroreflex and Renin–angiotensin system

The brain can regulate blood flow over a range of blood pressure values by vasoconstriction and vasodilation of the arteries.[49]

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.[50] Rising pressure is detected when the walls of the arteries stretch due to an increase in blood volume. 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.[51]
This information is then conveyed, via afferent nerve fibers, to the solitary nucleus in the medulla oblongata.[52] From here motor nerves belonging to the autonomic nervous system are stimulated to influence the activity of chiefly the heart and the smallest diameter arteries, called arterioles. 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.

Calcium levels
Main article: Calcium metabolism § Regulation of calcium metabolism
Calcium homeostasis

The plasma ionized calcium (Ca2+) concentration is very tightly controlled by a pair of homeostatic mechanisms.[53] The sensor for the first one is situated in the parathyroid glands, where the chief cells sense the Ca2+ level by means of specialized calcium receptors in their membranes. 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 (see also bone mineral), 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.[54]

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. The skeleton acts as an extremely large calcium store (about 1 kg) compared with the plasma calcium store (about 180 mg). Longer term regulation occurs through calcium absorption or loss from the gut.

Another example are the most well-characterised endocannabinoids like anandamide (N-arachidonoylethanolamide; AEA) and 2-arachidonoylglycerol (2-AG), whose synthesis occurs through the action of a series of intracellular enzymes activated in response to a rise in intracellular calcium levels to introduce homeostasis and prevention of tumor development through putative protective mechanisms that prevent cell growth and migration by activation of CB1 and/or CB2 and adjoining receptors.[55]

Sodium concentration
Main article: Renin–angiotensin system Further information: Sodium in biology, Tubuloglomerular feedback, and Sodium-calcium exchanger

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.[56] These cells also respond to rate of blood flow through the juxtaglomerular apparatus, which, under normal circumstances, is directly proportional to the arterial blood pressure, making this tissue an ancillary arterial blood pressure sensor.

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.[56][57][58] Renin is an enzyme which cleaves a decapeptide (a short protein chain, 10 amino acids long) from a plasma α-2-globulin called angiotensinogen. This decapeptide is known as angiotensin I.[56] It has no known biological activity. 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.[56][59] The reabsorption of sodium ions from the renal tubular fluid halts further sodium ion losses from the body, and therefore preventing the worsening of hyponatremia. 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). So there is no change in the osmolality of the ECF, and therefore no change in the ADH concentration of the plasma. 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.

Potassium concentration
Main articles: Potassium § Homeostasis, and Potassium in biology

High potassium concentrations in the plasma cause depolarization of the zona glomerulosa cells’ membranes in the outer layer of the adrenal cortex.[60] This causes the release of aldosterone into the blood.

Aldosterone acts primarily on the distal convoluted tubules and collecting ducts of the kidneys, stimulating the excretion of potassium ions into the urine.[56] It does so, however, by activating the basolateral Na+/K+ pumps of the tubular epithelial cells. These sodium/potassium exchangers pump three sodium ions out of the cell, into the interstitial fluid and two potassium ions into the cell from the interstitial fluid. This creates an ionic concentration gradient which results in the reabsorption of sodium (Na+) ions from the tubular fluid into the blood, and secreting potassium (K+) ions from the blood into the urine (lumen of collecting duct).[61][62]

Fluid balance
Main articles: Osmoregulation and Thirst

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.

Blood pH

Main articles: Acid–base homeostasis and Acid-base imbalance

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 1:20, at which ratio the blood pH is 7.4 (as explained in the Henderson–Hasselbalch equation). 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. The metabolism of these cells produces carbon dioxide, which is rapidly converted to hydrogen and bicarbonate through the action of carbonic anhydrase.[63] When the ECF pH falls (becoming more acidic) the renal tubular cells excrete hydrogen ions into the tubular fluid to leave the body via urine. Bicarbonate ions are simultaneously secreted into the blood that decreases the carbonic acid, and consequently raises the plasma pH.[63] The converse happens when the plasma pH rises above normal: bicarbonate ions are excreted into the urine, and hydrogen ions released into the plasma.

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

Cerebrospinal fluid (CSF) allows for regulation of the distribution of substances between cells of the brain,[64] 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.[65]

Neurotransmission

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.[66] An imbalance between excitation and inhibition is seen to be implicated in a number of neuropsychiatric disorders.[67]

Neuroendocrine system
Further information: Metabolism, Enterohepatic circulation, and Metabolic pathway
See also: Enzyme § Regulation

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.[68]
Three endocrine glands of the hypothalamic–pituitary–gonadal axis (HPG axis) often work together and have important regulatory functions. 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.[4]

Gene regulation
Main article: Regulation of gene expression

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.

Energy balance
Main article: Energy homeostasis

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).

