Homeostasis is defined as "The maintenance of a constant internal environment". It includes the maintenance of the chemical composition and of the temperature of body fluids at constant levels. Effective homeostatic control mechanisms allows organisms a degree of ndependance from the environment by seeking to provide a more stable internal environment than that outside the body. This has allowed for the colonisation of harsher environments, helping to ensure the survival of the species in case of large-scale environmental change such as global warming.

The typical control system consists of a number of components:
The "Norm" - The set level at which the system operates. For example, in the homeostatic control of temperature in humans, the norm would be body temperature - about 37 degrees celsius.
The receptor - This monitors the factor being controlled and detects any deviation from the norm. A significant deviation from the norm will provolk a response.
The communicator - It can be either hormones or it can be a part of the nervous system. The communicator is the system which communicates information regarding the deviation from the norm of the controlled factor from the receptors to the effectors.
The effectors - This is what brings about the neccecary change required to counteract the deviation from the norm.
The effector stops acting when the controlled factor has returned to the norm. This occurs by a method known as "Negative feedback". The effector counteracts the deviation from the norm but would end up countering it too much, resulting in another deviation on the opposite side of the norm (over-correction), if it wasn't stopped. The receptors detect the return of the controlled factor to the norm and the corrective mechanism (the effector) is switched off.

It is worth noting that in some systems, the feedback causes the receptor to increase, not decrease, its message to the effector. This is known as "Positive feedback" - and example of which is the hormone "Oxytocin" which is released during childbirth. It stimulates the contractions of the uterus, which in turn stimulates the production of more oxytoxin, leading to more rapid and violent contractions until the baby is born. I'm not sure how this system is stopped, but it's nothing to do with homeostasis and I'd better get back on topic.

Perhaps the most commonly understood of the two main types of homeostasis is the regulation of temperature, so I will begin with that.

Temperature regulation:

Temperature regulation is important as the rate of metabolic reactions are controlled by enzymes which only operate efficiently at temperatures close to their optimum. They will function very slowly at lower temperatures and they can become nonfunctioning (denatured) in temperatures which are too high, a process caused by breaking the specific bonds essential to maintain the precise, globular shape of an enzyme required for it to carry out the reaction which it is designed to speed up. Animals use two different methods of temperature control - they are classified as either Homoiotherms (Endotherms) or Poikilotherms (Ectotherms).
Poikilotherms, commonly referred to as "cold blooded creatures", allow their body temperature to fluctuate with the ambient external temperature. This has the disadvantage that they require a very specific habitat and thus cannot colonise extreme environments. A lot of aquatic animals are poikilotherms as water, having a very high specific heat capacity, takes such a great amount of heat energy to heat it up by a few degrees celsius and thus water is a very stable environment as the temperature in it does not vary by much and does not vary quickly - it takes a long time to heat water enough to cause a change that would impact on sea-dwelling organisms.
Homoiotherms, commonly known as "warm blooded creatures", can regulate their body temperature within narrow limits at a relatively high temperature by controlling heat gain and loss and by contolling their metabolic rate. Example temperatures would be 37-38 degrees celsius for most mammals and 40 degrees celsius for most birds.

Most of their body heat is derived from their immediate environment and their body temperature follows that of their surroundings. They control their body temperature by behavioral and physiological mechanisms. Essentially, they move to a warmer position if their temperature drops to an undesirable level. Land-dwelling poikilotherms such as lizards would tend to be seen basking in the sun and control their temperature then by changing their orientation relative to the sun to gain heat or by moving into shade or water in order to lose heat. Many poikilotherms have the ability to change their colour, which will change the ammount of heat absorbed as darker colours will absorb more heat, raising the body temperature, and lighter colours will reflect more and thus help lower temperature. They may also lose heat by evaporation of moisture.

