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the first stage of the general adaptation syndrome; the response of the sympathetic nervous system to either physical stress or a strong emotional state (see also stress reaction). It is an automatic and instantaneous response that increases the body's capability to cope with a sudden emergency. The physiologic changes that occur increase physical strength and mental activity. The blood pressure is elevated, the blood glucose level is raised for additional energy, the blood coagulates more readily, and the flow of blood to muscles needed for activity is increased, while those organs not needed for “fight” or “flight” receive a diminished blood supply. One of the most striking manifestations of this reaction is the involution of lymphoid tissues due to the action of adrenal hormones. Called also sympathetic stress reaction and fight-or-flight reaction.