FIGURE 14 The viscosity of blood appears to decrease as the diameter of the tube it flows through decreases. Because most of the peripheral resistance in the body resides in vessels with radii of 50 mm or less, the Fahraeus-Lindquist effect allows blood to flow more easily through the tissues. (Modified from Fahraeus R, Lindquist T, Am J Physiol 1931; 96:562.)

pulmonary system, where a large increase in cardiac output causes only a small increase in pulmonary arterial pressure. On the other hand, arterioles are not very distensible. Because most of the peripheral resistance resides in arterioles, flow through most organs remains a remarkably linear function of the perfusion pressure as depicted by Ohm's law. The basis for the distensibility of a blood vessel can be seen in Table 1. The walls of different blood vessel types consist of varying amounts of connective tissue and muscle. Also, the vessels differ in radius and wall thickness. Thus, distensibility differs among vessel types. Distensibility of blood vessels is measured as compliance (C), a change in volume (V) in response to a change in distending pressure (P):

Because of the compliance relationship, the pressure in a vessel cannot be raised without moving blood into it. As a result, pressure changes within the cardiovascular system are usually accompanied by shifts in the blood distribution. For example, if arterial pressure is increased, blood must be translocated into the arterial system at the expense of some other compartment such as the veins, whose pressure and volume must fall. Figure 15 reveals that there is a large difference between the compliance of arteries and veins. At any given pressure, much more volume is contained in the veins. This large capacitance contributes to the reservoir function of the veins. Thus, in the above example, a large rise in arterial pressure would usually be accompanied by a smaller drop in venous pressure. Notice that at zero pressure the vessels still contain appreciable blood. This volume is referred to as the unstressed volume. Thus, the total volume of blood in a vessel is given by:

(P x C) + unstressed volume

Pressure (mm Hg)

FIGURE 15 Volume-pressure relationships of the venous and arterial systems indicate that the venous compliance (indicated by the slopes of the lines) is much greater than that of the arteries. Also, the unstressed volume (the volume at zero pressure) is much larger for the veins.

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