The Homeostatic Control of Body Temperature
(An extract taken from ‘A Question of Biology – A Textbook Companion’ by Jon Clark)
1. Why is the control of body temperature so important?
Simply the body temperature must be maintained at around 37 degrees C because this is the optimum temperature for the function of the 10,000 enzymes that control the body’s chemical reactions, such as respiration.
Above about 42 degrees C enzymes denature (are destroyed) – therefore if the body temperature rises above this point then death will result. Below 25 degrees C enzyme controlled reactions become too slow to maintain life and death usually results.
So the maintainance of body temperature is literally a matter of life and death – you don’t get much more important than that!
2. How is the body’s core temperature maintained?
In homeotherms – creatures that can control their body temperature (i.e mammals and birds) the body core temperature – i.e the temperature inside the body around the main organs, is kept at around 37 degrees C. This is maintained by the hypothalamus, which controls the processes of sweating, shivering, vasodilation and vasoconstriction,and the rate of liver metabolism according to the principle of negative feedback. So, an environmental change
(such as going out in the sun) might raise body temperature, this is detected by the hypothalamus which will stimulate, through the nervous system, sweating at the sweat glands in the skin and vasodilation to allow more blood supply to the skin. Sweat will evaporate and cool the increased blood supply to the skin. This is negative feedback – an increase in blood temperature will cause a response to bring it down again.
Of course if the body is too cold the hypothalamus will act to raise the core temperature through vasoconstriction restricting the flow of blood to the skin and through causing shivering which creates heat by muscle contraction.
You may notice that you tend to sweat when you exercise. This is because your muscles are producing a lot of heat through increased respiration, this would raise the body temperature if the hypothalamus did not cause sweating and vasodilation to cool the blood.
3. What adaptations (other than homestatic temperature control) do homeotherms show to really cold and really hot environments?

Heat flows from hotter to colder areas – you will notice this when you leave a hot drink for a while and when you try and drink it, it has gone cold.
Heat is lost from the body to the colder environment by convection, conduction and radiation.
Try sleeping on a cold floor without a mattress and you will notice how quickly you become cold by heat being conducted from your body into the cold floor.
The greater the surface area of the organism in relation to its volume the more heat it will lose. The arctic fox has tiny ears – this is an adaptation to reduce the amount of heat it loses to the very cold environment. The desert fox has large ears, this is an adaptation to allow it to lose more heat through the large surface area of its ears – it needs to have this adaptation to enable it to radiate and convect the body heat it generates as it moves to its hot environment; otherwise it would overheat. African elephants have taken this adaptation to an extreme, they have huge ears which they flap around to increase heat loss by convection.
4. How does the size of an organism affect its heat loss to the environment?
As the overall size of an organism increases its surface area to volume ratio decreases. This means that a large organism will lose heat much more slowly to its environment than a small one – and of course a small organism will lose heat much more quickly.
The large size of a polar bear allows it to lose less heat to its cold environment (the polar bear is usually considered to be the largest of the bear species). Larger species of penguin tend to inhabit colder regions. African elephants need to compensate for their large size by having huge flat ears which greatly increase their surface area – allowing heat they generate, especially as they move, to be lost to their hot environment.
It has been observed that humans in desert environments e.g Kenya, Sudan, tend to be tall and thin -which increases surface area to volume ratio, and therefore heat loss. Humans in arctic, cold environments e.g Inuits or Eskimos, tend to be short and chubby – which reduces surface area to volume ratio, and therefore reduces heat loss.
Generally speaking the colder the environment the larger the organism
-in order to reduce heat loss; and vice versa, although of course there are many exceptions and different adaptations, such as the African elephant.
An interesting perspective is to think of the activity level of organisms of different sizes. Think of a tiny shrew, always active and running around - they need to generate body heat through movement to compensate for the large heat loss they incur through being small. Contrast this with a huge, slow moving elephant – which cannot afford to generate too much heat through

movement, as it would not be able to get rid of it. Think of the constantly busy, tiny hummingbird and the large, relatively static, emu.
5. What happens to humans when the homeostatic mechanisms that control body temperature are not sufficient?
The control system for body temperature will work within certain limits but cannot cope very well with extreme cold or heat – we are simply not adapted to cope with that.
The body maintains its core temperature in cold conditions by reducing circulation to the skin, the temperature of the body’s extremities- the fingers and toes – is not controlled, so they get cold. So people who walk to the North or South Pole often suffer frost bite to their fingers and toes.