Introduction:

Homeostasis, from Greek words for same and steady, refers to any procedure which living things utilize to actively keep fairly stable conditions essential for survival, or we can say it controls internal environment and likely to maintain stable, steady condition of living organism such as temperature or pH.

All organisms attempt to preserve relatively stable internal environment; i.e., all organisms try to maintain homeostasis with varying degrees of success. Living organisms can grow, survive and reproduce only in external environments which give adequate levels of water, nutrients, oxygen and carbon dioxide, and appropriate physical conditions like light and temperature. Several organisms can adapt to varying external environments, others are not capable to and can be injured or killed when conditions change. Biochemical reactions in living cells can happen only when pH, various salts and nutrients, and physical states are within certain limits. Higher developed multi-cellular organisms are more able to control their internal environment as they generally grow the protective outer layer and contain specialized cells.

Principles of Homeostasis in Animals:

Regulation of body temperature and blood glucose was utilized to show following principles; homeostasis gives the constant internal environment and independence from fluctuating external conditions, negative feedback tends to restore systems to original levels, possession of separate mechanisms controlling departures in different directions from original state provides greater degree of control, control mechanisms should be coordinated.

Regulating body temperature:

Humans, together with other mammals and with birds, are endothermic; they can maintain moderately constant body temperatures independent of environmental temperature. When temperature of the blood exceeds 37°C (98.6°F), neurons in part of brain known as hypothalamus detect temperature change. Acting through control of motor neurons, hypothalamus responds by promoting dissipation of heat by sweating, dilation of blood vessels in skin, and other mechanisms. These responses likely to counteract rise in body temperature. When body temperature falls, hypothalamus coordinates different set of responses, like shivering and constriction of blood vessels in skin, that assist to raise body temperature and correct final challenge to homeostasis. Reptiles try to maintain the constant body temperature by behavioral means - by placing themselves in changeable locations of sun and shade. That is why you often see lizards basking in sun. Sick lizards even give themselves "fever" by looking for warmer locations. Many invertebrates don't use feedback regulation to physiologically manage the body temperature. In its place, they employ behavior to adjust their temperature. Several butterflies, for instance, should reach certain body temperature before they can fly.

Regulating blood glucose:

When you digest the carbohydrate-possessing meal, you absorb glucose in blood. This causes the temporary increase in blood glucose concentration that is brought back down in few hours. Glucose levels within blood are continually monitored by sensor, islets of Langerhans in pancreas. When levels increase, islets secrete hormone insulin that stimulates uptake of blood glucose in muscles, liver and adipose tissue.

The Organs used for Homeostasis in Animals:

Osmoregulatory organs:

Animals have evolved diversity of mechanisms to deal with problems of water balance. In several animals, removal of water or salts from body is coupled with removal of metabolic wastes by excretory system. Protists use contractile vacuoles for given purpose, as do sponges. Other multicellular animals have system of excretory tubules (little tubes) which expel fluid and wastes from body.

In flatworms, tubules are known as protonephridia, and they branch all through body in bulblike flame cells. While the simple excretory structures open to outside of body, they don't open to inside of body. Rather, cilia within flame cells should draw in fluid from body. Water and metabolites are then reabsorbed, and substances to be excreted are expelled by excretory pores.

Other invertebrates have the system of tubules which open both to inside and to outside of body. In earthworm, these tubules are called as metanephridia. Metanephridia get fluid from body cavity through the process of filtration in funnel shaped structures known as ephrostomes. Term filtration is utilized as fluid is formed under pressure and passes by small openings, so that molecules larger than the certain size are excluded. This filtered fluid is isotonic to fluid in coelom, but as it passes by tubules of metanephridia, NaCl is removed by active transport processes. The general term for transport out of tubule and in surrounding body fluids is reabsorption. As salt is reabsorbed from filtrate, urine excreted is more dilute than body fluids (is hypotonic). Kidneys of mollusks and excretory organs of crustaceans (known as antennal glands) also make urine by filtration and reclaim certain ions by reabsorption. Excretory organs in insects are Malpighian tubules, extensions of digestive tract which branch off anterior to hindgut. Urine is not formed by filtration in the tubules, as there is no pressure difference between blood in body cavity and tubule.

