Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Chapter 8
Chapter 9
Chapter 10
Chapter 11
Chapter 12
Chapter 13
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Sitting comfortably in your room, you idly turn the pages of this book. The window is half open, and one of those zephyr-like breezes that poets are so fond of wafts gently around you. Suddenly, we know not why, the temperature drops by 10 degrees Celsius. Deeply absorbed though you are, within a few moments you have noti
ced the change.
You look up, frowning slightly, because you don’t find what is
happening very pleasant. You shiver a little, wrapping your arms
round yourself: after a few moments you go in search of a sweater, and
then close the window. You notice that your hands have gone rather pale
and cold in the interim. You might not notice that your heart rate has
gone up a few beats.You have just demonstrated another fundamental adaptive response……..
Keeping warm is a basic need for mammals. Nearly all mammals maintain their body temperature at around 37oC, though some species vary this by a degree or so. The extraordinary and rather embarrassing fact is that we don’t really know why there has been such evolutionary pressure for 37oC; why not 42o? Or 25o? There have been a number of theories (of course), but none is really entirely convincing. Most of them argue either that 37oC is the optimum for chemical reactions in the body to take place, or that 37oC is the easiest body temperature to maintain in an ambient one of around 25oC, which is the average air temperature in regions where mammals are thought to have evolved…..
Keeping cool is equally important. Just as your body can’t function if it gets too cold, it starts to fail if it overheats. The need to keep body temperature between rather strict limits is one of the best examples of homeostasis. Like other examples of homeostasis, these limits can be changed, or the defence mechanisms strengthened in situations where the demand is persistent - a protracted period in either a very cold or a very hot environment – by the process of adaptation or acclimatisation. So: survival may depend on how good you are at defending your body temperature against a challenge, or how well you can adapt to a persistent change in external temperature………
The importance of temperature control is reflected in the numbers of temperature sensors (detectors) you have. Your body is littered with them. They’re in your viscera, your brain and in your skin. Actually, your body doesn’t care too much about your skin temperature, though, of course, the skin can burn or freeze like any other part. But the skin functions rather well even when it’s hot or cold, unlike other parts of your body. Its temperature sensors are there not to protect it, but to tell the rest of the body – and particularly the brain – what it’s like outside……
Let’s put a small rodent, like a hamster, in a cold room and watch what happens. We are using several instruments, including a camera very sensitive to heat emissions (infra-red). The hamsters fluffs out its coat. After a little while, we notice that it begins to shiver, as we might expect. Then, to our amazement, the camera suddenly shows ‘hot-spots’ scattered over the hamster’s body – on its back, in its abdomen, in its chest. It’s as if the little animal had suddenly switched on its central heating. Which is exactly what has happened…….
The thyroid lies in the neck, and makes several related hormones that include thyroxine……… Thyroxine acts to regulate the rate at which the body’s metabolism works. So an injection of this hormone will stimulate oxygen consumption and increase the heat produced by the tissues. You can think of the thyroid as a regulator of the energy production of the body – like increasing the fuel flow to an engine, or turning up your boiler. Persistent cold stimulates the thyroid….
….At the beginning of this chapter we saw how you reacted to a sudden drop in temperature because you found it unpleasant being cold. Being uncomfortable and being in danger are not at all the same. In this case, as in so many others, your brain ensures you dislike a situation which may, unknown to you, hold danger. The danger, in this case, of hypothermia. Discomfort is not something we tolerate for long, unless there are special (and overriding) circumstances, so you reduced yours by closing the window and so on, thus (unwittingly) counteracting the threat to your body temperature. Animals do the same: put a rat into the cold, and he will press a bar (if he has been taught how) to turn on a warm lamp for a few seconds; then again when it goes off, and so on. As we have seen, animals given a choice of environments will move to one that is temperate, and avoid either extreme heat or extreme cold. They are, as we say, motivated to stay warm in the cold, cool in the heat. Neither you or the rat do these things because you ‘know’ that your body temperature is under attack; you do it because your brain ensures that being too cold or too hot is ‘unpleasant’ and you act to reduce or remove the unpleasantness. In so doing, you may have saved your life…..
But the human limbic system is surrounded by the human cerebral cortex, a very different structure, and one that, unlike the limbic system, shows huge development in man compared even to close relatives like apes, let alone little rodents like rats or hamsters. Whilst the strategy of keeping warm is the same for man and rat, the tactics are very different. Primeval man had no fur coat, so he had to think of using the pelts of other animals, and clothing was invented. Adapting clothes to the prevailing conditions may be superficially similar to a cat shedding its winter coat, but it’s actually different…..
Scientific and literary quotes.