”I am Your Body”

SKIN

     YOU THINK OF ME, your skin, as a not too interesting sausage wrapper, an inert parchment demanding much shaving, bathing, scratching, anointing and giving little. How wrong you are. I am an absolute essential.

I do jobs you never dreamed of. You doesn't think of me as a manufacturer of intricate chemicals, but I am. I produce at least one important vitamin-vitamin D-and activate the sex hormone, testosterone, produced by a males testes. I help regulate blood pressure. I keep water in (you would quickly die if I didn't). I also keep water out (you can swim for hours without becoming waterlogged). My complex nervous system detects pain, touch, heat, cold, and instantly passes findings along to your brain. I have been called the "frontier" of your body. But maybe "rampart" would be better, for I protect against a mighty horde of potentially deadly invaders-bacteria-that live or land on my surface.

I come in many forms. Your fingernails and toenails, the hair on your head, the callus on your sole, the wart you once had on a finger. And I am made up of three layers: the outer epidermis, the middle dermis, and the bottom subcutaneous tissue.

In most places around your body, my outer layer is paper-thin. You can see for yourself next time you burn a finger; my epidermis is the transparent tissue on the top of the blister. You can shave off a callus without drawing blood because there is no blood supply in my epidermis. Its cells are nourished by diffusion from below.

While a snake sheds its skin in dramatic fashion, the shedding of my epidermis is a slow, steady process. Each day many millions of baby epidermal cells are formed in the innermost part of my epidermis and begin pushing their way outward, changing as they go from jellylike cellular material to harder, horny keratin. My keratin layer consists of flattened, shingly cells-all dead. (Fragile living cells couldn't survive exposed to the hostile outside.) Each day millions are washed away when you shower, or are rubbed away by clothing. Thus you get a new outer skin every 27 days-the birth-to-death span of these.

Functionally, there isn't a great deal to be said for my fatty subcutaneous portion. It acts as a shock absorber to protect internal organs, serves as an insulator to conserve body heat and is responsible for pleasing body contours-more important to women than to men. Some experts don't consider this layer part of me at all-subcutaneous actually means "beneath the skin."

Look now at my tough "hide," or dermis. It is the strong but elastic envelope that holds everything together-that keeps vessels, fat, etc. from bulging or falling out. The dermis contains an intricate collection of nerves, blood vessels and glands. The mix varies at different places in your body. But under an average square centimeter of surface-an area the size of your little fingernail and about 1/8 inch thick - there will be some 100 sweat glands, 12 feet of nerves, hundreds of nerve endings, 10 hair follicles, 15 sebaceous glands and 3 feet of blood vessels!

My intricate network of blood vessels is particularly interesting. When you exercise on a hot day and these blood vessels dilate; you become flushed. I am trying to radiate heat to the outside to get rid of it. On a cold day the reverse takes place. My vessels shut down, detouring blood to the interior of your body; you turn pale. My blood vessels are also under the command of emotions. When angry, you flush-I've opened up the blood vessels of your face. Fear shuts them down-you get cold feet.

lt's no news, of course, that evaporation of sweat cools the body. But that hardly tells the full story of my complex air conditioning system. Let body temperature vary more than a few degrees from a normal 98.6 degrees, and you are a goner. To avoid this, I have a staggering number of sweat glands-two million, spread around your 18 square feet of body surface. Each is a tightly coiled little tube buried deep in my dermis, with a 50 inch-long duct rising to the surface. Tiny as they are, I have a total of six miles of these ducts.

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