Adapation is a biological umbrella term describing all forms of function regulation in the body in normal conditions and in pathology. In essence, life itself is continuous adaptation of the individual to the constantly changing environmental conditions. The adaptation of a biological species consisting of individuals depends on how pronounced is the ability of individuals to adapt to the changing environmental conditions. This is a complex of constantly self-regulating processes that allow a biological species to adapt to changing conditions of existence. All individual organisms that constitute a species possess these specific adaptive reactions, which manifest both in conditions of health and disease.
Adaptive reactions develop in both healthy and diseased individuals and are aimed at preserving homeostasis and adapting to new living conditions. However, health and illness are individual phenomena, and in a particular person, specific adaptive reactions are refracted through his/her features related to reactivity, age, gender, living conditions, etc. The organism must constantly adapt to changing temperature, atmospheric pressure, radiation effects, and the changing spectrum of microorganisms and viruses. Using reactivity, the body adapts to the external environment, preserving the homeostasis characteristic of all representatives of this species, and at the same time, changes in reactivity and homeostasis are stages in the pathogenesis of any disease. Adaptation is also necessary in those situations when physiological or pathological stress of an organ or system function occurs, as well as when reduction or perversion of these functions takes place. For example, prolonged physical activity in humans leads to growing mass of muscles including the myocardium, increasing heart force and lung volume, etc. In a prosthesis-dependent person with an amputated leg, a complex of adaptive processes develops in the skeleton and in certain muscle groups, allowing him/her to correct the altered body’s center of gravity; changes also occur in the vestibular apparatus and local circulation in the preserved part of the limb. Therefore, the problem of adaptation is not only biological, determining health, but also medical, i.e. the problem of pathology.
Compensation is a combination of the body’s responses to injuries or diseases, aimed at restoring impaired functions. Therefore, compensation is a type of adaptation developing in disease, therefore it is individual, since only a certain individual has a certain illness. Therefore, compensation processes are more specific than adaptation, and relate to these as a part to a whole.
The criterion of sufficiency or insufficiency of compensatory reactions is the degree of organ function restoration. In an individual, personal reactions as well as species-specific adaptive reactions are present. During a disease, both types of reactions are aimed at restoring homeostasis and recovery and can hardly be separated. Therefore, in clinical practice, both are often referred to as compensatory-adaptive processes. However, these concepts are not identical, since adaptation usually occurs in physiological conditions, and it is adaptive reactions that allow avoiding the disease and exclude the need for developing compensatory reactions. Moreover, while the biological significance of compensatory reactions is to restore the disturbed functions of organs and systems, restoration of function is not necessary for adaptive processes. For adaptation, the primary goal is to handle changes of living conditions related to the external and internal environment. Therefore, adaptive reactions are a necessary attribute of an individual’s entire life.
In general, the biological expediency of both adaptation and compensation is to restore homeostasis. The same goals determine the doctor’s actions since ancient times. However, there is a certain antagonism between compensation-adaptation mechanisms and the doctor’s actions. It lies in the fact that a pathological process is masked for a long time by the compensation and adaptation reactions of the body, containing the disease but not eliminating it. This makes the process invisible to the doctor and the patient, and thereby prevents timely diagnosis. This is the dualism and dialectics of adaptive and compensatory reactions. Therefore, one of the main tasks of a doctor is the need to discover the essence of disease, unmasking the mimicry of compensation and adaptation responses that may be a barrier to early diagnosis of human diseases.
The entire diversity of adaptive and compensatory changes of body organs and tissues in normal and pathological conditions can be brought down to four main reactions: atrophy, hypertrophy, regeneration and tissue reconstitution.