Stress is the response of the body to a threat. It initially causes a sympathetic response. Epinephrine is secreted by the adrenal medulla in response to hypothalamic innervation and norephinephrine is secreted by the sympathetic division of the ANS. This alarm stage causes increased heart rate, cardiac output, and blood pressure, and diverts blood flow to the heart and skeletal muscles in order to overcome or flee from the threat.
If the stressor is not removed, the body enters the resistance stage. To maintain a state of alert, long-term metabolic adjustments are made for increased energy needs. This resistance stage is driven by cortisol secreted by the adrenal cortex in response to ACTH produced in the anterior pituitary gland which has been stimulated by CRH from the hypothalamus. The series of reactive responses of hypothalamus-anterior pituitary gland-adrenals is the HPA axis.
Long-term stress places a severe strain on the body. In its final phase, the exhaustion phase, vital systems collapse due to the failure of the body to meet its continuing need for energy.
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