Aging is the series of biological changes, involving many different processes, that over time eventually lead to functional impairment and death of an organism. It is a very basic element in all of biology from single-celled organisms to the most complex and is closely intertwined with the cell and organism function. The simple passage of time determines chronological age, but it does not determine biological aging. Each species has a different life span, and the speed of aging is paced by the length of the life span, so that a rat grows old and dies in about three years, while the average adult human has a life expectancy of about 80 years (see old age). In complex organisms there are many ways in which aging takes place, but even in single-celled organisms there are a number of changes that lead to loss of reproductive capacity and eventual cell death.
As higher organisms such as mammals age, reproductive capacity declines and is eventually lost, muscular strength decreases, reflexes are diminished and slowed, senses such as vision and hearing lose acuity, pulmonary and cardiovascular capacity decreases, and organs such as the liver, stomach, and intestine lose some of their capacity to perform their usual physiological functions. In humans some of the most marked changes are in the external features. Aging humans often have graying or loss of hair, wrinkled skin, and stooped posture.
DISEASE VERSUS AGING
One of the intrinsic difficulties of defining what constitutes aging is disentangling the alterations resulting from the passage of time and those resulting solely from disease. Elimination of many of the diseases that killed people during childhood or young adulthood has led to an increasingly long average life span in the United States. However, elimination of these diseases has not influenced the maximum potential life span of humans, which appears to be about 115 yearsÑand appears to have been constant for about the past 100,000 years.
One big question is how to determine if an alteration of body function seen in old age is actually a result of disease or aging. The simple answer is that if a process is not seen in the entire population then it is not aging but disease. However, the differentiation between disease and aging is not always simple, since there is a good deal of variation among persons in extent and time of the initial display of aging processes. It becomes somewhat arbitrary as to what percentage of the population is required to show a specific change within a specified age range in order to make it an aging process. In addition, there are some diseases that occur more frequently in older people, including some forms of cancer.
Another problem in identifying aging processes is that changes seen widely in a population may result from cultural or environmental effects. In the United States most persons add girth and weight during middle age, thus making this appear to be an aging change.
However, the same change is not seen in populations with less food or in populations with a more active lifestyle during middle age. Thus the weight gain may be pervasive and even have an impact on a person's health and life span, but it is not an aging effect, since a large-scale weight reduction and exercise program could likely alter the weight gain without changing maximum life span potential.
There are some diseases such as progeria and Down syndrome in which an accelerated aging process is seen. These individuals show external signs of aging at early chronological ages. If proliferative cells are taken from people with these diseases and tested for reproductive cycles, as in the fetal lung tissue research of the 1960s, they show a reduced number of cell reproductive cycles. Such individuals provide models of accelerated aging and may provide insight as to how aging is controlled genetically.
LIFE SPAN
There is no defined set of characteristics that will predict longevity. The environmental niche ocupied by a particular species may influence longevity in ways that are not yet appreciated. Species with higher reproductive rates tend to have shorter life spans. Species with longer life spans have a longer period of care for their young, greater intelligence, or are predators. But there are exceptions to these general rules. For example, sharks and large sea turtles appear to have life spans of beyond 100 years, yet neither animal ranks highly in intelligence or caring for their offspring.
Since the start of recorded history people have tried to increase their life spans. Research has shown that caloric restriction, but not so strong as to retard growth, produces a longer life span in rodents. How the restriction on calories prolongs life is not known, but it may be on account of stimulation or delayed development of the immune system or stimulation of the animal's cellular repair systems. These results may be related to results recently obtained on single-celled animals. Protozoa exposed to various kinds of stress (low-dose radiation, reduced oxygen supply) lived longer than normal. It has been speculated that the stress may have either induced a greater than normal effort at DNA repair or that it pushed these organisms to a super (or "reserve") life span that would help the cell outlive an ecological crisis.