Shock is a consequence of insufficient blood flow for adequate oxygenation at the cellular level. Early recognition of the signs permits prompt therapy to maintain the effective circulating volume before irreparable organ damage occurs. Classification of shock on the basis of percentage of blood loss is helpful in guiding initial management. Replacement of red blood cells may be required for acute blood loss of more than 25 percent of the patient's circulating volume. Adequate arterial PO2 is essential for maintaining cellular oxygenation.