A 2019 review of weight-change interventions, including dieting, exercise and overeating, found that body weight homeostasis could not precisely correct for “energetic errors”, the loss or gain of calories, in the short-term.[69]

Clinical significance

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 plasma ionized calcium homeostat can be disrupted by the constant, unchanging, over-production of parathyroid hormone by a parathyroid adenoma resulting in the typically features of hyperparathyroidism, namely high plasma ionized Ca2+ levels and the resorption of bone, which can lead to spontaneous fractures. The abnormally high plasma ionized calcium concentrations cause conformational changes in many cell-surface proteins (especially ion channels and hormone or neurotransmitter receptors)[70] giving rise to lethargy, muscle weakness, anorexia, constipation and labile emotions.[71]

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.[5]

Various chronic diseases are kept under control by homeostatic compensation, which masks a problem by compensating for it (making up for it) in another way. However, the compensating mechanisms eventually wear out or are disrupted by a new complicating factor (such as the advent of a concurrent acute viral infection), which sends the body reeling through a new cascade of events. Such decompensation unmasks the underlying disease, worsening its symptoms. Common examples include decompensated heart failure, kidney failure, and liver failure.

Biosphere

In the Gaia hypothesis, James Lovelock[72] stated that the entire mass of living matter on Earth (or any planet with life) functions as a vast homeostatic superorganism that actively modifies its planetary environment to produce the environmental conditions necessary for its own survival. In this view, the entire planet maintains several homeostasis (the primary one being temperature homeostasis). Whether this sort of system is present on Earth is open to debate. However, some relatively simple homeostatic mechanisms are generally accepted. For example, it is sometimes claimed that when atmospheric carbon dioxide levels rise, certain plants may be able to grow better and thus act to remove more carbon dioxide from the atmosphere. However, warming has exacerbated droughts, making water the actual limiting factor on land. When sunlight is plentiful and the atmospheric temperature climbs, it has been claimed that the phytoplankton of the ocean surface waters, acting as global sunshine, and therefore heat sensors, may thrive and produce more dimethyl sulfide (DMS). The DMS molecules act as cloud condensation nuclei, which produce more clouds, and thus increase the atmospheric albedo, and this feeds back to lower the temperature of the atmosphere. However, rising sea temperature has stratified the oceans, separating warm, sunlit waters from cool, nutrient-rich waters. Thus, nutrients have become the limiting factor, and plankton levels have actually fallen over the past 50 years, not risen. As scientists discover more about Earth, vast numbers of positive and negative feedback loops are being discovered, that, together, maintain a metastable condition, sometimes within a very broad range of environmental conditions.

Predictive

Predictive homeostasis is an anticipatory response to an expected challenge in the future, such as the stimulation of insulin secretion by gut hormones which enter the blood in response to a meal.[38] This insulin secretion occurs before the blood sugar level rises, lowering the blood sugar level in anticipation of a large influx into the blood of glucose resulting from the digestion of carbohydrates in the gut.[73] Such anticipatory reactions are open loop systems which are based, essentially, on “guess work”, and are not self-correcting.[74] Anticipatory responses always require a closed loop negative feedback system to correct the ‘over-shoots’ and ‘under-shoots’ to which the anticipatory systems are prone.

Other fields

The term has come to be used in other fields, for example:

Risk
Main article: Risk homeostasis

An actuary may refer to risk homeostasis, where (for example) people who have anti-lock brakes have no better safety record than those without anti-lock brakes, because the former unconsciously compensate for the safer vehicle via less-safe driving habits. Previous to the innovation of anti-lock brakes, certain maneuvers involved minor skids, evoking fear and avoidance: Now the anti-lock system moves the boundary for such feedback, and behavior patterns expand into the no-longer punitive area. It has also been suggested that ecological crises are an instance of risk homeostasis in which a particular behavior continues until proven dangerous or dramatic consequences actually occur.[75]

Stress

Sociologists and psychologists may refer to stress homeostasis, the tendency of a population or an individual to stay at a certain level of stress, often generating artificial stresses if the “natural” level of stress is not enough.[76]

Jean-François Lyotard, a postmodern theorist, has applied this term to societal ‘power centers’ that he describes in The Postmodern Condition, as being ‘governed by a principle of homeostasis,’ for example, the scientific hierarchy, which will sometimes ignore a radical new discovery for years because it destabilizes previously accepted norms.

Technology

Familiar technological homeostatic mechanisms include:

A thermostat operates by switching heaters or air-conditioners on and off in response to the output of a temperature sensor.
Cruise control adjusts a car’s throttle in response to changes in speed.[77][78]
An autopilot operates the steering controls of an aircraft or ship in response to deviation from a pre-set compass bearing or route.[79]
Process control systems in a chemical plant or oil refinery maintain fluid levels, pressures, temperature, chemical composition, etc. by controlling heaters, pumps and valves.[80]
The centrifugal governor of a steam engine, as designed by James Watt in 1788, reduces the throttle valve in response to increases in the engine speed, or opens the valve if the speed falls below the pre-set rate.[81][82]

See also

Apoptosis – Programmed cell death in multicellular organisms
Cerebral autoregulation
Chronobiology
Enantiostasis
Geophysiology
Glycobiology
Homeorhesis
Homeostatic plasticity
Hormesis
Le Chatelier’s principle – Principle to predict effects of a change in conditions on a chemical equilibrium
Lenz’s law
Osmosis – chemical process
Proteostasis
Senescence – Deterioration of function with age
Steady state
Systems biology – Computational and mathematical modeling of complex biological systems
Vis medicatrix naturae

