Saturday, January 2, 2010

Your Temperature Control System

In some respects, the life of a reptile has a lot of appeal. When the sun rises on a Monday morning, springing out of bed is the last thing on any lizard’s mind. Much as you may need a second or third cup of coffee before you can string a coherent sentence together, a lizard can’t do much of anything until it’s spent a long, lazy morning basking in the sun, heating its body to operating temperature.

Warm-blooded humans like you don’t work that way. Your internal temperature stays at a balmy 98.6 degrees Fahrenheit (or thereabouts). This is quite a feat, because your body continuously generates heat—primarily by your muscles contracting during routine activity and by major organs like the liver. To cool down, your body needs to release some of that heat into the air around you.

Using a design that’s the human equivalent of a hot-water radiator, your body sends warm blood to the surface of your skin so it can radiate heat away to the cooler world outside. When you need to conserve heat, your body clamps down on this process, tightening the blood vessels in your skin. That reduces the flow of blood near the skin and slows your rate of heat loss.

This system explains why people become flushed when they’re hot (it’s from the increased blood flow). It also explains how frostbite inflicts its damage. The cold itself doesn’t harm your body—instead, the extremely reduced blood flow starves your cells of the oxygen they need to survive.

One thing this system doesn’t explain is the uniquely human habit of blushing, in which sudden embarrassment causes increased blood flow and pronounced reddening, particularly in the face. Scientists guess that blushing may be an involuntary skin signal designed to solve social problems. It works like this: If you get into a sticky situation with a more dominant member of your social tribe, blushing expresses your remorse and gets you off the hook without the need for physical violence. Experts agree that the best way to deal with blushing is to announce it and accept it (for example, by saying something along the lines of, “Oh drat, I’m about to blush again!”). Trying to hide it usually triggers a cycle of increased embarrassment and increased blushing, turning the skin of a sensitive person to a distinct shade of cranberry jelly.

The body’s heat-exchange system makes perfect sense, but on its own it’s just not enough. Sure, your body can radiate heat through your skin, but on a hot day it won’t lose a sufficient amount to keep you cool. To lose heat more efficiently, you need the help of sweat.
Sweat is part of your body’s messy air-conditioning unit. Your body sweats continuously, but you don’t notice the small amounts of moisture that trickle out because it’s truly miniscule, and your body reabsorbs some of it. But when the outside temperature rises or the activity in your body soars (say, when you run to catch the last bus home), your body ramps up its sweat production.

Sweat is mostly water, with a pinch of salt and tiny amounts of other waste products thrown in. As sweat evaporates, it takes some of the heat from your skin, noticeably cooling it. (And if you don’t think it’s noticeable, try taking a hot shower and then walk around the house without drying yourself.)

But the real point of sweat isn’t to cool your skin, but to cool your blood, thereby maintaining your internal body temperature. To accomplish this, your body uses the blood redirection trick you saw on page 24. When you sweat, your body sends more blood to the newly cooled surface of your skin. The blood gets a chance to cool down, and then it gets pumped back deeper into your body. This isn’t all that different from the way a refrigerator works—it circulates a special substance (ammonia gas) through coils at the back. Once this substance cools, it’s returned to the inside of the fridge so it can keep your rutabagas fresh.

Your skin is studded with several million sweat glands. They cover every square inch of your skin, with just a few exceptions (namely, your lips, nipples, and sexual equipment). The structure of a sweat gland is simple: It looks like a coiled tube that sits in the dermis (where your body produces sweat) and opens out through a pore. Some, but not all, sweat glands squirt their liquid out onto a hair, like your sebaceous glands do. If you live in a cold or moderate climate, you can produce about one quart of sweat every hour. Move to the tropics and a few weeks later your body doubles or triples its maximum sweatproducing capacity. At the same time, your sweat becomes less salty.

Stress also causes sweating. Other than the obvious purpose (to embarrass you in your third-grade public-speaking competition), sweating in response to stress works as part of your body’s fight-or-flight response. Essentially, your body assumes that you’re either going to run away from or attack the threat in front of you, so it prepares for the imminent increase in body heat by switching on your natural air conditioning.

Say what you like about farm animals and zoo dwellers, but humans are the undisputed sweating champions of the natural world. In fact, many mammals barely sweat at all. Cats and dogs, for example, sweat only on their paws. (This is why dogs pant—they can’t cool themselves sufficiently by sweating alone. The air they inhale cools the surface of their lungs and the blood that runs nearby.) Our habit of sweating probably explains why we don’t have thick fur covering our bodies like some other animals—if we did, it would interfere with our ability to evaporate sweat from our skin.

Source of Information : Oreilly - Your Body Missing Manual

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