Physiological Adaptation

an aggregate of physiological reactions which forms the basis for the adaptation of the organism to changes in environmental conditions and is aimed at maintaining homeostasis, a relative stability of its internal environment. As a result of physiological adaptation, the organism becomes more resistant to cold, heat, deficiency of oxygen, changes in barometric pressure, and other factors. The study of physiological adaptation is of great importance for understanding the self-regulatory processes of the organism and its interaction with the environment. Investigations of physiological adaptation became of great practical interest in connection with manned space flight. The responses of the organism to stimulations of considerable intensity have common nonspecific features and are called the adaptation syndrome. The process of physiological adaptation to unusual and extreme conditions passes through several stages or phases. At first, decompensation (impairment of functions) prevails, then incomplete adaptation (the organism’s active search for stable conditions corresponding to the new environmental conditions), and, finally, the phase of rather stable adaptation. This can be well observed, for example, in the case of adaptation to high altitudes. The changes in conditions in this case are complex, but most important is the insufficient partial pressure of oxygen connected with the overall decrease in barometric pressure. During ascent to high altitudes, one observes dizziness, disturbances of visual and auditory perceptions, dyspnea, and other phenomena characteristic of altitude sickness. As a result of physiological adaptation, the decompensation phenomena gradually subside, and adaptation to these unusual conditions sets in: the number of erythrocytes increases (in man, from 4–5 million to 8 million per cubic millimeter); the oxygen-binding capacity of the hemoglobin rises; pulmonary ventilation intensifies; and cardiac activity, the condition of the nervous system, and other functions become normalized.

The changes occurring in the organism during physiological adaptation involve all its levels, from the subcellular molecular level to the organism as a whole. Of considerable importance in physiological adaptation is training, both for the action of each specific factor and for environmental changes in general. Thus, training for high-altitude conditions, for the effects of acceleration, and for other conditions helps astronauts survive overloads during space flights; trained athletes cope better with new difficult conditions, such as enforced immobility.

Extremely important in physiological adaptation are the reactivity of the organism and its initial functional condition (age, level of training, and so forth); the responses of the organism to various influences change in relation to these factors. In physiological adaptation the plasticity of the nervous system is manifested, permitting the organism to reestablish contact and equilibrium with changed environmental conditions.

Under the influence of repeated and rather long-lasting action of extreme conditions, compatible with normal vital activity, an adaptive reorganization of functions sets in, which expands the organism’s range of existence. However, the variations of environmental conditions within which physiological adaptation can occur have definite limits characteristic of each species and each individual organism. The mechanisms that elucidate the process of physiological adaptation also make it possible to understand, to a certain extent, the adaptation of organisms during evolution. The return of the organism to its original condition after physiological adaptation is called deadaptation.

Of great biological importance is the physiological adaptation of the analyzers (sometimes called adaptation of the receptors or sense organs) to the action of specific stimuli—for example, the adaptation of the visual analyzer to light or darkness, that of the auditory analyzer to sound, that of the cutaneous analyzer to mechanical and temperature stimuli, and that of the vestibular apparatus to rotation. Physiological adaptation of the analyzers is connected with changes in the sensitivity of the peripheral sense organs (the receptors) and with processes occurring in the central nervous system. Thus, light adaptation, caused by prolonged exposure to bright light, leads to a decrease of the light sensitivity of the eyes, and dark adaptation leads to its increase. In darkness, the sensitivity of the eyes to light increases many thousandfold within an hour; this change is due both to the restoration of visual pigments and to changes in the neural elements and nerve cells of the cerebral cortex. Physiological adaptation in the auditory analyzer is manifested in an increased threshold of stimulation under the influence of high-intensity sound. A gradual change in sensitivity—that is, physiological adaptation—is also observed during the action of cold, heat, and other stimuli on the skin. Very important in this process is the rate of increase of the intensity of the stimulus.