The document discusses Selye's General Adaptation Syndrome (GAS) model of stress. The GAS describes the biological experience of stress through three stages: alarm reaction, resistance, and exhaustion. In the alarm reaction stage, the body initially reacts as if injured through a drop in blood pressure and body temperature before activating the sympathetic nervous system's fight-flight-freeze response. The resistance stage involves the release of cortisol to shut down unnecessary functions. Prolonged stress leads to the exhaustion stage where resources are depleted, leaving the immune system weakened. The GAS was the first model to describe stress from a biological perspective but does not account for individual differences.