References

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Further reading

Clausen, M. J.; Poulsen, H. (2013). “Chapter 3 Sodium/Potassium homeostasis, Chapter 5 Calcium homeostasis, Chapter 6 Manganese homeostasis”. In Banci, Lucia (ed.). Metallomics and the Cell. Metal Ions in Life Sciences. 12. Springer. pp. 41–67. doi:10.1007/978-94-007-5561-1_3. ISBN 978-94-007-5560-4. PMID 23595670. electronic-book ISBN 978-94-007-5561-1 ISSN 1559-0836 electronic-ISSN 1868-0402

External links

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Cell Homeostasis Flashcards | Quizlet

Cell Homeostasis Flashcards | Quizlet – Start studying Cell Homeostasis. Learn vocabulary, terms and more with flashcards, games and other study tools. Which homeostatic process requires energy to move particles across the plasma membrane? active transport. Which best defines homeostasis?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.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…

What is Homeostasis? – Scientific American – Define homeostasis. homeostasis synonyms, homeostasis pronunciation, homeostasis translation, English dictionary definition of homeostasis. n. A state of equilibrium, as in an organism or cell, maintained by self-regulating processes: The kidneys maintain homeostasis in the body by regulating…Homeostasis helps animals maintain stable internal and external environments with the best conditions for it to operate. There are three components to homeostatic regulation in animals: the receptor, the control center, and the effector.Homeostasis definition is – a relatively stable state of equilibrium or a tendency toward such a state between the different but interdependent elements or groups of elements of an organism, population, or group.

What is Homeostasis? - Scientific American

Which best defines homeostasis? – Brainly.in – Powered by Discourse, best viewed with JavaScript enabled.Homeostasis refers to the mechanism of the body to maintain a stable internal environment instead of changes taking place in the external environment. The balance that is attained is called dynamic equilibrium, which means as the changes occur body work to maintain a relatively uniform conditions.Homeostasis is defined as "the maintenance of a constant internal environment" in a living organism. Every organism carries out some form of regulation As well as getting rid of urea, the kidneys deal with water and salts in the blood. Depending on the diet, there is likely to be more than the body…

