Differential Timetables of Aging in Organs and Systems

Changes with aging lack uniformity, not only among individuals of the same species but also within the same individual: onset,

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Aging Organ Reproduction
Avoid disease

FIGURE 5 Successful or healthy aging. Successful aging is based on maintenance of mental and physical function, avoidance of diseases, and enjoyment of life.

rate, and magnitude of changes vary depending on the cell, tissue, organ, system, or laboratory value considered (28). A number of laboratory values, many of which often may remain unchanged with aging, or may decrease or increase, depending on the individual examined, are illustrative of the heterogeneity of changes with aging (Table 4). Laboratory values are usually presented as having a normal range. Older persons often have values in the extremes (high or low) of normal range (30).

In whichever organ and system considered, timetables of aging represent an approximation, for the onset of aging cannot be pinpointed precisely by any specific physiologic sign (such as menarche for ovarian maturation). A classic example of a unique timetable involving an organ that develops and ceases to function during a specific period of the life span is the ovary; its function begins at adolescence (in humans, approximately 10-12 years) and stops at menopause when ovulation ceases (in humans, approximately 50 years) (Chapter 10).

Aging is often a slow and continuous process, and, therefore, some of its effects are observed only when they have progressed sufficiently to induce changes that can be identified and validated by available testing methods. An illustrative example is atherosclerosis. Atherosclerotic lesions begin in childhood, when they remain functionally silent. The lesions become manifest in middle and old age, when the lesions have progressed sufficiently to induce pathological consequences that affect the entire cardiovascular function (Chapters 15 and 16).

Fasting blood sugar values are minimally affected by aging, with the exception of late-onset diabetes (insulin-resistant diabetes), where blood sugar levels are elevated (Chapter 13). However, even in the nondiabetic elderly, when blood sugar levels are tested under increased physiologic demand (e.g., a sugar load, as in the sugar tolerance test), the efficiency with which the organism is capable of maintaining levels within normal limits and the rapidity with which these levels return to normal are significantly reduced in old as compared to adults (Chapter 13). Other examples such as nerve conduction velocity, cardiac index (cardiac output/min/m2 of body), renal function (filtration rate, blood flow), and respiratory function (vital capacity, maximum breathing capacity) are less capable of withstanding stress in old compared to young individuals. An important general point, highly illustrated by the above examples, is that exposure to stress reveals age differences not otherwise detectable under steady-state conditions; it clearly demonstrates the declining ability of the old organism to withstand or respond adequately to stress (Chapter 9).

Other body functions begin to age relatively early in adult life. For example, deterioration of both vision (especially accommodation) and hearing begins in the teens and continues to progress until around 60 years of age (Chapter 8). Auditory deterioration may also be hastened by the environmental noise to which, in our civilization, individuals are often continuously exposed from young age. Some comparative studies in isolated populations living in a quiet environment (e.g., forest-dwelling African tribes) and who have maintained good auditory function into old age seem to support the view that continuous exposure to noise is harmful to hearing (Chapter 8).

Differential aging also includes declines in stature (or standing height), sitting height, breadth of shoulder, and depth of chest—all of which show a progressive reduction with aging in contrast to head diameters, which remain practically unchanged. The considerable and progressive diminution in stature that occurs with aging in all humans may be ascribed, at least in part, to alterations in bone structure (e.g., osteoporosis) (Chapter 20).

Body weight usually increases at the initial stages of senescence as a consequence of increased fat deposition

TABLE 4 Some Laboratory Values in Old Age

Unchanged Decreased Increased

TABLE 4 Some Laboratory Values in Old Age

Unchanged Decreased Increased

Hepatic function tests

Arterial blood test

Serum albumin

Alkaline phosphatase

Serum bilirubin

pH

HDL cholesterol (women)

Uric acid

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