As cells age, they function less well. Eventually, old cells die as a normal part of the body’s functioning. Old cells sometimes die because they are programmed to do so. The genes of cells program a process that, when triggered, results in the death of the cell. This programmed death, called apoptosis, is a kind of cell suicide. The aging of a cell is one trigger. Old cells must die to make room for new cells. Other triggers include an excess number of cells and possibly irreparable damage to a cell.
Old cells also die because they can divide only a limited number of times (remember Hayflick limit). This limit is programmed by genes. When a cell can no longer divide, it grows larger, exists for a while, 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.
An interesting point of knowledge
Disorders and disease, not aging, are usually responsible for the most loss of function.
How organs function depends on how the cells within them function. Older cells functionless. In some organs, cells die but are not replaced, and the number of cells decreases. The number of cells in the testes, ovaries, liver, and kidneys decreases significantly as the body ages. When the number of cells becomes too low, an organ cannot function normally. Not all organs lose a large number of cells. The brain is an example. Healthy older people do not lose many brain cells. Substantial losses occur mainly in people who have had a stroke or who have a disorder that causes the progressive loss of nerve cells (neurodegenerative disorders), such as Alzheimer disease or Parkinson disease.
A decline in one organ’s function, 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 function less because blood flow to them is decreased. This is considered the domino effect.
The eyes, followed by the ears, begin to change early in mid-life. Nearsighted and limitation in hearing are a slow but steady change of which nearly everybody is confronted. These signs are often only discovered in routine controls. But this has an interesting side effect. Nearsightedness or myopia is a refractive defect of the eye lens through which the image is formed in front of the retina, not on it. This defect can have people seeing closer objects clearly while distant ones are blurred. Farsighted is simply the opposite. When aging the retina is stiffening and the light enters it in a different way. Nearsighted people slowly get a better sight and glasses gradually lose strength. On the other side, may need reading glasses which takes away the advantage made.
Most internal functions decline with aging. However, even with this decline, most functions remain adequate because most organs start with considerably more functional capacity than the body needs (functional reserve). For example, if half the liver is destroyed, the remaining tissue is more than enough to maintain normal function.
Even though most functions remain adequate, the decline in function means that older people are less able to handle various stresses, including strenuous physical activity, extreme temperature changes in the environment, and disorders. This decline also means that older people experience side effects from drugs. Some organs malfunction under stress while others continue to function at the same level. These organs include the heart and blood vessels, the urinary organs (such as the kidneys), and the brain.