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Characteristics of Life – 字幕已打开!单击右下角的CC以关闭。 在Twitter(@AmoebaSisters)和Facebook上关注我们! 你见过那些玩具吗
放在水中几天,然后他们 尺寸增加? 他们总是在这件很棒的事情上
这样的包装,“增长了100万倍 在水中!”-好吧,我有点夸张了-但是
他们确实成长得惊人。 我只有一次。 那是小马。 我梦想着它变得特别大,
为了给它足够的空间,它花了几个 天在浴缸里。 尽管从技术上讲它确实变大了,
我梦imagine以求的梦想并不是真的 成为。 它可能会增加尺寸的事实
在我的脑海里给了我一些想法 它可能还活着。 因为活着的东西会增长,对吗? 结合我过分的想象力
我很难回到 这个玩具小马可能会增加的现实
大小,但是它还没有生命。 当我们谈论生物生长与发育时,
这种类型的尺寸增加是不一样的 事情。 如果我们要真正探索生物学,
对生活的研究使它变得至关重要 我们了解事物的特征
还活着 生活很难定义,但特征
可以探索生物。 我们想在这里提一提
开始,是什么使有机体“活着” 仍然引起生物学家的很多质疑。 嘿,记住,有许多生物尚未发现
在我们自己的星球上-并揭示了新信息 一直都在生物科学中
是我们的免责声明,此信息可以 变化,并且仍在辩论中。 有点像我们在分类中提到的内容
视频。 这就是为什么我们不会给出确定的原因
多少个特性的数值 生活中有,因为我们不想
暗示他们处于特定顺序或 我们列出的内容无法扩展—-
或其中不能包括例外情况 我们可能不会提及。 另外,我们谈论的特征可以
当然标题会有所不同。 看,我们不打算谈论清单
那个会在这里记住。 我们的目标实际上是让您思考
有机体活着意味着什么 和可以探索的生活特征
尝试学习生物时。 组织。 我们可以在这里看细胞理论
根据这个理论,生物 由细胞组成。 有些生物是单细胞的,因此它们可以
由一个单元组成。 但是更复杂的生物,例如小马
在这里,是多细胞的,因此 可以安排成许多单元的
组织-组织可以组成器官 器官可以是器官系统的一部分。 这些是组织的生物学层面! 至于我们的浴缸长马呢? 没有细胞。 体内平衡。 保持平衡的平衡非常重要
甚至可能发生许多生物过程 例如,酶通常需要
pH值范围甚至可以正常工作。 维持体内平衡可能意味着维持
一定的温度和一定的百分比 水浓度。 小马和您的人体有各种各样
保持同位性的反馈系统, 但即使是单细胞生物
变形虫-依靠其细胞膜来维持 内部动态平衡。 活生物体具有同稳态调节作用-
不会在浴缸里长出的小马继续前进。 代谢..
如果你还活着,你需要一些办法
捕获能量并为过程使用能量, 包括一些保持体内平衡的过程
如前所述。 活生物体中发生的化学反应
是新陈代谢的一部分。 因此,仅举一些反应的例子
是生物体内新陈代谢的一部分 植物是自养生物,可以捕获光
在已知过程中产生葡萄糖的能量 作为光合作用。 动物是通常需要的异养生物
吃东西,然后按顺序消化 获得葡萄糖。 这两个示例生物都崩溃了
葡萄糖在细胞呼吸中产生ATP 能源。 小马正在发生新陈代谢。 但不要放在浴缸的小马里。 再生产。 这可能非常简单,例如单细胞
可以复制DNA并分裂成细菌的细菌 2-现在您有两种细菌。 或者它可以更复杂,像这样生活
小马,其中涉及精子和卵细胞的结合 制作受精卵称为合子
最终会发展成一个婴儿 小马。 浴缸长出的小马没有繁殖。 增长与发展。 生命有机体具有遗传物质,
它的发展和增长的代码。 小马会长成
长大的小马,因为它的遗传物质 包含有关此发展和增长的说明。 浴缸长出的小马可能会因大小增加
入水,但没有增长 并根据基因指示进行开发。 对刺激的反应。 人们通常认为对刺激有反应
生活的特征。 可以有内部和外部刺激。 如果这只小马觉得需要吃饭,可以
由于许多身体系统的协调 内部提醒小马饿了。 如果这只小马感觉到危险,那么这个外部
刺激可能导致它逃跑。 浴缸小马不是这样。 但你知道,对刺激的反应可能
不明显。 我以前有很多仙人掌
小子 虽然我不会特别称呼他们
我小时候很兴奋,但我确实注意到 如果我把它们放在窗台上,没有
旋转它们,它们至少会弯曲- 几个星期的时间。 太神奇了,因为他们正在回应
点亮。 植物对光的反应就是对
刺激。 演化。 总结生活的工作定义
Gerald Joyce博士是生物学家小组的成员 将生活描述为“自我维持的系统
有达尔文进化的能力。” 引人入胜的报价。 许多科学家认为进化的过程
作为生活的特征,尽管 将在一段时间内发生。 生活人口中的基因频率
生物可能会随着时间的流逝而发生变化, 例如自然选择。 一些基因可以编码某些特质
导致更高的生殖适应性,而 一些基因可能编码较低的性状
生殖健康-因此可能是 选择反对。 随着时间的流逝,这些会导致适应。 因为浴缸小马甚至无法繁殖
首先…您不会看到的。 人生特征绝对值得探索。 请记住,可以调整这些特性-或可能会有例外。 有些事情很难分类。 带病毒。 在我们的病毒视频中,我们谈论病毒如何
大多数科学家认为, 不活着 病毒及其遗传物质可以
复制-尽管它们需要主机才能复制。 它们可以进化。 但总体而言,他们似乎缺乏很多
生活的其他特征 通常归为生活类。 考虑一下
潜在的额外地球生命 是,不是地球上的生命。 它仍然具有这些特征吗
我们讨论过的生活? 有时,随着科学的发展,我们发现自己面临更多的问题。 嗯…就是变形虫姐妹们的了
我们提醒您保持好奇。 .

