The venules and veins return the blood from the microcirculation to the right atrium. However, they do not serve merely as passive conduits. Instead, they have a crucial active role in stabilizing and regulating the venous return of blood to the heart.

The venous system differs from the arterial system in two important respects. First, the total volume (and cross-sectional area) of the venous system is much greater than that of the arterial system. This is because there are many more venules than arterioles; venules also tend to have larger internal diameters than arterioles. Second, the veins are quite thin walled, and can therefore expand greatly to hold more blood if their internal pressure rises.

As a result of its large cross-sectional area, the venous system offers much less resistance to flow than the arterial system. The pressure gradient required to drive the blood through the venous system (15 mmHg) is therefore much smaller than the pressure needed in the arterial system (80 mmHg). The average pressure in the venae cavae at the level of the heart (the central venous pressure) is usually close to 0 mmHg (i.e. atmospheric pressure). The flow of blood back to the heart is aided by the presence of one-way venous valves in the arms and especially the legs, which prevent backflow.

Venous Arterial Compliance

The graph in Figure 22 (upper right) illustrates the relationship between pressure and volume in a typical vein and artery. The slope of the volume – pressure curve is referred to as the compliance. Compliance is a measure of expandability. Veins are much more compliant than arteries at low pressures (0–10 mmHg). Small increases in venous pressure in this range therefore cause large increases in venous blood volume.

One reason for high venous compliance is that their thin walls allow veins to collapse at low internal pressures. Only small increases in pressure are needed to ‘reinflate’ a collapsed vein with blood until it has nearly rounded up. At higher pressures, however, venous compliance decreases dramatically (see graph) because the slack in rigid collagen fibres in the venous wall is rapidly taken up. This limit on the expandability of the veins is important in limiting the pooling of blood in the veins of the legs that occurs during standing.

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