Figure 1. As blood travels through the arterial system, resistance from the walls of the blood vessels reduces the pressure and velocity of the blood.
Because blood pressure is so low in venules and veins, two mechanisms assist the return of blood to the heart (venous return):
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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.
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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.