There are TWO sorts of organisms when it comes to thermoregulation in terms of the mechanisms they have to maintain a constant body temperature. These are ectotherm(s/ic) organisms AND
endotherm(s/ic) /homeotherm(s/ic) organisms. Temperature control is important as enzymes need to have a constant temperature to catalyse the reaction, if the temperature is low-then enzymes don’t work as fast as the substrates do NOT have enough kinetic/movement energy required for successful collisions with the enzymes active site. Similarly, if the temperature is too high, there is a change in the tertiary/3D structure of the enzyme and so this means that enzymes denature.
Neither situation is good and they both can lead to death. There must be ways of controlling body temperature and this is achieved by changes in behaviour ONLY by ectothermic organisms and by behaviour AND physiology with the endothermic organisms.
Ectothermic organisms
Examples of ectothermic organisms include fish, amphibians, reptiles etc. and these organisms have the same body temperature as the temperature of the environment. This is directly proportional.
This image here shows the effects of this. E.g. if the temperature of the environment is 10 degrees Celsius for example, their body temperature is about the same.
However, these organisms are able to maintain a body temperature at an appropriate level, by changes in behaviour.
Behavioural changes if body temp is too low
1.
Basking in the sun
2.
Change in body direction, so that a large surface area is exposed to the sun
3.
Lie flat on the ground- the ground is warm
Behavioural changes if body temp is too high
1.
Burrowing
2.
Change in body direction, so that there is a larger surface area not exposed to the sun
3.
Avoid contact with ground.

Endothermic organisms
Examples of these organisms include only mammals and birds. These organisms are able to maintain a constant body temperature ( roughly about 37 degrees Celsius) because of their physiology.
However, physiology can go wrong if the temperature is too high, leading an increase in body temperature, a condition known as hyperthermia.
The straight graphs illustrates the effects of physiology in endotherms.
How body temperature is maintained
These organisms are able to control their metabolism/respiration rate. Most of the energy released from respiratory substrate is converted into heat/thermal energy. So by increasing rate of respiration, you can release more heat and vice versa . So this means that organisms can have
optimal enzyme activity. They also have behavioural adaptations e.g. changes in sweating rate.
When body temperature is hot
1.
Increase in sweating rate- this only is able to work because water has a high latent heat of
vaporisation/evaporation (Amount of energy required to make water turn into a gas). To reduce body temperature , the bead of sweat on the surface of skin generated by the extraction of water, salts etc. from the blood into a sweat gland requires heat from the blood in order to become a gas. This means that a LOT of heat is required because of the high latent heat of evaporation.
2.
Vasodilation ( widening of blood vessels)- This means more heat can be lost by radiation (i.e. in the form of electromagnetic radiation) , convection etc.
3.
Erector(pili) muscle in dermis (part of the skin) relax- Hair can trap a lot of air, this tends to raise body temperature, so by relaxing this smooth muscle, you aren’t able to trap a lot of air, hence cooling you down.
When it is cold
1.
Decrease in sweating rate
2.
Vasoconstriction (shortening of blood vessels)
3.
Erector (pili) muscle in dermis contract- trapping a lot of air.
When body temperature is too cold
These methods above are not sufficient enough to raise body temperature. We need methods fast enough so that we don’t get hypothermia (low body temperature). In the skin, there are peripheral
thermoreceptors, controlling the temperature of the extremities. These are stimulated when

temperature is low and send nerve impulses to the thermoregulatory centre in the hypothalamus to assess the situation. The hypothalamus in turn sends nerve impulses to a number of effector organs. One of these organs are the adrenal glands-secreting the hormone adrenalin. This hormone is able to increase metabolic rate so more heat is released and hence increasing body temperature.
Also the thyroid gland is stimulated to secrete thyroxine- a hormone that controls metabolic rate.
There are also behavioural changes to back up this physiological response such as huddling (to keep warm) and shivering (constant contraction and relaxation of skeletal muscle) which does the same thing.