In post 20.16 we saw how the body was able to control its chemical composition; in this post I am going to write about how it controls its temperature. This post is mostly about temperature control in humans.
We need to maintain our bodies at about 37oC to provide the optimum conditions for the chemical reactions (post 16.33) that our lives depend on. The heat to maintain this temperature (post 16.35) is generated mainly by using our muscles and major organs, like the liver, brain and heart. If our temperature is too high, we need to lose heat; if it is too low, we need to generate more heat.
How do we know whether we are too hot or too cold? A part of our brain, called the hypothalamus, contains specialist cells that sense when we are too hot or too cold. They then activate the mechanisms to increase our temperature, if we are too cold, or decrease our temperature, if we are too hot.
We increase our temperature by generating more heat and by minimising heat loss. We can generate more heat by using our muscles to move our arms and legs. Another way to use muscles is by shivering. When we shiver, we are moving the muscles under our skin. We can minimise heat loss by contracting our blood vessels to reduce their surface area; this process is called vasoconstriction.
Contraction of the blood vessels decreases their surface area and so reduces the rate of heat loss by conduction (post 19.14). Another way to reduce heat loss is to reduce blood flow to regions that can easily loose heat – like our arms and legs. Since blood is usually hotter than the surrounding air, reducing the flow of blood reduces the rate at which heat is lost. We don’t reduce blood flow to the body core because it contains vital organs, so we feel the cold in our hands and feet before we feel it in the rest of our bodies. Normally hair lies flat against our skin. If we are cold, muscles at the base of each hair pull the hair upright. Upright hairs trap a thicker layer of air around the skin. Since air has a low thermal conductivity (post 19.14) heat loss by conduction from the surface of the body is then reduced. This way of conserving heat is more useful to animals whose bodies are covered by a thick layer of hair or fur or wool. But we can see the same mechanism in humans. When muscles pull hair upright, they create little bumps in the skin, called “goose bumps” or “goose pimples” at the base of each hair, as shown in the picture above.
How do our bodies lose heat when we are too hot? The most obvious mechanism is by sweating. When we are too hot, the sweat glands in our skin secrete sweat – which is mostly water. Because we are warm, the water in sweat evaporates – it turns from a liquid to a gas. This requires energy in the form of heat (called latent heat of vaporisation – see post 16.37). So, evaporation of sweat removes heat from the body.
Dogs don’t have sweat glands. So they lose heat by rapid breathing, called panting, to expel hot air. Humans use this mechanism too. Dogs usually stick out their tongues when they pant. Unlike their skin, their tongues are not covered by fur (that traps air, as explained above) and so can lose heat more easily.
Temperature control, sometimes called thermoregulation, is another example of homeostasis (post 20.16) – the processes by which the body controls our internal environment to keep us healthy. This is not the only example of the automatic control systems that we use – when we move, we use active damping to ensure that we control the movement (post 18.9). Of course, these mechanisms can go wrong. Problems with the nervous system can reduce our ability to damp movements leading to jerky motion, as occurs in Parkinson’s disease. Problems can also occur in homeostasis. If we get too cold, the mechanisms to generate heat may be inadequate. Our temperature then drops to a dangerously low level; this is called hypothermia. Hypothermia can be a problem in mountains in the winter and, because the brain does not function as well as normal, can lead to people making irrational decisions – like taking their gloves off because their hands are cold and so, they reason, their gloves aren’t useful. If we get so hot that our mechanisms for reducing temperature can’t cope, our body temperatures can rise to a dangerously high level; this is called hyperthermia. Hyperthermia can be caused by serious infections by bacteria or viruses; this form of hyperthermia is called fever. But normally our bodies’ control systems are adequate to function effectively, in other words, to stay healthy.
20.16 Homeostasis 1
18.9 Damping and muscles