Secretion of K+ develops the osmotic gradient which causes water to enter tubules by osmosis from body's open circulatory system. Most of the water and K+ is then reabsorbed in circulatory system through epithelium of hindgut, leaving only small molecules and waste products to be excreted from rectum along with feces. Malpighian tubules therefore give very proficient means of water conservation. Kidneys of vertebrates, unlike Malpighian tubules of insects, develop tubular fluid by filtration of blood under pressure. Additionally to containing waste products and water, filtrate has several small molecules that are of value to animal, amino acids, comprising glucose, and vitamins. The molecules and most of the water are reabsorbed from tubules in blood, whereas wastes remain in filtrate. Extra wastes may be secreted by tubules and added to filtrate, and final waste product, urine, is eradicated from body. It may look odd that vertebrate kidney must filter out about everything from blood plasma and then spend energy to take back or reabsorb what body requires.

Evolution of the vertebrate kidney:

Kidney is complex organ composed of thousands of repeating units known as nephrons, each with structure of bent tube. Blood pressure forces fluid in blood past filter, known as glomerulus, at top of each nephron. Glomerulus keeps proteins, blood cells, and other helpful large molecules in blood but permits water, and small molecules and wastes dissolved in it, to pass through and in bent tube part of nephron. As filtered fluid passes through nephron tube, helpful sugars and ions are recovered from it by active transport, leaving water and metabolic wastes behind in fluid urine. Though same essential design has been kept in all vertebrate kidneys, there have been some alterations. As original glomerular filtrate is isotonic to blood, all vertebrates can produce the urine which is isotonic to blood by reabsorbing ions and water in equal proportions or hypotonic to blood - which is, more dilute than blood, by reabsorbing relatively less water blood.

(i) Freshwater fish:

Kidneys are believed to have evolved first among freshwater teleosts, or bony fish. As body fluids of freshwater fish contain greater osmotic concentration than surrounding water, these animals face 2 serious problems: (a) water tends to enter body from environment; and (b) solutes likely to leave body and enter environment. Freshwater fish deal with first problem by not drinking water and by excreting large volume of dilute urine that is hypotonic to the body fluids.

(ii) Marine bony fish:

Though most groups of animals appear to have evolved first in sea, marine bony fish (teleosts) maybe evolved from freshwater ancestors. They faced important new problems in making transition to sea as their body fluids are hypotonic to surrounding seawater. As a result, water likely to leave the bodies by osmosis across the gills, and they also lose water in urine. To reimburse for this continuous water loss, marine fish drink seawater in large amount. Many divalent cations (mainly Ca++ and Mg++) in seawater that marine fish drinks remain in digestive tract and are eliminated through anus. Though, are absorbed in blood, as are monovalent ions K+, Na+, and Cl-. Many monovalent ions are keenly transported out of blood across gill surfaces, whereas divalent ions which enter blood are secreted into nephron tubules and excreted in urine. In these two ways, marine bony fish eradicate ions they get from seawater they drink. Urine they excrete is isotonic to body fluids.

(iii) Cartilaginous fish:

Elasmobranchs, comprising sharks and rays, are by far most common subclass in class Chondrichthyes (cartilaginous fish). Elasmobranchs have solved osmotic problem posed by the seawater environment in different way than have bony fish. Instead of having body fluids which are hypotonic to seawater, so that they have to constantly drink seawater and actively pump out ions, elasmobranchs reabsorb urea from nephron tubules and maintain the blood urea concentration which is 100 times higher than that of mammals. This added urea makes the blood about isotonic to surrounding sea. As there is no net water movement in isotonic solutions, water loss is prevented. Therefore, these fishes don't require drinking seawater for osmotic balance, and kidneys and gills don't have to remove large amounts of ions from bodies. Enzymes and tissues of cartilaginous fish have developed to bear high urea concentrations.

(iv) Amphibians and reptiles:

First terrestrial vertebrates were amphibians, and amphibian kidney is identical to that of freshwater fish. This is not amazing, as amphibians spend the important portion of time in fresh water, and when they are on land, they usually reside in wet places. Amphibians make very dilute urine and compensate for the loss of Na+ by actively transporting Na+ across skin from surrounding water. Reptiles live in diverse habitats.

Marine reptiles, comprising some crocodilians, sea snakes, sea turtles, and one lizard, have kidneys similar to their freshwater relatives but face opposite problems; they likely to lose water and take in salts. Like marine teleosts (bony fish), they drink seawater and excrete isotonic urine.