Homoiotherms can generate heat internally by altering the metabolic rate - the rate at which the metabolic processes which generate heat such as respiration take place. Their temperature is controlled by voluntary and involuntary mechanisms:
Voluntary: Heat may be gained voluntary by exercise/movement and lost by moving into shade. The development of heightened intelligence in humans has allowed for the development of technology such as heaters and warm clothes, which certainly helps. I prefer wrapping a blanket around myself over exercise ;).
Involuntary: Heat can be gained by a number of involuntary methods - most of which are actually methods of reducing heat loss rather than methods of generating heat. Vasoconstriction of peripheral blood vessels in the skin will help to reduce heat loss by conduction from the blood to the outside environment via the skin. Vasoconstriction is a process by which the blood vessels contract, becoming smaller so that the blood flow in the area is decreased. The peripheral blood vessels are those small arterioles which provide a better blood supply than the skin tissues actually need and so contracting them to lower the blood flow for a while doesn't cause problems. Another method of gaining heat is by shivering - the muscles shallowly contract in sequence so that the body vibrates. Heat is generated this way by friction and by the reaction of the body to the increased activity, which is to increase the metabolic rate as if you were exercising. It's worth noting here that shivering is not an entirely voluntary reaction, no matter what anyone tells you. I know this for a fact, because I currently have shivering under voluntary control - I can choose to shiver or not when cold.

The main method of heat production being the metabolic processes such as respiration, heat can be gained by the increasing of the metabolic rate. This is the reason people say that a little alcohol will keep you warm - the liver gives out quite a bit of heat when working hard to break down the alcohol. The mamalian body will also not produce sweat when overheating and body hairs will raise to trap air between them. Air is a good insulator and thus trapping air traps heat - the same reason a string vest can keep someone warm (some air is trapped in the holes between the strings).
Heat can be lost involuntarilly by much the opposite ways. The metabolic rate can be slowed to lower heat production, the peripheral blood vessels in the skin can vasodilate, increasing blood flow and so increasing heat loss to the environment, sweat can be produced from the sweat glands, which cools down the skin by evaporation and the hairs on the skin are lowered.

In homoiotherms, the core body temerature is regulated by the hypothalmus, which is a small body at the base of the brain. Within the hypothalmus is a regulatory centre, which has two parts: The "Heat gain centre" and the "Heat loss centre". The hypothalmus monitors the temperature of the blood passing through it and in addition recieves nervous information from the receptors in the skin about external temperature changes. Any changes which bring about a decrease in blood temperature will cause heat conservation and, conversely, any changes which cause the blood temperature to increase will have the opposite effect. Thermoreceptors are part of the nervous system and are specialised sense receptor cells which detect temperature changes. Therefore, temperature regulation involves interaction of the nervous system.

Temperature limits:
Experiments carried out on human test subjects and observations on hypothermia patients have allowed science to build up a picture of the temperature tolerance of the human body. As the external temperature drops, a range of thermoregulatory mechanisms are used to conserve heat. Below a certain point, simple measures such as vasoconstriction and raising body hairs are no longer sufficient and the metabolic rate begins to increase. This is known as the "low critical temperature". If the temperature keeps on falling, the metabolic rate will rise to produce sufficient heat to maintain the core temperature. Eventually, chemical reactions can no longer take place and the subject dies - this is the "low lethal temperature".

As the external temperature rises, thermoregulating mechanisms such as sweating can keep the core temperature stable until the "high critical temperature", which is a variable based on the level of humidity since heat loss by evaporation works better in less humid environments, is reached. If the external temperature continues to increase, the metabolic rate starts to go up as the body's reactions double for every 10 degrees celsius rise in temperature. Once this happens, positive feedback occurs since the metabolic rate increasing produces more heat, which will increase the metabolic rate further. Death usually occurs when the core temperature reaches about 42 degrees celsius - the "high critical temperature". At this temperature, enzymes controlling the vital metabolic reactions are denatured and fail to function.

Chemical composition control:

As important as the body's tissue fluids being kept within narrow limits in temperature is the body fluids being kept close to a certain concentration. This involves the regulation of the dissolved components of the fluids, such as salt and glucose. This occurs by a system known as "Osmoregulation" - the regulation of water content and solutes. Another important factor, to be noted, in the control of chemical composition is the release of wastes such as CO2 (Carbon Dioxide) produced during respiration and the urea preoduced in the liver by the breakdown of amino acids by the removal of an amine group, a process known as "Deamination". However, since the wastes are usually produced at a steady rate and removed at the same rate, their concentration can be mostly ignored as an influence on osmotic balance of body tissues.

