Chapter 7: Stress and Adaptation – The Body’s Response to Change

Maintaining this constancy relies on thousands of interconnected control systems operating primarily through negative feedback mechanisms, where the system counteracts deviations to return to a set point, such as regulating blood glucose or hormone levels. When homeostasis is severely challenged, the resulting demand activates a coordinated response mechanism called stress, which Hans Selye formalized as the General Adaptation Syndrome (GAS). The GAS proceeds through three predictable stages: the alarm stage (immediate sympathetic nervous system and HPA axis activation), the resistance stage (optimized defense selection), and the exhaustion stage (resource depletion and potential systemic damage). The physiological response is orchestrated by intricate neuroendocrine pathways, integrating the central nervous system (including the cerebral cortex, hypothalamus, and the Locus Coeruleus-NE system) with the endocrine system. Key mediators, including catecholamines (epinephrine/NE) and cortisol, are released to mobilize energy, enhance cardiovascular function, and modulate the immune system. The cumulative physiological cost of sustained or repeated activation is measured by the allostatic load. An individual's success in achieving adaptation—the ability to regain balance—is determined by conditioning factors such as physiologic reserve, time, age, health status, nutrition, sleep cycles, and psychological attributes like hardiness and social support. When stress is chronic, it contributes to various health problems, including the disabling condition of Posttraumatic Stress Disorder (PTSD), which is characterized by intrusion (flashbacks), avoidance, and hyperarousal, often accompanied by altered central neural function and abnormal cortisol dynamics. Treatment strategies focus on avoiding risky coping behaviors and utilizing effective nonpharmacologic methods, such as relaxation techniques, guided imagery, music therapy, massage, and biofeedback, to reduce sympathetic activity and musculoskeletal tension.