”I am Your Body”

BLOODSTREAM

     THINK BIG when you think about me. I am a transport system with 75,000 miles of route-more than a global airline. I am also a garbage man and delivery boy with 60 trillion customers - that's 17,000 times the number of people that are on earth. My customers are the cells in your body. I haul away their wastes and provide them with the essentials of life. I am your bloodstream.

You think of me as a sluggish river, and are hardly aware of the frenzied activity under way within me at all times. In the second it takes you to blink your eyelids, 1.2 million of my red cells reach the end of their 120-day life span and perish. In that same second, your marrow, mostly in your ribs, skull and vertebrae, produces an equal number of new cells. In a lifetime these bones will manufacture about half a ton of red cells. During its short life, each red cell will make something like 75,000 round trips from your heart to other parts of your body.

How do I get around your body? Your heart is my main pump - and not a very good one as far as I am concerned. It pumps in surges, and it is therefore up to my big arteries to even the irregular flow, expanding with the pumping stroke, contracting between beats, so that blood arrives as a steady stream in my extremities. By the time the blood is ready to return to the heart through my veins, pressure has dropped to near zero. Left on its own, the blood would never get back.

Yet I keep it moving-from toes back to the heart, aided by muscles outside my system. An awkward arrangement, but it works. As your leg muscles contract, they squeeze veins and push blood upward (regularly spaced valves prevent backflow). That's why walking is an excellent stimulator of circulation. (If the valves leak, the veins are apt to stretch and become clogged with clotted blood. That's a varicose vein - often painful, always troublesome.)

Basically, the fluid that flows through my intricate pipeline system consists of red cells, a bewildering array of leucocytes, or white blood cells - granulocytes, lymphocytes, monocytes - as well as platelets and a variety of other, soluble constituents such as cholesterol, sugar, salts, enzymes and fats, plus liquid plasma to float everything in. To assure safe blood volume and pressure, my liquidity must always be maintained at the proper level. To be on the safe side, I absorb virtually all the water that Joe drinks - excreting any excess via urine, sweat and exhaled air. When water is in short supply, I conserve every drop and cry for help. That's why the badly wounded always beg for water.

Everyone is familiar with my basic blood groups - O, A, B and AB. But my blood also contains a great variety of other factors (M, N, P, Rh, etc.), and new ones are identified all the time. There is a growing probability that your blood may be quite as distinctive as your fingerprints, different from all other bloods on earth. In fact, it might be possible to take a blood sample from each person in a large stadium right now, and then a year from now take another sample and assign each fan his proper seat on the basis of individual blood characteristics.

To distribute oxygen and food to cells, I operate like a municipal water-supply system. The heart pumps, blood is pushed through arteries that grow ever smaller, and finally the flow gets to the capillaries. These gossamer cobwebs, which link arteries and veins, are where the real action takes place.

Capillaries are so small that red blood cells must squeeze through single tile, occasionally even twisting themselves into odd shapes to make it. But in the second or so required for passage there is a whirlwind of activity. It's like unloading a delivery truck, and then reloading it with items no longer wanted. The big thing to be unloaded of course is oxygen, and carbon dioxide from cellular combustion that is the main waste product reloaded in its place.

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