Introduction to Anatomy & Physiology: Crash Course A&P #1 – أود أن تُمعنوا النظر
في أنفسكم لثانية واحدة.
لا أقصد أن تراجعوا حياتكم،
فذلك ليس من شأني، وإنما انظروا إلى أجسامكم. ارفعوا أيديكم وحرّكوها. خذوا رشفة ماء
احبسوا أنفساكم واستنشقوا الهواء. هذه الأشياء بسيطة جدًا لمعظمنا
لدرجة أننا لا نفكر بها إطلاقًا، ولكن كل منها أعقد بكثير مما تبدو. فكل حركة نقوم بها وكل يوم نعيشه هو نتيجة عمل مجموعة من الأنظمة معًا
لتؤدي وظائفها بشكل سليم. باختصار يا صديقي، أنت كائن رائع. فأنت معُقد ومُنتج ورائع بتعدد أشكالك أكثر مما تتصور. هل تعرف مثلًا
أن طول أمعائك إذا ما تم مدّها يعادل ارتفاع مبنًى مكوّن من ثلاثة طوابق؟ أو أنك عندما تصبح مسنًا،
ستكون قد أنتجت كمية كافية من اللعاب لملء حوض سباحة؟ أو أنك تفقد سنويًا
ثلثي كلغ من الخلايا الميتة؟ وأنك ستفقد أكثر من 50 كلغ منها خلال حياتك؟
إنها قطع صغيرة جافة من جسدك تتحرك في أرجاء منزلك
وتستقر على رفوف كتبك وتُغذّي عث الغبار. أنت عالَم صغير. وأنا هنا لأساعدك في التعرف
على ذلك العالم وذلك الجسم الذي يأويك من خلال فرعي المعرفة المتلازمين، علم التشريح،
وهو دراسة بنية أعضاء الجسم والعلاقات بينها، وعلم وظائف الأعضاء
الذي يُفسر عمل تلك الأعضاء معًا لتؤدي وظائفها وتُحافظ على حياة الجسم. علم التشريح يشرح ما هو الجسم
وعلم الأعضاء يشرح ما يفعله الجسم، ومعًا يشكلان علم أجسامنا. إنه علم مُعقد بلا شك
ويعتمد على الكثير من فروع المعرفة الأخرى، مثل الكيمياء وحتى الفيزياء.
وعليك استيعاب الكثير من المصطلحات الجديدة، كثير منها باللغتين اللاتينية واليونانية. لكن هذه الحلقة
لن تكون مجرد سرد لأسماء أعضائك أو رسمًا بيانيًا
يشرح كيف تزوّدك شريحة بيتزا بالطاقة. لأن فرعي المعرفة هذين يتمحوران فعليًا
حول سبب كونك على قيد الحياة الآن وكيف أصبحت على قيد الحياة، وكيف تؤذيك الأمراض
وكيف تتعافى من العلل والإصابة. إنهما عن الأمور المهمة من المنظور الأشمل؛
أمورٌ إما نفكر فيها معظم الوقت، أو نحاول ألا نفكر بها:
كالموت والجنس والأكل والنوم بل وعملية التفكير ذاتها. إنها عمليات يُمكننا فهمها
من خلال علم التشريح وعلم وظائف الأعضاء. إذا انتبهتَ جيدًا وإذا أتقنتُ أنا عملي،
فستخرج من هذه الحلقة ولديك فهم أعمق ليس فقط لطريقة عمل الجسم،
وهو ما يشمل كل شيء بدءًا من المصافحة إلى النوبات القلبية،
لكنك أيضًا ستلاحظ أن أهميتك لا تقتصر على مجمل أعضائك. لقد توصلنا إلى فهم الجسم الحي
من خلال دراسة عدد كبير من الجثث. وقد قمنا بذلك سرًّا لوقت طويل. فقد كان تشريح الجثث البشرية محظورًا
في العديد من المجتمعات لعدة قرون. وبالتالي سلكت دراسة علم الترشيح دربًا
طويلًا وبطيئًا، بل ومريبًا في أغلب الأحيان. جمع الطبيب اليوناني غالين من القرن الثاني
ما استطاع من معلومات حول جسم الإنسان من خلال إجراء عمليات التشريح على الخنازير. ودا فينشي كان يتفحّص الجثث ليرسم لوحاته
على نحو بارع من الناحية التشريحية، إلى أن منعه البابا من فعل ذلك. وسُمح لعلماء التشريح المرخّصين
في القرنين السابع والثامن عشر بإجراء عمليات تشريح بشرية صارمة التنظيم،
وقد لاقت رواجًا كبيرًا حتى إنها أتيحت للعامة برسم للدخول،
وبحضور شخصيات مثل مايكل أنجلو ورامبرانت. دراسة تشريح الجسم البشري باتت هوسًا في أوروبا
حتى إن سرقة القبور أصبحت وظيفة مربحة، وإن كانت غير قانونية،
إلى أن أقرت بريطانيا قانون التشريح عام 1832 وفّر القانون للطلاب الكثير من الجثث
كانت تعود للقتلة المعدومين. واليوم لا يزال طلاب علمي التشريح
ووظائف الأعضاء يستخدمون الجثث للتعلم؛ أي لدراسة ما يوجد بجسم الإنسان
من خلال تشريح الجثث بشكل فردي ومباشر. وهو إجراء قانوني تمامًا.
الجثث تعود لمتطوعين، وهذا ما يعنيه الناس عندما يقولون
إنهم سيتبرعون بأجسامهم للأبحاث العلمية. فماذا أوضحت لنا كل هذه الجثث؟ من الأفكار المهمة التي شهدناها مرارًا وتكرارًا
