Please explain warm bloodedness

… in a general way to a non-science person.

This is one of those things that I was thinking about, and realized I had very little understanding of.

I understand that mammals and birds are warm blooded, in that they maintain their body temperature by their metabolism. I understand that most fish and reptiles are not warm blooded, although their are a few kinds of these that have a higher body temperature as a result of other processes.

For us warm blooded types, how exactly do we stay that way? Does our brain monitor our body temperature, and adjust accordingly? Where exactly is our “thermostat” and how does it work? I think I’m amazed that our bodies can keep our temperatures in the same, fairly small, range of temperature regardless of the environment and our activities.

The brain controls the regulation of body temperature, in birds and mammals. The regulatory mechanisms are digestion, and muscular movement to create heat, and perspiration and respiration to dissipate heat. In addition birds and heavily furred mammals can alter the configuration of hair and feathers to increase and decrease their efficiency as insulators or evaporators.

Some mammals also use basking and wallowing to augment the body functions, in climates which make that behavior useful. These behaviors are the same ones used by reptiles to maintain their own body temperature. The ability to be warm, and active during darkness, and early in the day offer distinct advantages whether predator, or prey.

The specifics of the mechanism is very detailed, and complicated, although simplistically one can say you shiver when you are cold, and sweat when you are hot. Your hair stands on end when you are cold, so it insulates better, and lays flat, and wicks your sweat better when you are hot. Running a fever makes you lethargic, and being slightly cold makes you a bit hungry. None of these descriptions is complete, however, and the full explanation is the subject of a longish chapter in a good comparative physiology text.

A warm blooded amimal attempts to keep its core temperature constant. This lets it operate the same in both hot and cold weather, as long as the core temperature is maintained.

Your skin acts as a radiator. As your body generates heat, the capilaries near the skin increase blood flow to the skin, heating it above your core temperature, and you lose heat through convection to the air. When it is hot enough, you also sweat. The evaporation of the sweat, cools the skin, drawing more heat from the blood. This is like spraying water on the radiator of a hot car to cool it faster. You can also lose heat through your breath.

If it gets too hot, or you run low on water and can’t sweat, your core temperature rises and your body shuts down [heat stroke].

If it is cold, your body reduces the blood flow to the skin. The capilaries in the skin shut down. Your skin may turn blue but your warm blood doesn’t get near your skin. Your skin temperature will drop below your core temperature. Of couse your skin needs blood to live, so if the blood flow is shut off for a while, it dies [frost bite]. As lone as your core temperature stays OK, you will live, but could lose ears, nose, fingers and toes.

So you can think of the capilaries as the thermostat. They open wide when it is hot, and close down when it is cold.

Also, remember that “cold-blooded” animals often have body temperatures much warmer than mammals do. A lizard that has been basking in the sun will be VERY warm, and can run around as actively as any mammal. The difference is that it doesn’t get the warmth through internal metabolism and doesn’t have to keep their core temeperature constant.

Warm-blooded and cold-blooded are really kind of vague. The important questions are whether the heat is generated internally or externally, and whether body temperature is kept constant or allowed to vary. Very large reptiles can keep a fairly constant body temperature through sheer thermal mass, although they don’t generate the heat internally. This is not a simple binary concept.

Thank you very much for your responses, which have all seemed to hit just the right amount of actual science to make things clearly understood.

I’m now wondering how and why we evolved to the temperature that we are. Is there a magic ratio between body size and ideal temperature? I have heard that some people consistantly run a little above or a little below the average 98.6 degrees. Is there a reason for this, or is it just a random deviation? Is it genetic – if I’m a little on the warm side, is it more likely that my children will be?

Or perhaps the real question is can anyone recommend a book about human physiology that would cover this that is aimed at a general audience?

I don’t know the scientific basis for answering it, but I do know my own normal temperature isn’t quite as high as 98.6, it’s probably closer to 98.2, and that my mother is like that too. Perhaps I inherited it from her, but it could be a coincidence having to do with being in the same enviromental conditions.

Human body temperature actually varies a degree or two during the course of a day. An individual’s average temperature probably depends on donzens of factors. More than likely a few of them are genetic.