What is general adaptation syndrome by Hans Selye?

General Adaptation Syndrome (GAS) is a model proposed by Hans Selye to describe the body's physiological response to stress. It consists of three stages: the alarm reaction, where the body recognizes a stressor and initiates a fight-or-flight response; the resistance stage, where the body adapts to the stressor and attempts to maintain balance; and the exhaustion stage, which occurs when the body's resources are depleted, potentially leading to health issues. Selye's work highlights how prolonged stress can impact physical and mental health.