أن وظيفة الخلية أو العضو أو الكائن الحي بأكمله
تعكس شكل تكوينه دائمًا. فالدم يتدفق في اتجاه واحد عبر قلبك،
لأن الصمامات تمنعه من التدفق في الاتجاه المعاكس. وبنفس الطريقة،
فإن عظامك قوية وصلبة، لتحمي وتدعم جميع أعضائك اللينة. الفكرة الأساسية القائلة
إن وظائف البنية تعتمد على شكلها المحدد، يُطلق عليها تكامل البنية والوظيفة. وهذا ينطبق على جميع مستويات تراكيب الجسم،
من الخلية إلى النسيج وصولًا إلى الجهاز. وتبدأ العملية من المتناهي الصغر: الذرّات. كالكرسي الذي تجلس عليه،
أنت عبارة عن تكتّل من الذرات، حوالي 7 أقتيليونات تحديدًا. لحسن حظنا أننا تناولنا أساسيات الكيمياء التي يجب على الطلاب الجدد في علم الوظائف
معرفتها في حلقات الكيمياء سابقًا، وسوف أحيلك إلى تلك الحلقات
كلما تحدثنا عن عمل الأشياء على المستوى الذري. لكن المستوى التالي
بعد كيمياء الذرات والجزيئات يشمل أصغر وحدات بناء الكائنات الحية،
ألا وهي الخلايا. تشترك جميع الخلايا ببعض الوظائف الأساسية،
لكنها أيضًا تأتي بأحجام وأشكال مختلفة، حسب وظيفتها. فمثلًا إحدى أصغر خلايا الجسم
هي خلية الدم الحمراء، ويبلغ عرضها قرابة 5 ميكرومترات.
قارن ذلك بالعصبون الحركي المنفرد الذي يمتد على طول الساق بالكامل،
من إصبع قدمك الكبير حتى أسفل عمودك الفقري، أي نحو متر بين الطرفين.
تجتمع الخلايا عادةً مع خلايا مشابهة لتشكل المستوى التالي من التنظيم: وهو الأنسجة
كالعضلات والأغشية وبطانات التجاويف والأنسجة العصبية والأنسجة الضامة.
عندما يجتمع نوعان أو أكثر من الأنسجة، فهي تُشكل أعضاءً حية، كالقلب والكبد
والرئتين والجلد، وكلها تؤدي وظائف محددة لتحافظ على عمل الجسم. تعمل الأعضاء وتتجمع معًا
لتنجز المهام وتشكّل أجهزة الأعضاء. وهي طريقة توحّد الكبد والمعدة والأمعاء
في جهازك الهضمي لنقل الطعام من الفم إلى فتحة الشرج. وأخيرًا تجتمع كل المستويات السابقة
لتشكيل أعلى مستويات التنظيم وهو الجسم نفسه. أنا وأنت وكلبك،
جميعنا كائنات عضوية متكاملة تشكّلنا عبر التنظيم المتقن لتريليونات الخلايا
التي تعمل في نشاط دائم. وقدرة هذه الأنظمة الحية
في المحافظة على وضع داخلي مستقر مهما كانت التغيرات التي تحدث خارج الجسم
تسمّى "الاستتباب"، وهو موضوع رئيسي آخر
في علمي التشريح ووظائف الأعضاء. البقاء على قيد الحياة
يعتمد على التوازن بين المواد والطاقة، أنت تحتاج للقدر المناسب
من الدم والماء والغذاء والأكسجين لتوليد وتوزيع الطاقة،
وكذلك درجة حرارة مثالية وضغط دم مناسب وتحريك فعّال للفضلات عبر جسمك.
لا بد من الحفاظ على توازن كل ذلك. وبقولنا إن البقاء على قيد الحياة
يعتمد على ذلك يعني أن السبب الأساسي للوفاة هو خسارة قدر هائل لا رجعة فيه
من مستوى الاستتباب. فشل الأعضاء وانخفاض الحرارة والاختناق
والجوع والجفاف، جميعها تؤدي لنهاية واحدة، عبر الإخلال بتوازنك الداخلي
الذي يسمح لجسمك بمواصلة تصنيع الطاقة. لنفترض أن ذراعك قد بُترت؛
إذا لم يعالَج هذا الجرح البليغ، ستنزف حتى الموت، أليس كذلك؟ ولكن ما معنى ذلك حقًا؟
ماذا سيحدث؟ وكيف سأموت؟ إذا لم تعالَج إصابة الشريان،
فستؤدي إلى هبوط حاد في ضغط الدم وهذا بدوره سيمنع وصول الأكسجين
إلى جميع أجزاء الجسم. إذًا فالنتيجة الحقيقية لهذه الإصابة،
أو سبب الوفاة الفعلي، هو فقدان الاستتباب. يُمكنك أن تعيش حياة صحية دون ذراع،
لكنك لن تعيش بدون ضغط الدم، لأن الخلايا لن تحصل على الأكسجين بدون الدم،
وبدون الأوكسجين، لا يمكن تصنيع الطاقة، فتموت الخلايا. ومع هذا العدد الكبير من الأجزاء المتصلة
اللازمة لبقائك حيًا، ستلاحظ أننا نحتاج إلى لغة دقيقة
لتحديد أجزاء الجسم ووصف ما يحدث لها. لن يوصي الطبيب بإجراء عملية جراحية لمريض قائلًا إنه يعاني من ألم في البطن. بل عليه أن يقدم شرحًا مفصلاً،
شيء أشبه بخريطة شفهية. طوّر علم التشريح مع الوقت
