Even more useful terms are Ectothermic or Endothermic, which suggest two different mechanisms of thermoregulation. Ectotherms generally obtain heat from their external surroundings. Their body temperature varies, corresponding at any time with the temperature of their external environment. Endothermic animals, on the other hand, have relatively constant body temperatures. Their body temperature is independent of that of their external environment.
Monkeys and walruses, for example, both have body temperatures of about 38"aC, despite living in very different habitats. However if body temperature rises above its optimum level (usually around 40"aC in mammals) then the enzyme rate inside the body will go into sharp decline. This is because enzymes are proteins, and become denatured. One of the first organs to be affected is the brain. Since the brain controls breathing and the circulation, the rise in body temperature disrupts the normal functioning of these important systems.
If the body temperature decreases dramatically (hypothermia) then this will slow metabolic activity and impairs brain function. Here is a graph to show the relationship between the body temperature and environmental temperature for a cat (endotherm) and a lizard (ectotherm) Also we need to clarify what is meant by an extreme climate. In this investigation I will be using two different climates, The Desert and The Arctic When an endotherm is subjected to severe cold it is liable to lose heat energy but this can be counteracted in a number of ways; "h It could raise the hairs into a more vertical position by the contraction of the erector pili muscles.
Air can then be trapped in the spaces between the hairs and, being a poor conductor of heat, it serves as an insulatory layer round the animal. This is an involuntary response brought about by the nervous system. "h The arterioles leading to the superficial capillaries constrict and as a result blood flow to the surface of the skin is greatly reduced (vasoconstriction). This is brought about by the sympathetic nervous system and is useful in uncovered regions such as the ears where the surface-volume ratio is particularly large and so particularly susceptible to cold.
"h The metabolic rate can be increased therefore heating the inside of the body. A general increase in the metabolic rate is brought about by the hormones adrenaline and thyroxin which are produced in large amount during cold conditions. There is then a general increase in muscle tone, which is then followed by spasmodic contractions (shivering) The response to high temperature is basically the reverse of the above processes; i.e "h Hairs lie flat against the body "h The arterioles dialate (vasodialation) allowing blood to flow back to the surface of the skin so heat can be lost by diffusion. "h Sweating occurs and the evaporation of the water from the skin cools the skin and blood. "h Panting occurs.
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