The body changes with aging because changes occur in individual cells and in whole organs. These changes result in changes in function, in appearance, and thus in the experience of aging.

The Aging of Cells

As cells age, they function less well. Eventually, they must die, as a normal part of the body's functioning.

Cells may die because they do not divide normally or because they are damaged. Cells may be damaged by harmful substances in the environment, such as radiation, sunlight, and chemotherapy drugs. Cells may also be damaged by certain by-products of their own normal activities. These by-products, called free radicals, are given off when cells produce energy.

Many cells die because the genes they contain program a process that, when triggered, results in death of the cell. This programmed death, called apoptosis, is a kind of cell suicide. Reasons for cell suicide include replacing old cells with new ones and eliminating excess cells.

Also, cells die because they can divide only a limited number of times. This limit is also programmed by genes. When a cell can no longer divide, it grows larger, exists for a while, and then dies. The mechanism that limits cell division involves a structure called a telomere. Telomeres are used to move the cell's genetic material in preparation for cell division. Every time a cell divides, the telomeres shorten a bit. Eventually, the telomeres become so short that the cell can no longer divide. The telomeres of cancer cells, unlike those of normal cells, do not shorten each time the cell divides. Consequently, cancer cells can divide forever.

The Aging of Organs

How well organs function depends on how well the cells within them function. Older cells function less well. Also, in some organs, cells die and are not replaced, so the number of cells decreases. The number of cells in the testes, ovaries, liver, and kidneys decreases markedly as the body ages. When the number of cells becomes too low, an organ cannot function normally. Thus, most organs function less well as people age. However, not all organs lose a large number of cells. The brain is one example. Healthy older people do not lose many brain cells. Substantial losses occur mainly in people who have had strokes or who have Alzheimer's disease or Parkinson's disease.

A decline in one organ's functioned, whether due to a disorder or to aging itself, can affect the function of another. For example, if atherosclerosis narrows blood vessels to the kidneys, the kidneys functions less well because blood flow to them is decreased.