Osmoregulation and excretion in terrestrial mammals:
Animal cells require the maintenance of a steady state of the intracellular fluids of cells (tissue fluid) for efficient functioning. Aquatic organisms gain or lose water by osmosis through all permeable parts of the body surface depending on whether the environment is dilute or concentrated. Osmosis, for those who don't know, is the movement of water through a selectively permeable membrane. It always occurs in a direction to equalise the concentration on both sides of the membrane. For example, if there is a higher concentration inside an amoeba than in the water it is in, water will move into it by osmosis until the concentrations are equal inside and outside the amoeba. As you can imagine, if it absorbs too much water, the amoeba's membrane could burst under the stress of the water and the amoeba would die. Osmoregulation is required in mammals to prevent the cells inflating and causing pressure on each other, which can cause problems (particularly in the brain), and to keep the blood plasma volume at a constant level.

Terrestrial organisms have the problem of losing water by evaporation. Terrestrial mammals, in particular, have developed a number of ways to counteract this and maintain a steady osmotic state. Skin and hair with keratin in it is used by many mammals to provide a more waterproof coat. Humans have an oil called sebum on their skin which acts as an osmotic barrier, preventing water which comes into contact with the skin from interfering with the osmotic balance to any great degree. Lungs have been developed internally so that a wet surface for gaseous exchange (as is needed, since the oxygen and CO2 can only pass into the blood when dissolved) is provided without exposing a large area to dessication. Lungs are lined with mucus and thousands of tiny hairs to prevent as much moisture as possible from being released on exhaling. The metanephric kidney, which uses many tiny nephrons as the site for ultrafiltration of the blood, has provided a method for filtering ions and harmful materials out of the blood while keeping as much water as is needed. The urine produced by terrestrial mammals is also hypertonic, meaning it has a much higher concentration than the body tissue fluid. This means it contains a lot less water than it should - it has been concentrated - and less water is lost due to excretion in this way. A steady state is obtained by the loss and gain of water occuring at the same rate. Most mammals replace the water lost by excretion and evaporation with water they drink or water contained in foods that they eat.

Excretion in the mammalian body involves the removal of the waste product of respiration (CO2), mineral salts, excess water and the nitrogenous excretory product "Urea". Urea is formed in the liver and has the chemical composition "CO(NH2)2". It is formed by the process of deamination and the combining of two ammonia molecules with one molecule of CO2. The entire process is called the "Urea cycle" or "Ornithine cycle". Many marine invertebrates and all fresh water animals excrete ammonia, however urea is much less toxic than ammonia and although it is less soluble, less water is required for its elimination because tissues can tolerate higher concentrations of it.

The mamalian kidney:
This is the main organ of nitrogenous excretion. Its functions include the removal of metabolic waste products such as urea and creatine, the regulation of chemical composition of the body fluid (its role in homeostasis) by removal of substances in excess of immediate requirements such as Na+, K+ and Ca+ ions, the maintenance of of optimal water potential (concentration) of body fluids - a process referred to as osmoregulation - and the regulation of the pH of body fluids. I have already discussed the importance of removing urea from the bloodstream and why it is there, so I'll just add a note here that it is poisonous and if it isn't removed, can build up over time and eventually kill someone. The removal of ions in excess in the bloodstream is required because the ions, being dissolved, affect osmotic balance. Too many Na+, K+ and Ca+ ions in the blood will lead to too high a concentration of body tissue fluid and blood so they will have to be removed to maintain a steady osmotic balance without introducing so much water that it may cause problems. The pH of urine can vary between 4.5 and 8.0, which ranges from acidic to slightly alkaline. This is because the kidney will remove acidic components of the blood in order to maintain a steady pH within the body tissue fluid. This is required because enzymes only function correctly at specific pHs.

