Mike Underwood, age 12, of Huntsville, Alabama, for his question:
How many cells are in a man's body?
Robert Hooke, an Englishman, first identified the cell as a unit. He was the top microscopist of his day. The discovery was made 304 years ago with a thin slice of cork magnified by the best existing microscope. Hooke never realized what miracles his cell unit held for future scientists.
About six million cells are crowded together in one square inch of human skin. If the skin covering the body of an average sized man could be spread out flat, it would cover a rug one yard wide and two yards long. This is a total of 2,592 square inches of skirl, each with an average quota of six million cells. In a drop of human blood, there are more than five million floating cells. The average man has about five quarts of blood. It contains a total of about 25 trillion red cells, 1 1/2 billion platelets and some 35 billion assorted white cells.
The average weight of a man's brain is about three pounds. It is linked to about five ounces of nerve cells that spread throughout the body. The brain and its nervous system contain about 10 billion cells. Blood cells and nerve cells perform very different duties. But the body needs dozens of other kinds of specialized cells to work together as a living organism. The billions of liver cells are different from the billions of lung cells. A man has more than 600 muscles made of special muscle cells. His body is supported by a skeleton of 206 separate bones, and each bone is built from stacks of special hard walled cells.
The entire human body is built from cells too small for the human eye to see. Most scientists measure them in micron ,units which come 25,000 to an inch. It would, of course, be impossible to count the total cells in a man's body, one by one. But it is possible to estimate the total number. The cells in a small sample of an organ can be counted and the number multiplied by the size of the entire organ. For example, you would find six million cells in a square inch of skin. The total number of skin cells would be this number multiplied by the total area of skin, which is about 18 square feet. The same estimating job can be done with the liver, the muscles, the intestines and every other part of the body.
The final adding fob is stupendous. There are 25 trillion red cells, so the final number must run into the trillions. And,since we are estimating it from small samples, we cannot expect it to be perfectly accurate. Several researchers have tried to do the fob and though no two estimates are exactly alike, they do not vary by more than a trillion or so. In round figures, the total number of cells in a man's body is estimated to be about 60 trillion.
Most of these cells, remember, are busy laboratdkries::of chemical activity. Most of them wear out and are replaced by new ones. Certain intestine cells work themselves to death in about 36 hours. A white blood cell is replaced with a new one after 13 days, a red cell has a life expectancy of three to four months.
Nerve cells, however, are built to last a century. They cannot be replaced and they work as hard or harder than any other type of cell in the body. The fiber of one nerve cell may be six feet long, reaching all the way from the toes to the brain. It may be in contact with 25,000 other nerve cells. It can pulse signals at 200 miles an hour and the entire nervous system can cope with 36 trillion impulses an hour. Yet a lifetime of this busy communication work does not wear out the body's durable nerve cells.