The document discusses the physiology of stress in humans. It explains that stress is the body's response to any difficult event or stressor, whether physical, psychological, or social. The body's stress response evolved in our ancestors to help them fight or flee from threats. When stressed, the sympathetic nervous system is activated, causing changes like increased heart rate, blood pressure, and blood sugar that provide energy for fighting or fleeing. While this response was useful for surviving threats in the past, modern stressors are often chronic and the stress response can be harmful if constantly activated. Prolonged stress can negatively impact the immune system over time.

1. PHYSIOLOGY OF STRESS