Selye's General Adaptation Syndrome Experiments

In 1936 the endocrinologist Hans Selye, while conducting unrelated experiments injecting laboratory rats with ovarian extracts in search of a new hormone, noticed that the rats developed a consistent pattern of physiological changes, including enlargement of the adrenal glands, shrinkage of the thymus and lymph nodes, and ulceration of the stomach lining, regardless of which specific substance had been injected. Selye went on to show experimentally that this same pattern of bodily changes appeared in response to a wide range of different harmful or demanding stimuli, including cold exposure, surgical injury, forced exercise and other physical stressors, leading him to conclude that the body mounts a broadly similar, nonspecific physiological response to almost any sufficiently severe or prolonged demand placed upon it. Selye named this pattern the general adaptation syndrome and described it as unfolding in three stages: an initial alarm stage resembling a fight-or-flight response, in which the body mobilizes its defenses; a subsequent resistance stage, in which the body adapts to the ongoing stressor and appears outwardly normal while its physiological resources remain under strain; and, if the stressor persists long enough, an exhaustion stage, in which the body's adaptive resources are depleted and illness or organ damage can result. Selye's experiments and his general adaptation syndrome model established stress as a coherent biological and psychological construct in its own right and became a founding influence on the later field of health psychology and the study of stress-related illness.