Читать книгу Aging - Harry R. Moody - Страница 98

The human body may be viewed as a remarkable assembly of components functioning at various levels of organization. Systems of molecules, cells, and organs are all marvelously integrated to preserve life. The eminent nineteenth-century physiologist Claude Bernard emphasized that these integrated components act to maintain a constant internal environment despite variable external conditions. Bernard saw life as a conflict between external threats and the ability of the organism to maintain the internal milieu.

These fundamental observations have stood well the test of time. Indeed, the human organism cannot survive if the body temperature is more than a few degrees from normal, if acid-base balance is disturbed by a single pH unit, or if more than 20% of the body water is lost. Body chemicals are regulated closely, often to within 2% or 3% of an average value. A change in one direction in body constituent is often followed by a complicated set of responses that act to restore equilibrium.

Bernard also noted that living beings change from a period of development to a period of senescence or decline. He stated that “this characteristic of a determined development, of a beginning and an end, of continuous progress in one direction within a fixed term, belongs inherently to living beings.”

The regulation of bodily functions within precise limits was termed homeostasis by Cannon (1932). Living organisms under threat from an extraordinary array of destructive sources maintain their internal milieu despite the perturbations, using what Cannon called the “wisdom of the body.” Dubos (1965) has pointed out that this “wisdom” is not infallible. Homeostasis is only an ideal concept; regulatory mechanisms do not always return bodily functions to their original state, and they can sometimes be misdirected. Dubos sees disease as a “manifestation of such inadequate responses.” Health corresponds to the situation in which the organism responds adaptively and restores its original integrity.

The ability of the body to maintain homeostasis declines inevitably with decreasing organ reserve. Figure 3 shows the decline for lungs, kidneys, heart, and nerves. The decline is not the same for all individuals, nor is the decline the same for all organs. For example, nerve conduction declines more slowly than does maximal breathing capacity. And some organs, such as the liver, intestinal lining cells, and bone marrow red cells, seem to show even less decline with age.

The important point, however, is that with age there is a decline in the ability to respond to perturbations. With the decline in organ reserve, the protective envelope within which a disturbance may be restored becomes smaller. A young person might survive a major injury or a bacterial pneumonia; an older person may succumb to a fractured hip or to influenza. If homeostasis cannot be maintained, life is over. The declining straight lines of Figure 3 clearly mandate a finite life span; death must inevitably result when organ function declines below the level necessary to sustain life….