One of the most important examples of homeostasis is the regulation of body temperature. Homeotherms like mammals maintain their body temperatures at around 37°C. If their temperature rises too high, there is a risk of enzymes and other proteins being denatured. If the temperature falls too low, biochemical reactions will be too slow for it to remain active. Endotherms maintain a stable body temperature using both physiological and behavioral means. Mammals rely mainly on physiological means. Behavioral methods are when temperature receptors in the skin send nerve impulses to the voluntary centre of the brain. Once behavioral methods have been exhausted the temperature of blood starts to fall or rise. In humans temperature is controlled by the thermoregulatory centre in the hypothalamus. It receives information from two sets of thermoreceptors: receptors in the hypothalamus monitor the temperature of the blood as it passes through the brain; the core temperature, and receptors in the skin monitor the external temperature. Both pieces of information are needed so that the body can make correct adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature. Temperature control is achieved by negative feedback. The thermoregulatory centre is part of the autonomic nervous system, so the various responses are all involuntary.
When a mammal is cold there are four main physiological responses. Firstly shivering; muscles contract and relax repeatedly, generating heat by friction and from metabolic reactions. Vasoconstriction; the arterioles leading to capillaries on the surface layers of the skin constrict to reduce blood flow through the capillaries, this means less heat is carried from the core to the surface of the body, maintaining core temperature. It also means the amount of heat lost through the skin by radiation and conduction is reduced. Hair rising; muscles contract, raising skin hairs and trapping an insulating layer of still, warm air next to the skin. Increased metabolic rate; glands secrete adrenaline and thyroxine respectively, which increase the metabolic rate in different tissues, especially the liver, so generating heat.
When a mammal is cold there are two main physiological responses. Firstly vasodilation; arterioles leading to the capillaries dilate, also the shunt vessels are closed off. This results in increased blood flowing closer to the surface of the skin and so more heat is lost to the environment by radiation and conduction. Glands secrete sweat onto surface of the skin, where it evaporates. This is an endothermic process and water has a high latent heat of evaporation, so it takes heat from the body.
The water potential of the blood must be regulated to prevent loss or gain of water from cells. Blood water homeostasis is controlled by the hypothalamus. It contains osmosreceptor cells, which can detect changes in the water potential of the blood passing through the brain. ADH is stored in the pituitary gland, and its target cells are the endothelial cells of the collecting ducts of the kidney nephrons. Water molecules can only cross their membranes via water channels. ADH causes these water channels to open. When water is lost from the mammal for example, by sweating, low water potential is detect by osmoreceptors in the hypothalamus this secrets ADH which opens up the channels allowing more water to be reabsorbed and therefore increasing the water potential of the blood. The opposite of this mechanism occurs when there is a gain of water by, for example drinking.
Excretion is a vital part of a mammal’s routine. If a build up of toxic wastes are not got rid off then it would disrupt the subtle internal conditions and thus disrupting many chemical processes. The kidney is an organ that is involved in the excretion of waste products. It removes urea and other toxic wastes from the blood, forming a dilute solution called urine in the process. The two kidneys have a very extensive blood supply and the whole blood supply passes through the kidneys at regular intervals, ensuring that waste materials do not build up. The renal artery carries blood to the kidney, while the renal vein carries blood, now with far lower concentrations of urea and mineral ions, away from the kidney. The urine formed passes down the ureter to the bladder and is later excreted from the body.
Mammals in desert conditions have to control their water loss to survive so they often have special adaptations. They usually have long loops of Henle, so the surface area is greater so more sodium ions accumulate in the medulla. This means the medulla have especially low water potentials and therefore allowing more water to be reabsorbed back into the system. This makes their urine more concentrated.
As it can be seen by the processes carried out by the kidneys nitrogenous compounds cannot be stored by the body and so has to be removed. The liver like the kidney also assists in the removal of such compounds. Excess amino acids have their amino group removed by the liver. Deamination occurs in the liver; this is where the nitrogen containing amino groups from the excess amino acids are removed, forming ammonia and organic acids. The organic acids are respired off converted to carbohydrate and stored as glycogen. Ammonia is far too poisonous too mammals to be excreted directly so it reacts with carbon dioxide to be excreted as urea a safer form. This then passes to the kidney which is excreted as urine. The liver is also responsible for the regulation of lipids. The lipids are removed from the blood by either the break down of them or modification and transportation to fat depots. Bile compounds, minerals and cholesterol are also regulated.
Another form of excretion arises in the lungs. This organ excretes waste carbon dioxide as well as water. Carbon dioxide is a waste product of respiration and is released from the lungs during expiration along with water vapour. The lungs are specialized organs which contain millions of microscopic air sacs called alveoli. The large numbers of alveoli means a large surface area to volume ratio for gas exchange. The alveoli consist of a single celled epithelium so the thickness of the membrane is reduced and the diffusion pathway is small. A steep concentration gradient between the alveoli and the capillary surrounding it is maintained. This all enables Fick’s law to be at its optimum so diffusion is at its maximum.
Maintaining constant internal conditions is vital to the survival of any organism especially a mammal. Mammals have evolved through time to consist of many mechanisms that can counteract any change that arises. This has allowed mammals to become more complex and able to survive regardless of their external environment unlike other members of the animal kingdom.