The kidneys of terrestrial reptiles reabsorb much of salt and water in the nephron tubules, assisting fairly to conserve blood volume in dry environments. Like fish and amphibians, they can't produce urine which is more concentrated than blood plasma. Though, when urine enters their cloaca (common exit of digestive and urinary tracts), extra water can be reabsorbed.

(v) Mammals and birds:

Mammals and birds are only vertebrates capable to produce urine with higher osmotic concentration than their body fluids. This permits the vertebrates to excrete waste products in small volume of water, so that more water can be kept in body. Human kidneys can produce urine which is as much as 4.2 times as concentrated as blood plasma, but kidneys of some other mammals are even more proficient at conserving water. Kidneys of kangaroo rat are so capable it never has to drink water; it can get all water it requires from its food and from water produced in aerobic cell respiration. Production of hypertonic urine is achieved by loop of Henle portion of nephron found only in mammals and birds. The nephron with long loop of Henle extends deeper in renal medulla, where hypertonic osmotic environment draws out more water, and so can make more concentrated urine. Many mammals contain some nephrons with short loops and other nephrons with loops which are much longer. Birds, though, contain relatively few or no nephrons with long loops, so they can't generate urine which is as concentrated as that of mammals. At most, they can only reabsorb sufficient water to create urine which is approximately twice concentration of their blood. Marine birds solve problem of water loss by drinking salt water and then excreting excess salt from salt glands near eyes.

Ammonia, urea, and uric acid:

Amino acids and nucleic acids are nitrogen-containing molecules. When animals catabolise the molecules for energy or convert them in carbohydrates or lipids, they generate nitrogen-containing by-products known as nitrogenous wastes which should be eliminated from body. First step in metabolism of amino acids and nucleic acids is removal of amino (-NH₂) group and combination with H+ to form ammonia (NH₃) in liver. Ammonia is fairly toxic to cells and thus is safe only in extremely dilute concentrations. Excretion of ammonia is not problem for bony fish and tadpoles that eliminate most of it by diffusion through gills and less by excretion in extremely dilute urine. In elasmobranchs, adult amphibians, and mammals, nitrogenous wastes are eradicated in far less toxic form of urea. Urea is water-soluble and so can be excreted in large amounts in urine. It is carried in bloodstream from its place of synthesis in liver to kidneys where it is excreted in urine. Birds, reptiles, and insects excrete nitrogenous wastes in form of uric acid that is only a little soluble in water. Consequently of its low solubility, uric acid precipitates and therefore can be excreted utilizing very little water. Uric acid forms pasty white material in bird droppings known as guano. Ability to synthesize uric acid in groups of animals is also significant as their eggs are covered within shells, and nitrogenous wastes build up as embryo grows within egg. Formation of uric acid, while the lengthy process which needs significant energy, creates the compound which crystallises and precipitates. As the precipitate, it is not capable to influence embryo's growth although it is still inside egg. Mammals also generate some uric acid, but it is a waste product of degradation of purine nucleotides, not of amino acids. Several mammals have enzyme called as uricase, which converts uric acid in more soluble derivative, allantoin.

Feedback Mechanism and Homeostatic Control System:

Feedback is the mechanism, procedure or signal which is looped back to control system within itself. Such a loop is known as feedback loop. In systems having input and output, feeding back part of output so as to increase input is positive feedback (regeneration); feeding back part of output in such a way as to partly resist input is negative feedback (degeneration).

Usually, the control system has input from the external signal source and output to external load; this states a natural sense (or direction) or path of propagation of signal; feedforward sense or path explains signal propagation from input to output; feedback explains signal propagation in reverse sense. When sample of output of system is fed back, in reverse sense, by different feedback path in interior of system, to contribute to input of one of its internal feedforward components, particularly active device or substance which is consumed in irreversible reaction; it is known as feedback. Propagation of signal around feedback loop takes limited time as it is causal.

Feedforward, feedback and regulation are self connected. Feedforward carries signal from source to load. Negative feedback assists to maintain stability in system in spite of external changes. It is connected to homeostasis. For instance, in population of foxes (predators) and rabbits (prey), increase in number of foxes will cause the decrease in number of rabbits; smaller rabbit population will maintain fewer foxes, and fox population will fall back. In electronic amplifier feeding back negative copy of output to input will likely to cancel distortion, making output a more correct replica of input signal.

