Shock, in physiology, state of acute circulatory insufficiency of the blood. It is the result of the inability of the heart to pump an adequate volume of blood at sufficient pressure for normal blood passage through the major organs of the body (see Circulatory System). Shock may be caused by heart failure, injury, burns, hemorrhage, or major surgery. Sudden infection or poisoning and abnormal extracellular fluid volume resulting from excessive loss of water and electrolytes (ionic conductors) from the gastrointestinal tract, kidneys, or skin may also cause shock.

Shock is generally characterized by apathy, weakness, shallow breathing, rapid heartbeat, feeble pulse, lowered blood pressure, and coldness and clamminess of skin. During the early stage consciousness is retained, but alertness is diminished. Sudden peripheral circulatory failure, however, affects the brain and causes fainting. In less severe shock compensatory constriction of the blood vessels helps restore circulation, but if shock persists, compensatory mechanisms fail and local anemia damages vital organs, such as the brain, heart, liver, and lungs.

First aid for shock includes having the victim lie down, keeping him or her warm but not overheated, stopping any bleeding, and, if the person is not breathing, administering artificial respiration. A physician or paramedic may administer oxygen and sedative drugs and take measures to lower the body temperature if it is very high. Medical personnel may give intravenous glucose and salt solutions, and transfusions of plasma or whole blood may be required when shock is due to blood loss. In medical centers, treatment is monitored by direct measurements of arterial blood pressure and arterial blood gases. Sometimes blood pressure is increased by vasopressors (drugs that constrict blood vessels).