|MadSci Network: Anatomy|
Kristi, Like so many, this question is surprisingly complex. I didnít know much about this, beyond basic medical understanding, so I did some reading. Turns out there is a tremendous amount of research done to address cold adaptation, because there is a lot of practical application. If we understood the cold response better, we could potentially help athletes, rescue workers, the elderly (who have a lower cold tolerance), immersion victims, and so on. I must say, though, that I have not learned too much beyond my basic medical knowledge. First off, I assume that you are talking about the situation where the whole body is cold. Some of the information below is actually not correct if you are talking about just a part of the body being cold (like sticking your hand in ice water). The way to think about this problem is as follows. When it is cold around you, the environment is stealing heat from your body. There are two ways to respond to this - defend the heat you have, and make more heat. In the acute sense ("acute" means in the short term, when you find yourself being in the cold when you are normally warm), your body has several mechanisms to defend the heat it has. The first is not physiological, but behavioral - you go where itís warm, put on a coat, etc. The most important physiological defense response is called "peripheral vasoconstriction". Basically, you have blood vessels running all through your body, including through skin on the surface of your body (we call these "peripheral"). The blood running through these vessels is, of course, warm. But when your skin is surrounded by cold air or cold water, the heat in that blood is easily lost to the environment (much more easily than from deep inside your body). Since the blood is constantly circulating, this can rapidly cool down your whole body. To prevent this, your body responds by closing down a lot of these peripheral blood vessels (using a kind of blood vessel called an arteriole that has muscles in the wall of the vessel), thereby limiting the amount of blood going through the skin and slowing the rate of heat loss. This is why your skin turns white when you are cold - very little blood is going through it. A final mechanism to defend body heat is often forgotten, but important - you STOP sweating. Water on the surface of the body is a very effective way of stealing heat from the body, and so you donít sweat when youíre cold. Increased heat production in the acute sense is also achieved by several mechanisms. The first mechanism is again not strictly physiological, but rather neurological - it is called shivering. Yes, shivering is a very complicated process that your body executes "on demand" when you are too cold. Muscles make heat when they work (like when you exercise). This is a basic principle of physics - when energy is taken from a storage source (such as fat) and broken down to do work, it can be converted to another form of energy, but ultimately it is all dissipated as heat. Thus, when your muscles do the work of contracting back and forth repeatedly, they generate heat. It is very interesting that different people have very different thresholds for shivering, and in general people who shiver more easily have a higher tolerance for cold (but even that is complicated, since once you adapt to the cold over the long term, you generally shiver less). The major physiologic mechanism of generating more heat follows the same logic as shivering, but is a little harder to grasp, perhaps. It is called "metabolic heat production". Again, when stored food is broken down to do work some of the energy is dissipated as heat. In this case, the work we are talking about is chemical work, like making new proteins, changing sugar into fat, and so on. Your body does this kind of chemical work constantly, which is what generates most of the heat in your body. When you are cold, this chemical work is stepped up a bit, and so more heat is generated. Perhaps the most interesting example of metabolic heat production is in a special kind of tissue called "brown fat". Brown fat is a lot like normal fat in that lipids are stored, and then broken down for energy when needed. The special thing about brown fat is that it breaks down the lipids in a very wasteful way. Normally, the energy from fat is used by tissues to do work (which again does generate some heat). But brown fat breaks down the lipids in a way that does no work at all, and so the energy in the lipids is converted entirely to heat (to be complete, the mitochondria in brown fat lack the ability to make ATP as the final product of oxidative phosphorylation). Adult humans have only a small amount of brown fat, and so this probably does not contribute much to heat production for us. On a mechanistic note, many of the above processes are mediated by a part of the nervous system called the sympathetic nervous system. This is the system that is primarily responsible for responding to stressful situations. Things get quite a bit harder to pin down when we consider the bodyís response to cold in the chronic sense ("chronic" means over a long period of time, like when you live in a cold environment), what is called "cold adaptation" or "cold acclimatization". Basically, several sources I have seen indicated that there really is no single, easily definable pattern of human adaptation to cold in the long term. Researchers have studied various human populations that live in cold environments, as well as experimental subjects who were put in a cold environment for a long time. Some people and populations adapt by increasing their metabolic rate by longer term mechanisms like increasing the rate of thyroid hormone production (this hormone stimulates metabolism). Others seem to defend their body heat more aggressively by making more fat, which acts to insulate the body better. Still others learn to tolerate slight decreases in their core body temperature. Still others never really adapt at all, in a strict sense, since no physiological changes are apparent - thus they adapt primarily by behavioral mechanisms, or perhaps by simply increasing the efficiency of the acute defense mechanisms. This list can go on, but I think you get the idea. In the end, the reason this question continues to interest and confuse researchers is that it seems like there should be a single, simple answer. But from everything I have read, there just isnít one. Bottom line, we are warm blooded animals, and our primary mechanisms for surviving in the cold are fire, clothing and shelter, and these are far more important than any physiological mechanism. If we are exposed too much, our body temperatures drop and we die, and this is true whether you live in Alaska or Hawaii. Now if you were a frog or a fish..... I hope that answers the question that you asked (and not some other question!). Tom Wilson, MD PhD
Try the links in the MadSci Library for more information on Anatomy.