FIGURE 21 The influence of a change in compliance on pulse pressure in the aorta. The slopes of the lines relating pressure and volume define compliance. When compliance is reduced, pulse pressure increases for the same stroke volume.

aorta. Note, however, that the mean pressure and thus the total energy are unchanged as the distance from the heart increases. As the pressure pulse progresses through the arterioles, the combination of arteriolar resistance and compliance attenuates the pulse such that it is almost completely damped out at the level of the capillaries and beyond (see Fig. 17).

Forces in the Blood Vessel Wall

The side pressure exerted on the wall of a blood vessel acts to distend the vessel. The hydrostatic pressure in the interstitium that would oppose that distension is normally very low. This distending force must, therefore, be balanced by forces within the vessel wall or else the vessel would rupture. The magnitude of the tension (T) in the wall opposing the transmural pressure (Pt) is influenced by vessel radius (r). The relationship among the three factors is given by the law of LaPlace:

Note that the wall tension increases not only with the internal pressure, which is obvious, but also with the radius of the vessel, which is not so obvious. The LaPlace relationship can be used to explain many of the structural features found in the cardiovascular system. For large arteries such as the aorta, both pressure and radius are large so that a very large force is generated in the aortic wall. However, the aortic wall is thick and composed of strong connective tissue and muscle. In the thin-walled veins, the radii are still large but the wall tension is low because the pressures are low.

The capillaries pose an interesting case. The walls of these vessels are a single layer of endothelial cells devoid of connective tissue. Although the pressures are much higher than those found in veins, this higher pressure is offset by the very small radii of the capillaries. Because of the small radius, the wall tension is very low. As a result, the capillaries seldom burst, whereas rupture of the thick-walled aorta is a relatively common medical event.

Distribution of Flow and Volume in the Vascular System

The volume of flow must be the same in all divisions of the cardiovascular system. That is, cardiac output must equal the volume of flow through the aorta, the arteries, arterioles, capillaries, and veins. The velocity of flow, on the other hand, differs markedly. This is because the total cross-sectional area of each of the divisions is different. The flow velocity is highest in the root of the aorta. As Table 1 indicates, the cross-sectional area of the aorta in the body is 9.6 cm2. According to Eq. 2, if the cardiac output is 5000 cm3/ min (83 cm3/sec), then the mean velocity will be approximately 8.65 cm/sec. The flow velocity is the slowest in the capillaries. The total cross-sectional area of all the capillaries (remember that the capillaries are in parallel with each other) is about 5000 cm2. Because flow through the capillary beds must also be 83 cm3/sec,

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