alarm reaction

Click to Play

CNET Top 5: Reactions to 3DTV

Click to Play

Insect Bite Allergic Reaction Response

Click to Play

Adverse Reactions to Immunizations in Children

Click to Play

How to Reduce Stress by Altering Your Stress Reactions

View more Science videos

The initial stage in the body's response to stressful stimuli, characterized by adaptive physiological changes, such as increased hormonal activity and increased heart rate.

The response of the sympathetic nervous system either to physical stress or to a strong emotional state. Called also stress reaction and fight or flight reaction. It is an automatic and instantaneous response that increases the body's capability to cope with a sudden emergency.
The physiological changes occurring during this reaction 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.

The first stage of the general adaptation syndrome of Hans Selye; occurs in response to severe physical and psychologic distress. Complete mobilization of body resources occurs in association with activity of the pituitary and adrenal glands and the sympathetic nervous system. See also syndrome, general adaptation.