CHAPTER 8: COMMON FUNCTIONAL LOSS ASSOCIATED WITH AGING

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 shows some decline 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 time-dependent physiological loss in humans occurs because aging is a random or stochastic process caused by a loss in molecular fidelity (recall that stochastic means that a process has a random probability distribution or pattern that can be analyzed statistically but may not be predicted precisely). Because of the random nature of aging, it is very difficult to predict the amount of time-dependent loss in any specific physiological system in any specific individual.

The inability to precisely define the amount and timing of human time-dependent physiological loss means that we must use generalities to describe functional decline. The amount of time-dependent 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. Moreover, keep in mind as you read that time-dependent function decline in humans can be influenced by (1) events that took place during early growth and development; (2) environment and lifestyle choices that influence the start and rate of aging; and (3) acceleration of time-dependent functional loss caused by the loss of or decline in reproductive ability.

In this chapter and the next, we are concerned with time-dependent functional decline in various human physiological systems. This chapter focuses on physiological decline that does not, in general, increase the risk of disease. Physiological systems that are more likely to develop time-dependent diseases leading to an increase in mortality rate—the circulatory system, nervous system, and skeletal system—are discussed in Chapter 9. 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 in these systems are more likely to be a function of environmental insult such as smoking and alcohol abuse than a result of time.