”I am Your Body”

THIGHBONE

    
YOU THINK OF US bones as dead material, simply an inert Tinkertoy framework for your living body. To a degree you are right. Without us you would collapse into a blob of jelly, unable to walk, talk or eat. But we are anything but dead or inert. We are organs, with a host of responsibilities beyond supporting your body. We contain virtually all of the body's mineral supply - 99 percent of your calcium and 88 percent of your phosphorus for example, plus smaller amounts of copper, cobalt and other essential trace elements. As a high-turnover warehouse we operate 24 hours a day, moving inventory in and out.
We also have a busy manufacturing division - our marrow. In a single minute, 180 million of your red blood cells die of old age. Your spleen and your liver supply a few replacements, but the vast bulk come from us. In the spongy interiors of our marrow chambers we also produce most of the white blood cells that protect you from infection.
I am your right femur, or thighbone. I will speak for the other bones because I am the largest, longest and strongest - strong enough, in fact, to bear the weight of a compact car. We bones are a big family. There are 206 of us in your body - some people have more, some less. As a matter of fact, you had more bones as a child than you do today. When born, you had 33 vertebrae in your spine. Then four of the lower ones fused to make the coccyx and five fused to form the sacrum. You might have had 11 pairs of ribs, or 13. Actually, most people have 12.

We bones come in all sizes and shapes: from the tiny stapes bone in your middle ear (which makes hearing possible) all the way up to me. My job, shared with my partner in your left leg, is to carry your body weight. We bones are bound together by ligaments. Tendons hook us to muscles; like strings on marionettes, they make motion possible.

We are made of two basic types of tissue: cancellous, which is light and porous, and compact, which is dense and superbly strong. Your spine and pelvis consist mainly of the first type, while I am primarily of the latter, as are other leg and arm bones. We discovered, millions of years before today's construction geniuses caught up with us, that weight for weight, a tube is stronger than a rod. This principle makes me, ounce for ounce, stronger than a solid piece of steel.

When you were born your bones were softer. This, of course, facilitated birth. The intricate process of calcification made them harder. We bones contain millions of cells called osteoblasts, which extrude afibrous protein - firm but pliable - called collagen. Among the fibers are minute air spaces, containing a gluelike mixture called ground substance. As these spaces fill in with microscopic particles of mineral (mainly calcium, phosphorus and carbonate) bone is formed. This job completed, your legs became strong enough to support you.

During your childhood, we bones had to support your body and grow ourselves - quite a task, something like enlarging a house without disturbing the occupants. During this period, certain areas at the ends of the long bones were composed of soft cartilage. New cartilage continued to form while the older, inner levels hardened into bone. But as you matured, those areas of cartilage solidified and no further growth was possible.

While we can't grow in length, like muscles we can enlarge or lose mass, grow stronger or weaker. Take up weight lifting and I would become stronger, denser, and thicker; if you lie in bed for several months and I would weaken.

My role in storing and releasing calcium is crucial. It is via the blood that I transact all of my business - I have, of course, my own surprisingly rich supply of blood vessels. I expose my mineral crystals to the current, plucking excess calcium from the blood or supplying it when there is a lack. The surface of crystal we bones expose to the bloodstream is vast; all flattened out, it would cover 100 acres of land!

Our hoard of calcium is relatively enormous - 2.2 pounds of it. But at any given moment there is only 1/40 ounce of calcium circulating in your bloodstream. Yet this tiny amount plays a key role. Without it, no impulses would travel along nerves, and blood would refuse to clot. Muscle contraction would cease - and so would your heartbeat. Too much calcium can be just as serious, possibly contributing to the formation of kidney stones. Next steps: uremic poisoning and death.

I mention these grisly facts simply to indicate how important it is that I have a supply of calcium ever ready, and that I feed exact amounts of it into your blood. The principal controls are glands in your neck. If blood levels of calcium drop, the parathyroids start secreting a hormone - my go signal. Too much calcium, and a hormone from your thyroid causes me to absorb calcium.

You thinks the only problem we bones face is fractures. As a matter of fact, being broken is generally a minor worry. Breaks come in four basic forms: closed, where the break is clean and the bone does not protrude through the skin; open, where it does protrude; green-stick, where the bone splits longitudinally without breaking entirely; and comminuted, where bone is shattered into small fragments.

Until fairly recently, breaks were treated mainly with "plaster and time." For the elderly, six months in bed with a broken hip often meant general deterioration, pneumonia, even death. Today's orthopedic surgeons aim to get people out of bed as quickly as possible. To facilitate bone patching, they have pins, screws, and plates. A frozen elbow, finger or knee? There are artificial joints. A shattered hip? Install a new ball and socket, very often made of plastic and metal. Does a girl think she is too tall? They can shorten stature by removing up to three inches of me. However, shortening of leg bones is not standard. Too short? Bones can be lengthened, but the procedure is very difficult.

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