The human body is constantly replacing and regenerating cells to maintain proper and efficient function. I understand that different cells—such as blood cells, skin cells, and bone cells—are being replaced at different rates. How often are the body's cells replaced, and how is this accomplished?
Cell replacement is accomplished by cell division, which makes more cells to replace cells that have been damaged by wear and tear or senescence (cells growing old and failing to function).
In some cases, a single cell of a certain type can divide to give rise to two daughter cells of the same type. This process allows one cell to replace cells of its own type. For example, a liver cell can divide to generate two liver cells. Another example is endothelial cells, which line the inside of our blood vessels; a single endothelial cell will duplicate itself by dividing into two cells.
In another case, a special kind of cell, called a stem cell, is used for cell replacement. These cells also divide into two daughter cells. Some of these daughter cells become stem cells that resemble the mother cell. Other daughter cells become the cell type that needs to be replaced. For instance, our skin contains stem cells called basal cells. These basal cells divide into daughter cells. Some of the daughter cells become basal cells that retain the ability to divide, and other daughter cells become specialized into skin cells. That way, some of the cells are used for cell replacement, while other cells retain the ability to divide in the future so that we don't run out of skin cells.
In still another case, a single cell called a pluripotent stem cell can divide to generateand thus replacelots of different cell types. For example, our bone marrow contains cells called hemopoietic stem cells that can divide to generate the whole range of blood cells. A single hemopoietic stem cell can divide several times to generate red blood cells as well as different kinds of white blood cells.
The rates at which cells are replaced vary quite a bit. For example, in the inner lining of the small intestine, cells turn over in a week or less. In the pancreas, the turnover time may be as long as a year or more.
Sometimes the rate of cell division to replace cells depends on the state or size of the tissue. Liver cells rarely divide, but if there is injury to the liver and the liver is somehow reduced, liver cells will divide to get the liver back to the right size. Similarly, in the skin, the basal cells will divide only to keep the skin at a certain thickness. If the outer layers of dead skin peel away more frequently, the basal cells on the inside will divide more frequently to make up for the lost skin. If a person takes good care of his or her skin and there is little damage and fewer cells to replace, the basal cells will reduce the frequency of division.
For more information on tissue maintenance, I recommend the following textbook:
Alberts, B., and others. Molecular Biology of the Cell . 3rd ed. New York: Garland Publishing; 1994. See chap. 22, pp. 1139-93.