where the pressure is greatest, and slowest in the capillaries, where the area is greatest, the sectional area of the capillaries, known as the peripheral area because it is farthest from the heart, being larger than that of the large arteries. Thus rapidity of flow varies with pressure and with area.
Blood Pressure.—Liquids, moreover, are incompressible and exert pressure on the walls of the tubes through which they pass. The amount of pressure depends upon the inflow and outflow, increasing directly with the inflow and inversely with the outflow, that is, the smaller the outlet the greater the pressure, and vice versa. The pressure is also greatest nearest to the inflow and gradually decreases with distance until at the point of outflow there is practically no pressure. So, in the arteries the blood pressure is greatest in the large vessels nearer the heart and gradually decreases as they branch into smaller and smaller vessels. In passing through the capillaries, owing to their small size and resultant increased friction, the blood meets with more resistance, the peripheral resistance, and this resistance usually regulates the pressure in the arteries. The greater the peripheral resistance, as a rule, the greater the arterial pressure. The pressure in the capillaries is very slight and in the veins there is practically no pressure. In fact, in the large veins near the heart the pressure is negative and the blood is almost sucked into the heart.
Pressure, then, is greatest in the arteries and least in the veins, while the rate of flow is fastest in the arteries—300 to 500 millimeters a second—and slowest in the capillaries—75 millimeters a second—being a little faster again in the veins—200 millimeters a second.
Blood pressure is gauged by opening a vessel and inserting a monometer, the pressure being determined by the height to which the mercury is raised. In man the pressure in the arteries is 120 to 160 millimeters. It is considerably heightened during inspiration by
