HEALTH AND DISEASE AS STATES OF ADAPTATION

The ability of the body to adapt both physically and psy­chologically to the many stresses that occur in both health and disease is affected by a number of factors, including age, health status, psychosocial resources, and the rapidity

with which the need to adapt occurs (see Chapter 9). Gen­erally speaking, adaptation affects the whole person. When adapting to stresses that are threats to health, the body uses those behaviors that are the most efficient and effective. It does not use long-term mechanisms when short-term adaptation is sufficient. The increase in heart rate that ac­companies a febrile illness is a temporary response de­signed to deliver additional oxygen to tissues during the short period that the elevated temperature increases meta­bolic needs. On the other hand, hypertrophy of the left ventricle is a long-term adaptive response that occurs in persons with chronic hypertension.

Adaptation is further affected by the availability of adaptive responses and the ability of the body to select the most appropriate response. The ability to adapt is depen­dent on the availability of adaptive responses—the greater number of available responses, the more effective the ca­pacity to adapt. Adaptive capacity is decreased with ex­tremes of age and with disease conditions that limit the availability of adaptive responses. The immaturity of the infant impairs the ability to adapt, as does the decline in functional reserve that occurs in the elderly. For example, infants have difficulty concentrating urine because of the immaturity of their renal tubular structures and there­fore are less able than an older child or adult to cope with decreased water intake or exaggerated water losses. Simi­larly, persons with preexisting heart disease are less able to adapt to health problems that require recruitment of cardiovascular responses. Adaptation also is less effective when changes in health status occur suddenly rather than gradually. For instance, it is possible to lose a liter of blood through chronic gastrointestinal bleeding without devel­oping signs of shock. On the other hand, a sudden hem­orrhage that causes the loss of an equal amount of blood is apt to produce hypotension and circulatory shock. Even in advanced disease states, the body retains much of its adaptive capacity and is able to maintain the internal envi­ronment within relatively normal limits.