第八章：人类衰老的生理学

We all know what human aging looks like. A 20-year-old looks different from a 40-year-old, who looks different from a 60-year-old. Looks can be deceiving, however, when it comes to the internal physiological changes that accompany the external signs of aging. Although research on populations has shown that every physiological system declines with age, the amount of decline, the systems affected, and the age at which the decline begins are highly variable and specific to each individual. This variability in age-related physiological loss in humans occurs because aging is a random, or stochastic, process caused by a loss in molecular fidelity. Because of the random nature of aging, it is very difficult to predict the amount of age-related loss in any specific physiological system in any specific individual.

The inability to precisely define the amount and timing of human agerelated physiological loss means that we must use generalities to describe functional decline. The amount of age-related loss in a specific physiological system described in this chapter is based on averages for populations, using data from both cross-sectional and longitudinal studies. Assigning a discrete age at which a particular physiological decline begins is impossible. Although we will refer to time-dependent changes in physiology using the term “age-related,” keep in mind the following important and, as yet, nonmeasurable variables that affect the start and rate of functional decline in humans: (1) the trajectory or rate of aging may be significantly influenced by events that took place during early growth and development; (2) environment and lifestyle choices introduce significant variation in the start and rate of aging; and (3) the loss of or decline in reproductive ability seems to accelerate age-related functional loss.

In this chapter and the next, we are concerned with age-related functional decline in various human physiological systems. This chapter focuses on physiological decline that does not, in general, increase the risk of disease or mortality. Physiological systems that are more likely to develop age-related diseases leading to an increase in mortality rate or morbidity—the circulatory system, nervous system, and skeletal system—are discussed in the next chapter. Not all systems and organs are covered in this text. For example, we exclude the lungs (respiratory system) and the liver (including the gallbladder), which show extremely minor changes with aging, changes that are limited, for the most part, to alterations in their circulatory components that lead to age-related disease.