Hydrostatic pressure created by the heart forces blood to move through the arteries. Systolic blood pressure, the pressure measured during contraction of the ventricles, averages about 110 mm Hg in arteries of the systemic circulation (for healthy, young adults). The diastolic blood pressure, measured during ventricle relaxation, is about 75 mm Hg in these arteries. As blood travels through the arterial system, resistance from the walls of the blood vessels reduces the pressure and velocity of the blood (see Figure 1). Blood pressure drops sharply in the arterioles and falls to between 40 and 20 mm Hg in the capillaries. Blood pressure descends further in the venules and approaches zero in the veins.
Because blood pressure is so low in venules and veins, two mechanisms assist the return of blood to the heart (venous return):
The muscular pump arises from contractions of skeletal muscles surrounding the veins. The contractions squeeze the veins, forcing the blood to move forward, the only direction it can move when valves in the veins close to prevent backflow.
The respiratory pump is created by the expansion and contraction of the lungs during breathing. During inspiration (inhaling), pressure in the abdominal region increases while pressure in the thoracic cavity decreases. These pressures act upon the veins passing through these regions. As a result, blood flows toward the heart as it moves from regions of higher pressure (the abdomen) to those of lower pressure (the chest and right atrium). When the pressures are reversed during expiration (exhaling), backflow in the veins is prevented by valves.