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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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?
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….
Discomfort is not something we tolerate for long, unless there are special (and overriding) circumstances, so you reduce yours in the cold by closing the window etc , 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. 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.