the sum total of the nonspecific defense reactions that arise in the organism of animals and man upon the action of external and internal stimuli of considerable strength and duration. These reactions promote the restoration of disturbed equilibrium and are aimed at maintaining homeostasis—the constancy of the internal environment of the organism.

The concept of the adaptation syndrome was proposed by the Canadian scientist H. Selye in 1936. The factors evoking the adaptation syndrome—for instance, infection, abrupt temperature changes, physical and mental trauma, a high muscle load, loss of blood, ionizing radiation, and the action of many drugs—are called stressors, and the condition of the organism that develops during their action is known as stress. The main symptoms of the adaptation syndrome are an increase in the size of the adrenal cortex and intensification of its secretory activity, decreases in the size of the thymus, spleen, and lymph nodes, changes in blood composition (leukocytosis, lymphopenia, and eosinopenia), a disturbed metabolism (with a prevalence of degradation processes and leading to loss of weight), and a decrease in blood pressure.

The adaptation syndrome develops in two or three stages. The first stage, or alarm reaction, lasts six to 48 hours and is divided into phases of shock and counter-shock; during this stage, increased amounts of adrenal hormones (glucocorticoids and adrenaline) are produced and enter the blood, and the body reorganizes and adapts to the difficult conditions. The second stage is resistance, during which the immunity of the organism to various influences is greater; by the end of this stage, the condition of the organism is normalized and recovery sets in. If the action of the stimuli is of great strength and duration, the third stage, exhaustion, sets in; this stage may end in the death of the organism.

Reflex processes (defense, vasomotor, and other reflexes) are the initial component of the body’s adaptation to unusual conditions; then humoral stimuli (adrenaline, histamine, and decomposition products of damaged tissues) go into action via the blood, lymph, and so on. All this triggers the mechanisms that ensure the adaptive reaction of the organism, primarily the reticular formation of the brain and the system hypothalamus-hypophysis-adrenal glands. The cells of the hypothalamus produce a releasing factor, under the influence of which the formation and release into the bloodstream of adrenocorticotropin by the hypophysis is increased; the adrenocorticotropin stimulates the activity of the adrenal cortex—that is, the production of glucocorticoids. Simultaneously, other humoral and neural mechanisms and the nervous system as a whole are drawn into the reaction.