NOTE: This article is about thermoregulation in mammals - specifically humans. You do not have to know about this in other species.
Thermoregulation is very important in mammals as the body temperature must be maintained at around 37oC. This is very high compared to the ambient temprature of most parts of the world, thus heat is lost all the time by conduction, convection, radiation and evaporation. In order to keep the temperature constant the body must do two things – produce heat inside the body and prevent too much heat being lost. The temprature must be kept constant as with higher temperatures enzymes are denatured and with lower temperatures enzymes and molecules function much slower due to particle theory – both these result in metabolic reactions slowing down.
The first two stages for either cooling or warming in mammals occour in the hypothalamus region of the brain. This contains many cells which monitor the temperature of the blood. From here nerve impulses are sent out to muscles and the skin.
If the blood temperature in a mammal rises above 37oC, the hypothalamus will send messages to the muscles and skin to tell them to do different things to normal, because the body's core temperature is too high.
One cooling mechanism is sweating. This is done by the sweat glands – when receptor(s) sense a temperature rise, the hypothalamus sends nerve impulses to the sweat glands, which will then start to secrete sweat. Sweat is mostly water, along with some dissolved ions, such as Cl- and Na+. This solution then travels from the sweat glands to the surface of the skin, via the sweat pore. This means the body loses heat by evaporation as the water molecules will gain kinetic energy and eventually become gas. This overall reduces the temperature of the skin, as the less energetic water molecules and therefore the ones with a lower temperature are left behind – meaning the overall temperature will drop. However, sweating is often bad for the body, because lots of water, as well as some mineral ions are lost – therefore it is important to drink water in hot weather, as well as replacing the mineral ions lost during sweating as well. Also, many mammals are just not capable of sweating – only really humans and horses do it in the sense we may imagine (cat and dogs sweat through their feet!).
Another cooling mechanism is flattening their hairs. Mammals (and birds) are the only type of chordates which have these hairs, used for keeping themselves warm - birds have feathers; which are also used for insulation, as well as flying. However, birds’ feathers do not do the same thing as mammals’ hairs – they do not rise in temperature rise. When the receptors register a temperature rise, the hypothalamus will send a message to the hairs to lie flat. This is because with the hairs flat, there is less insulation, and more heat can be lost - with the hairs flat, there is more air flow – thus more heat can be lost by convection.
A further cooling mechanism in mammals is vasodilation. This is where smooth muscles around the arterioles relax. This allows the blood vessels to widen. Therefore, there is more blood flowing in the artery. Therefore, some blood is redirected to the capillaries in the skin. These are closer to the surface of the skin – thus allowing more heat loss to the environment by conduction and radiation. suck on that.
When the blood temperature or the skin temperature drops below the magical 37oC, the hypothalamus will send out messages to, usually the same, muscles and skin. However, this results in completely different things happening.
Firstly, the sweat glands stop producing sweat – thus reducing heat loss by evaporation.
Another thing done by the body to reduce heat loss is shivering – by muscles in several different parts of the body. This is actually the muscles contracting and relaxing very fast. This gives off heat, because respiration is actually an exothermic reaction in muscle cells and the muscle cells need to respire to carry out this rapid contraction – therefore increasing the temperature of the blood. Heat loss is also reduced, as the shivering reduces loss by convection. Amazingly, shivering is more effective at producing heat than exercise. Shivering also needs glucose – because the muscles need it to respire. Therefore the mammal in cold weather must eat more food.
Also, the walls of the arterioles do the opposite of what they do in hot weather: they contract. This means the blood is taken away from the skin, and to the warmer centre of the body. This warms the core temperature, and reduces heat loss by radiation and conduction. This is called vasoconstriction – however, it is impossible to reduce all heat loss this way. This is also why in very cold weather; your extremities go numb and paler. This is also the early stage of frostbite, although this can only occur when water inside the cells starts to freeze.
Also, the erectorpili muscles, found just under the surface of the skin, contract. These tiny muscles are each attached to one hair follicle. Thus, these hairs all stand on end. All these hairs on end reduce heat loss by convection, and, they also act as a insulating layer. Convection is reduced as air gets trapped between them – meaning it cannot form a convection current. Also, this reduces heat loss by conduction – as the air is a really bad conductor. This is also how goose bumps are seen (only in humans), as the hairs are very small and there are very few, thus we can see the muscles contracting.