The aging process may vary considerably both between individuals and among different species. Over the years, innumerable theories have been proposed in an attempt to explain this phenomenon. While none of these theories conclusively defines the physiological mechanisms of aging, many do seem plausible. Of the mammalian species, the human life span is greatest. In fact, in recent decades, entire populations of humans have been increasing in average age. This shift in age distributions has occurred primarily in the world's developed countries. Undoubtedly, such change will one day have implications for health care providers. In nursing, for example, a re evaluation of health care priorities could become necessary. The term, "senescence," can be defined as a "decline in the functional capacity of the body that occurs with age" (Mera, 1992, p. 271). Rates of aging are generally highly variable. Among mammals, life span ranges from a low of a few months to a high of 80 years or more (Charlesworth, 1003, p. 11). In addition, even within species, individuals of the same chronological age may show widely different senescence changes in various anatomic regions.
Since the beginning of recorded history, many theories for aging have been proposed. For example, Hippocrates thought that old age was caused by an imbalance of the body's humors (e.g., blood, phlegm, and bile) with a gradual loss of body heat. Since the time of the Greeks, several additional aging theories have been postulated. Each of these various hypotheses can be classified according to the following four major theoretical subgroups: biological clock or programmed theories, immunological theories, DNA damage theories, and other cellular component (OCC) damage theories. Despite their differences though, each of the individual aging theories is in some way related to the others (Morse & Rabinowitz, 1989, p. 5). One theory of aging, the "clinker" the...