Final answer:
Stressful situations trigger physiological changes through the interaction of the nervous and endocrine systems, initiating the fight-or-flight response and releasing stress hormones like cortisol.
Step-by-step explanation:
When we experience situations as stressful, physiological changes occur in our bodies most notably due to the interaction of the endocrine system and the nervous system. The fight-or-flight response, often associated with the initial reaction to stress, is mediated by the sympathetic nervous system and the adrenal medulla. This response was crucial for survival in our evolutionary past as it helped organisms deal with immediate threats by preparing them to either fight or flee. Modern-day stresses, which often do not involve physical threats, still activate this response, leading to increased heart rate, respiration, and various other physiological changes. Concurrently, the hypothalamic-pituitary-adrenal (HPA) axis, part of the endocrine system, gets activated, releasing stress hormones like cortisol, which can affect virtually every organ system, including a notable impact on suppressing the immune system.