The main function of the kidney is in osmoregulation - the homeostatic maintenance of the water potential (osmotic balance) of cell tissue fluids. The kidney has a rich blood supply and regulates the blood composition at a steady state, ensuring the composition of tissue fluid is maintained at the optimal conditions of the cells. Each human kidney contains over a million nephrons and humans have two kidneys. The human kidney can filter approximately 120 cubic centimetres of blood per minute. They are located at the back of the abdominal cavity, on either side of the vertebral column. It is held in position by a thin layer of tissue called the "Peritoneum" and is usually surrounded by Fat, which cushions it to help avoid damage. They are each about 7 - 10 cm long and 2.5 - 4 cm wide and are richly supplied with blood from the renal artery and trained by a renal vein. Urine formed in the kidney passes down a pair of ureters to the urinary bladder, where it is stored until released via the urethra. That's enough on the structure and position of the kidney - I'm supposed to be discussing osmoregulation in homeostasis. That brings me to the important part of the kidney in osmoregulation - those million little nephrons.

The Nephron is the basic structural an functional unit of the kidney - the kidney's sole purpose as a whole is to support the position and role of the nephrons which make up most of it. Each kdney, as mentioned before, contains about a million nephrons. Each nephron is approximately 3cm long, providing an absolutely massive internal surface area for the exchange of materials. The nephron has four main functional regions with different features specialised to that particular function. I will try to keep these simple as I know I am going into far too much detail here on the workings of the kidney than is required to understand the role of the kidney in osmoregulation.
The bowman's capsule and glomerulus: - Each arteriole forms a "knot" of blood vessels, named the glomerulus, inside the bowman's capsule. There is a high pressure pulse wave due to ventricular action which forces fluid through the glomerulus from the blood into the bowman's capsule. This fluid is essentially blood plasma without the proteins, which are too big to pass through into the bowman's capsule. From here, it moves into the proximal convoluted tubule.
The proximal convoluted tubule: - Glucose and amino acids are reabsorbed back into the blood here in this long, twisted tube with a very rich blood supply. The absorbtion is complete, occuring by active transport, so the filtrate no longer has any glucose or amino acids in it at this point. Approximately 80% of the water is also reabsorbed here. It then moves onto the loop of henle.
The loop of Henle: - This is a long, thin tube which extends deep into the medulla of the kidney. Its length determines how concentrated the urine will be. It does this by setting up a salt gradient - tissues deeper in the medulla are more salty.
The distal convoluted tubule and collecting duct: - In here, some substances are released into the filtrate to eliminate toxic substances such as creatine and drugs such as penicillin. Further water is drawn out of the filtrate as it passes through the collecting duct.
That's a very basic, sketchy idea of what the nephron does. I'm only really including it for completeness and se that I can refer to parts of the nephron later without imagining huge question marks appearing above the head of anyone who would actually bother to read this.

Blood concentration control:
One question, I have not yet answered: How does the ammount of water removed by the kidneys vary?
The concentration of the blood increases - perhaps by the person losing a lot of water due to sweating during exercise or hot weather. This change is detected by osmoreceptors in the hypothalmus, a small area at the base of the brain. The hyothalmus secretes ADH (Anti-Diuretic Hormone) into the pituitary gland, from which it is released into the blood stream. ADH causes the collecting ducts on the nephrons in the kidney to become more permeable to water. The result is that more water is reabsorbed from the urine back into the blood. The urine will, if you recall the path that it takes in and after it leaves the nephron, be a lot more concentrated. If the blood becomes less concentrated, less ADH is produced and the collecting ducts don't reabsorb as much water so the urine will be of greater volume. This is another negative feedback system as the rate of reabsorbtion of water from the nephron ultrafiltrate corrects the deviation of the blood concentration detected by the hypothalmus, resulting in the corrective mechanism slowing down or shutting off.

This node is a bit like the "node your homework" idea, but perhaps more "node your revision". I hope that some day somebody will find it useful. Please send corrections to me and if I catch them, I'll edit the node. References: This node was created based on revision notes for the A-Level Biology course by CCEA. The notes were created by an excellent Biology teacher by the name of Dr Francis.

Edits performed on the node:
24th May 2004 - Edited footnote to include references and shuffled two sections about to make the node flow better - Thanks isogolem. 24th May 2004 - Corrected the spelling of celcius to celsius and since E2 is a US site, changed the spelling of behavior and evapouration to suit - Thanks to some guy I can't remember - timeshredder or something.