The diameter of the blood vessels is controlled by impulses from the nervous system which cause the muscular walls to contract. The nervous system also affects the rapidity and strength of the heartbeat, and thereby the blood pressure as well. Shock, which is associated with a dangerously low blood pressure, can be produced by factors that attack the strength of the heart as a pump, decrease the volume of the blood in the system, or permit the blood vessels to increase in diameter. There are five main types of circulatory shock. Low-volume shock occurs whenever there is insufficient blood to fill the circulatory system.
Neurogenic shock is due to disorders of the nervous system.
Two types of shock, allergic shock and septic shock, are due to reactions that impair the muscular functioning of the blood vessels. Cardiogenic shock is caused by impaired function of the heart. This is a common form of shock that occurs when blood or plasma is lost in such quantities that the remaining blood cannot fill the circulatory system despite constriction of the blood vessels. The treatment of low-volume shock requires replacement of the lost volume. This form of shock, often called fainting, may be brought on by severe pain, fright, unpleasant sights, or other strong stimuli that overwhelm the usual regulatory capacity of the nervous system.