مجموعة موحدة من المصطلحات التوجيهية تشرح مكان أحد الأعضاء في الجسم
بالنسبة لعضو آخر. تخيّل شخصًا يقف أمامك،
وهذا يسمى بالوضعية التشريحية الكلاسيكية حيث يكون الجسم منتصبًا
ووجهه إلى الأمام ويداه على جانبيه والكفان موجهان إلى الأمام. تخيّل الآن تشريح هذا الشخص
إلى مقاطع أو مستويات مختلفة، لكن لا تبالغ في تخيله. ينزل المستوى السهمي بشكل عمودي
ويقسم الجسم أو العضو إلى أجزاء يسرى ويمنى. إذا تخيّلت مستوى موازٍ للمستوى السهمي
ولكن مُزاح إلى أحد الجانبين، فذلك المستوى المجاور للسهمي. ويقسم المستوى الجبهي أو الأمامي
كل شيء بشكل عمودي إلى أمامي وخلفي. أما المستوى المستعرض أو الأفقي
فيقسم الجسم إلى جزأين علوي وسفلي. انظروا إلى ذلك الجسم من جديد
وستلاحظون المزيد من التقسيمات، كالفرق بين الأجزاء المحورية والزائدية. كل الأشياء الواقعة على امتداد وسط الجسم،
كالرأس والرقبة والجذع، تسمّى أجزاء محورية، بينما الذراعان والساقان
أو ما يُعرف باللواحق فهي الأجزاء الزائدية والمتصلة بمحور الجسم. كل شيء في مقدمة جسمك يُعتبر أماميًا أو بطنانيًا، وكل شيء في الخلف يُعتبر خلفيًا أو ظهريًا. إذًا فعيناك أماميتان ومؤخرتك خلفية
ولكن يقال أيضًا إن عظم القص أمامي بالنسبة إلى العامود الفقري
وأن القلب خلفي بالنسبة إلى عظم القص. والأجزاء في أعلى جسمك مثل رأسك
تُعتبر علوية أو قحفية بينما الأجزاء الأكثر انخفاضًا
تعتبر سفلية أو ذنبية. فالفك علوي بالنسبة إلى الرئتين لأنه
يقع فوقهما، بينما الحوض سفلي بالنسبة للمعدة لأنه يقع أسفلها. وهناك المزيد: إذا تخيّلت أن خط الوسط
يسير نزولًا على امتداد محور الجسم، فإن الأعضاء القريبة من خط الوسط تُدعى وسطية،
بينما الأجزاء البعيدة عن الخط تسمى جانبية. فالذراعان جانبيان نسبة للقلب،
والقلب وسطي نسبة للذراعين. وبالنظر إلى الأطراف،
أو الأجزاء الزائدية من جسمك، توصف المناطق الأقرب إلى وسط الجذع بالدانية
والأجزاء البعيدة بالقاصية. في علم التشريح، ركبتك دانية نسبة للكاحل
لأنها أقرب إلى الخط المحوري بينما المعصم قاصٍ بالنسبة للكوع
لأنه أبعد عن الوسط. حسنا، سؤال مفاجئ! تخيلوا أنني آكل شطيرة. ليتني كنت كذلك.
ولكن تخيلوا أني آكلها بشراهة وسهوت حتى نسيت نزع عود الأسنان عنها فابتلعته مع قضمة من الديك الرومي
واللحم المقدد والخبر المحمص. بقي جزء من عود الأسنان عالقًا في مكان ما هنا،
فالتقط الطبيب صورة بالأشعة السينية وقرر حاجتي لعملية جراحية. باستخدام اللغة التشريحية، كيف ستوجّه الجراح
نحو ذلك الوتد الخشبي الصغير الموجود في جسمي؟ قد تشرح مكانه بأنه يقع:
"بمحاذاة الخط الوسطي، خلف القلب لكن أمام الفقرات.
وأسفل الترقوة لكن أعلى المعدة." وهذا سيدل الجراح إلى المكان الذي سيبحث فيه،
أي في المريء، فوق المعدة بقليل!
لقد حذرتكم من البداية: هناك مصطلحات كثيرة! لكن هذه المصطلحات ربما أنقذت حياتي.
وبهذا نكون وصلنا لنهاية درسك الأول، وقد بدأتَ تتحدث كأنك خبير من الآن. تعلمتَ اليوم أن علم التشريح يدرس
بنية أعضاء الجسم، بينما علم وظائف الأعضاء يصف كيف تجتمع هذه الأجزاء
لتؤدي وظائفها. تحدثنا أيضًا عن بعض أساسيات هذين الفرعين،
ومنها تكامل البنية والوظيفة، والتنظيم الهرمي للجسم، وكيف أن توازن
المواد والطاقة المعروف بالاستتباب هو ما يبقينا على قيد الحياة.
ثم ختمنا الحلقة بتمهيد عن المصطلحات التوجيهية وكلها مرتبطة بعود أسنان. شكرًا لمتابعتكم، وخاصة لمشتركينا عبر Subbable
الذين يجعلون محتوى Crash Course متاحًا ليس فقط لأنفسهم، بل ولكافة
المتابعين في العالم. إذا أردتم دعمنا، فزوروا موقع subbable.com. كتبت هذه الحلقة كاثلين ييل
وحررها بليك دي باستينو، باستشارة الدكتور براندون جاكسون.
وإخراج ومونتاج نيكولاس جنكينز، إشراف نص فاليري بار، وتصميم صوت مايكل أراندا
والرسومات الجرافيكية إعداد فريق Thought Cafe. .