For instance, in organism, most positive feedback gives for fast autoexcitation of elements of endocrine and nervous systems (specially, in stress responses conditions) and are thought to play the main role in morphogenesis, and development of organs, all have which are, in essence, rapid escape from initial state. Homeostasis is particularly visible in nervous and endocrine systems if measured at organism level. However, in case of morphogenesis, feedback may only be sufficient to describe increase in momentum of system, and may not be enough in itself to account for movement or direction of parts.

Feedback is clearly different from reinforcement which happens in learning, or in conditioned reflexes. Feedback combines instantly with immediate input signal to drive responsive power gain element, without changing basic receptiveness of system to future signals. Reinforcement changes basic receptiveness of system to future signals, without combining with immediate input signal.

Positive feedback control:

Positive feedback is the process in which effects of small disturbance on (perturbation of) system comprise the increase in magnitude of perturbation. Like, A produces more of B which in turn generates more of A. On the contrary, system which responds to perturbation in a way which reduces its effect is said to show negative feedback.

Positive feedback is likely to cause oscillation, divergence from equilibrium, system instability, and often exponential growth. When there is more positive feedback than there are stabilizing tendencies, systems will usually accelerate towards the non-linear region, that may stabilize system or (depending on system) even damage or demolish it. Positive feedback may finish with system latched in new stable state. In positive feedback, the term positive refers to mathematical sign of direction of change rather than desirability of outcome. To avoid the confusion, it is at times better to use other terms like self-reinforcing feedback.

Positive feedback mechanisms are developed to accelerate or improve output created by the stimulus which has already been activated. Unlike negative feedback mechanisms which begin to preserve or control physiological functions within the set and narrow range, positive feedback mechanisms are developed to push levels out of normal ranges. To get this objective, the series of events starts a cascading process which builds to increase effect of stimulus. This process can be advantageous but is hardly ever used by body because of risks of acceleration's becoming uncontrollable.

One positive feedback instance event in body is blood platelet gathering, which, in turn, causes blood clotting in response to the break or tear in lining of blood vessels. Another instance is release of oxytocin to strengthen contractions which happen in childbirth.

Biochemical control takes place when gathering of product stimulates production of the enzyme liable for that product's production. Positive feedback control takes palce when information produced by feedback increases and accelerates response.

Negative feedback control mechanisms:

Negative feedback mechanisms comprise of decreasing output or activity of any organ or system back to its normal range of functioning. The good example of this is controlling blood pressure. Blood vessels can sense resistance of blood flow against walls when blood pressure increases. Blood vessels act as receptors and they pass on the message to brain. Brain then sends the message to heart and blood vessels, both of which are effectors. Heart rate would decrease as blood vessels increase in diameter (called as vasodilation).

Another significant example is when body is deprived of food. Body would then reset metabolic set point to lower than normal value. This would let body to continue to function, at slower rate, although body is starving. Thus, people who deprive themselves of food during attempting to lose weight would find it simple to shed weight primarily and much harder to lose more after. This is because of body readjusting itself to lower metabolic set point to let body to survive with its low supply of energy. Exercise can change this outcome by increasing metabolic demand.

Both feedbacks are evenly significant for healthy execution of one's body. Complications can happen if any of two feedbacks are affected or modified in any way.

Negative feedback is stopping of synthesis of the enzyme by accumulation of products of enzyme-mediated reaction. Negative feedback control happens when information produced by feedback reverses direction of response and controls secretion of most hormones.

Negative feedback loop is biochemical pathway where products of reaction reduce production of enzyme which controlled their formation.

Homeostatic imbalance:

Several diseases are the consequence of disturbance of homeostasis, the condition called as homeostatic imbalance. As it ages, each organism will lose effectiveness in the control systems. Inefficiencies slowly result in unstable internal environment which increases risk for illness. Additionally, homeostatic imbalance is also liable for physical changes related with aging.

Diseases which result from the homeostatic imbalance comprise dehydration, diabetes, hypoglycemia, hyperglycemia, gout, and any disease occurred by the toxin present in bloodstream. In perfect situation, homeostatic control mechanisms must prevent the imbalance from taking place, but, in some people, mechanisms don't work proficiently sufficient or quantity of substance exceeds levels at which it can be directed. In such cases, medical intervention is essential to restore balance, or permanent damage to organs may result.

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