Force originating in the pumping action of the heart, exerted by the blood against the walls of the blood vessels; the stretching of the vessels in response to this force and their subsequent contraction are important in maintaining blood flow through the vascular system. In human beings, blood pressure is usually measured indirectly over the brachial or femoral artery: the highest (systolic) pressure, normally about 120 (100 to 140) millimeters of mercury, occurs during contraction of the ventricles; the lowest (diastolic) pressure, normally about 80 (60 to 100) millimeters, occurs during ventricular relaxation. Blood pressure in the capillaries is usually about 20 to 30 millimeters, while the pressure in the large veins may become negative (lower than atmospheric pressure). Hypertension occurs when the body's smaller blood vessels (the arterioles) narrow, causing the blood to exert excessive pressure against the vessel walls and forcing the heart to work harder to maintain the pressure. Although the heart and blood vessels can tolerate increased blood pressure for months and even years, eventually the heart may enlarge (a condition called hypertrophy) and be weakened to the point of failure. Injury to blood vessels in the kidneys, brain, and eyes also may occur. 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 plasma blood minus the red and white blood cells and the platelets. Blood volume is reduced in a number of conditions involving loss of salt and water from the tissues as in excessive sweating and diarrhea and its replacement with water from the blood. Loss of water from the blood to the tissues may result from exposure to cold temperatures. Also, 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.