State in which the circulatory system fails to supply enough blood to peripheral tissues to meet basic requirements. Symptoms—weak, rapid pulse; low blood pressure; and cold, sweaty skin—are not all present in every case. Causes include low blood volume, caused by bleeding or fluid loss from burns or dehydration; inability of the heart to pump enough blood, due to heart attack, pulmonary embolism, or cardiac tamponade (compression of the heart by fluid in the membrane around it); and blood-vessel dilation as a result of septicemia, allergy (including anaphylaxis), or drugs. All result in reduced capillary blood flow; reflexes increase heart rate and constrict small blood vessels to protect the blood supply to essential organs. Without treatment of the underlying cause, these mechanisms fail; since the cause is not always clear, cases tend to require different and occasionally contradictory treatment (e.g., intravenous fluids can save the life of a patient with massive blood loss but can overload a weakened heart).

This entry comes from Encyclopædia Britannica Concise.
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