I often experience headaches and a rapid heartbeat when under stress. This is due to the fight-or-flight response of the body during stressful events.
Stress stimulates nerve impulses from the hypothalamus to the sympathetic division of the nervous system, including the adrenal medulla. The adrenal medulla secretes epinephrine and norepinephrine, and these hormones supplement and prolong the fight-or-flight response. Moreover, in this response, huge amounts of glucose and oxygen are brought to the brain, muscles, and heart in order to quickly mobilize the resources of the body for immediate activity. The brain becomes highly alert, the heart pumps blood vigorously, and the muscles become tense.