Shock is an acute state in which circulatory function is inadequate to supply sufficient amounts of O2 and other nutrients to tissues to meet metabolic demands. In most cases, cardiac output is low. In early shock, compensatory regional vasoconstriction (skin, skeletal muscle, splanchnic circulation) may temporarily maintain normal blood pressure and adequate blood flow to vital organs. As shock progresses, compensatory mechanisms fail and widespread cellular damage occurs. Insufficient O2 delivery to tissues causes anaerobic metabolism and lactic acidosis. If shock persists, irreversible injury to vital organs occurs; death ensues despite vigorous therapy that may temporarily return cardiovascular measurements to normal.