HYPOTENSION

also called Low Blood Pressure, condition in which the blood pressure is abnormally low, either because of reduced blood volume or because of increased blood-vessel capacity. Though not in itself an indication of ill health, it often accompanies disease conditions. Extensive bleeding is an obvious cause of reduced blood volume that leads to hypotension. There are other possible causes. A person who has suffered an extensive burn loses blood plasmathe blood minus the red and white blood cells and the platelets. The blood volume is reduced in a number of conditions involving loss of salt and water from the tissuesas in excessive sweating and diarrheaand its replacement with water from the blood. Loss of water from the blood to the tissues may also result from exposure to cold. Finally, a person who remains standing for as long as one-half hour may temporarily lose as much as 15 percent of the blood water into the tissues of the legs. Orthostatic hypotensionlow blood pressure upon standing upseems to spring from a failure in the autonomic nervous system. Normally, when a person stands up, there is a reflex constriction of the small arteries and veins to offset the effects of gravity. Hypotension from an increase in the capacity of the blood vessels is a factor in fainting and in other forms of syncope (q.v.). Hypotension is also a factor in poliomyelitis, in shock, and in poisoning with a depressive drug, such as a barbiturate.