The skin as you all know is the largest organ in the human body. It has a variety of functions including providing a water-tight barrier to minimise evaporation from the cells; it is also a habitat for billions of bacteria that live on the skin, the so-called skin flora and it contains a variety of sensory receptors that provide information about the external world to our central nervous systems. The iGCSE specification requires you to know about the role of skin in thermoregulation.
The skin is made up of an outer layer of dead cells called the epidermis that contains sensory nerve endings. Beneath this is the dermis which is made of living cells and blood vessels, sweat glands, hair follicles and other specialised sensory receptors, e.g. for touch. Underneath the dermis there is a subcutaneous tissue that in humans is packed full of adipose cells that store lipids.
The skin is involved in thermoregulation both as a receptor and more significantly as an effector.
The skin’s role as a receptor in thermoregulation
The brain receives information about temperature from two sets of thermoreceptors. There are receptors in the hypothalamus that measure the temperature of the blood passing through the brain. This provides information about core body temperature. In the skin there are two types of thermoreceptors, called hot and cold receptors, that together monitor the external temperature. Information from both these sets of receptors is used by thermoregulatory centres in the hypothalamus to regulate your body temperature.
The skin’s role as an effector in thermoregulation
The skin is the principle effector organ for thermoregulation. This is because it is found at the boundary between your cells and the external environment and so heat gain and heat loss happen through it. The skin has three ways of altering the heat gain/loss depending on nerve impulses from the CNS.
Humans have sweat glands spread over almost all the surface of the skin. These glands secrete a watery liquid, sweat that contains a solution of salts and a tiny amount of urea dissolved in large volumes of water. Sweat is only produced when the body temperature is too high as the evaporation of sweat from the surface of the skin leads to a cooler skin. How does this process work?
The main idea to understand is that the sweat itself is not in any way cool. Sweat is made in sweat glands from blood plasma so if the blood is getting too hot, the sweat will be hot as well. But it takes energy to evaporate water (to turn it from the liquid to the vapour state) and this energy (called the latent heat of vapourisation) is taken as heat energy from the skin. So as sweat evaporates, it uses thermal energy from the skin to turn the water molecules in sweat into a vapour. This evaporative cooling leaves the skin cooler once the sweat has evaporated than it was at the start.
Hairs on the skin play an important role in thermoregulation in many mammals but not really in our species. If the body temperature drops, the CNS causes hair erector muscles to contract and pull the hair to a more vertical position in the follicle. If an animal’s hairs stand on end, a thicker layer of air is trapped between them and so the body is better insulated against heat loss. Humans are relatively hairless and the only thing that really happens in us when the hair erector muscles contract is that we get “goose bumps”.
3) Shifting patterns of Blood flow in the skin
This is the main effector mechanism in human thermoregulation but it is also the one that tends to catch exam candidates out. Please make sure you understand this process fully and can explain this section of work very well indeed. If the body is getting too cold, the pattern of blood flow switches in the skin so less blood flows in the capillary beds near the surface of the skin and more blood is retained deeper in the skin structure. This is achieved by narrowing the arterioles that supply the capillary beds near the surface (arterioles and arteries have plenty of muscle in their walls that can contract to narrow the lumen of the blood vessel) This narrowing of arterioles is called vasoconstriction.
The converse happens when the body is getting too warm. The muscle in the walls of these arterioles now relaxes to widen the lumen, thus allowing more blood to flow in capillary beds near the surface. This vasodilation allows more heat to be lost from the blood by conduction, convection and radiation and so the blood leaving the skin has lost more heat to the external environment.
You will notice that at no point in these explanations of vasoconstriction and vasodilation do I mention capillaries in the skin moving deeper or nearer the surface. For some reason every year, GCSE candidates think that the reason you look redder when you are hot is because capillaries in the skin move nearer the surface. This cannot be true – blood vessels have a fixed position in the body for a start – but now you should understand that you look redder when you are hot because the capillaries that happen to be near the surface are having a greater volume of blood per minute flowing through them because of vasodilation. If you find yourself in the exam writing about capillaries moving in response to a change in temperature, please stop writing, take a deep breath, count to ten and then cross it all out and start again!