The body responds to stress in certain physiological ways. When a threat or danger is perceived, the body responds by releasing hormones that will prepare it for the fight-or-flight response. The main physiological structure involved in this response is known as the HPA (hypothalamic–pituitary gland–adrenal gland) axis. Such interactions of the endocrine hormones have evolved to ensure that the body’s internal environment remains stable; however, stress can disrupt this stability. Stimuli that disrupt homeostasis in this way are known as stressors.
Source: How the Body Responds to Stress