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) millimetres of mercury, occurs during contraction of the ventricles; the lowest (diastolic) pressure, normally about 80 (60 to 100) millimetres, occurs during ventricular relaxation. Blood pressure in the capillaries is usually about 20 to 30 millimetres, while the pressure in the large veins may become negative (lower than atmospheric pressure).

Arterial blood pressure varies among individuals and in the same individual from time to time. It is lower in children than in adults and increases gradually with age. It tends to be higher in those who are overweight. During sleep it decreases slightly, and during exercise and emotional excitement it increases. See also hypertension; hypotension.