Best Foods for Halitosis and Gingivitis – „Най-добрите храни при
халитоза и гингивит“ Да, наситените мазнини предизвикват
възпалителна реакция.
Да, възпалението е признато за един от основните
ключови причинно-следствени фактори при пародонталното заболяване. И така, това би могло да обясни защо намаляването
на приема на месо и млечни при хората може да стимулира пародонталното здраве. Но храненето на растителна основа
не само предлага по-ниски нива на наситени мазнини, холестерол
и животински протеини, но също и по-високи нива на сложни
въглехидрати и диетични фибри, витамини, минерали,
антиоксиданти, фитохимикали, така че не е нужно непременно
да знаем какъв е механизмът. Да, наситените мазнини се свързват
с развитието на пародонтозата, но в същото време приемът на фибри
може да играе защитна роля. Но няма как да знаем така или иначе,
докато не направим проверка. Ефектът от намеса в храненето в едно
рандомизирано контролирано проучване. На 7-месечна възраст повече от хиляда
бебета били разделени на групи, като половина се хранели с диета с ниско
съдържание на наситени мазнини и холестерол, за да се види дали ще имат по-малко
сърдечно-съдови заболявания когато пораснат. Те са все още едва на около
20 години, но като деца и юноши, тези, които били в групата на по-здравословно хранещите се,
накрая се оказали с по-добро производство на слюнка. Накарали ги да дъвчат восъчни кубчета и тези, които били в групата на по-здравословното хранене
от бебешка възраст, произвеждали повече слюнка. А слюнката е съществено важна за
поддържането на орално здраве, например за изчистването на захарта
и киселината по-бързо от зъбите. Това, което те мисля, че се случило, е че
по-голямото увеличение на потока на слюнката се дължи на по-големия прием на богати
на фибри храни: пълнозърнести, зеленчуци, плодове, горски плодове, които
изискват повече дъвчене, което от своя страна е известно,
че усилва производството на слюнка. Така че може би техните тела просто
са свикнали да произвеждат повече? С други думи, в допълнение
към общите ползи за здравето, фибрите могат да имат и ползи
за оралното здраве също така, но не непременно заради фибрите, а само
заради самото действие на дъвчене. Това ми напомня за това проучване, при което
едно-единствено хранене, богато на фибри, успява да намали лошия дъх
в рамките на часове. Лошият дъх се причинява от тези
газообразни серни съединения, произведени от определен тип бактерии, които са концентрирани
върху гърба на езика ви. И когато ядем, причината, поради която
лошият дъх се подобрява, може да се дължи на един вид „самопочистване“
на устата докато дъвчем. И така, логично е, че храните, които
трябва да се дъвчат по-интензивно, имат по-силен самопочистващ
ефект върху гърба на езика отколкото храни, които изискват
по-малко дъвчене, Но няма как да знаем докато
не направим проучване за това. Две много подобни ястия, само че едното
съдържало пълнозърнесто руло, повече фибри, повече дъвчене и сурова ябълка с
конфитюр, докато другото имало само бял хляб с желе и варени
ябълки, по-малко дъвчене. После просто измерили халитозните
вещества в дъха на хората два часа след хранене, след
това осем часа след хранене. И дори и след хранене с ниско съдържание
на фибри нивата на лош дъх спаднали, но при храненето с високо съдържание на
фибри те спаднали значително повече и останали ниски дори осем часа по-късно. Така че причината, поради която храненето с високо съдържание
на фибри може да подобри пародонталното заболяване, може да се дължи на фибрите,
по-ниския прием на наситени мазнини или само на дъвченето, но има
и още една възможност. Може би причината е в зеленчуците,
съдържащи нитрати. Знаем, че поглъщането на хранителни нитрати
под формата на зеленолистни и червено цвекло е доказано, че оказва много
благоприятни и клинично значими ефекти върху общото здравословно състояние,
включително поддържане на добър кръвопоток и намаляване на възпалението като цяло. Така че, хей, може ли подобрената циркулация на
кръв към венците и противовъзпалителните ефекти да имат полза при пациентите
с пародонтоза? Едно рандомизирано, двойно тайно,
контролирано с плацебо клинично проучване върху ежедневната консумация
на сок от маруля? Защо сок от маруля? Това
звучи толкова отвратително. Но не се притеснявайте. За да се подобри възприемането от пациентите,
сокът от маруля бил подправен с вкус на мед от цвят лайка
и подсладен със спленда. Това просто звучи още
по-лошо! Но подействало. Проучването с клинична интервенция
демонстрира отслабващ ефект от хранителния нитрат върху
възпалението на венците. Вижте това. В плацебо групата повечето от зъбите им
нямали гингивит, около 60%, но 40% са имали лек гингивит и почти
никой не е имал умерен гингивит. И след като са пиели плацебо сок от
маруля в продължение на две седмици – не знам дали това звучи
по-добре или по-зле – няма никаква реална промяна,
както може да се очаква. Но в групата на марулите хората в
началото били всъщност малко по-зле. Около половината има леко от
умерено заболяване на венците. Но след около две седмици пиене на истински сок
от маруля настъпили значителни подобрения. Умерената форма на заболяването изчезнала,
честотата на леката форма намалява наполовина, а три-четвърти от зъбите
нямали никакъв гингивит. В заключение, нашите открития показват,
че приемът на зеленолистни и червено цвекло може да бъде клинично полезно допълнение
при контрола на хроничния гингивит и всички видове хронични заболявания. Това, което е полезно за устата –
да не се пуши, здравословно хранене – е полезно и за останалата
част от тялото ни. Толкова много стоматолози, които виждат
хората по-често отколкото лекарите, трябва да съветват пациентите си
да живеят по-здравословно. И наистина, почти всички запитани
зъболекари хигиенисти смятат, че наистина имат роля в това да помагат на
пациентите си да подобрят храненето си, но все пак това изобщо не се случва. Ако попитате пациентите, по-малко
от един на десет ще ви кажат, че са получили съвети за
храненето си от своите зъболекари. Защо? Защото макар че стоматолозите
са мотивирани да включат храненето като част от грижата си за пациентите, повече се чувствали
неквалифицирани да осигурят насоки по отношение на храненето. Това никога не е спирало лекарите! Но е истина; храненето се пренебрегва
в стоматологичното образование, също както и в медицинското образование. И в повечето случаи единственото, което
хората получават, е биохимия на витамините, цикълът на Кребс отново и отново, за разлика
от приложимата клинична наука за храненето. Наистина не е толкова сложно. Или е? Схващате ли? Защо е трудно да постигнем необходимите
нива на прием на нитрати? .