Small Vessel Compliance May Explain Peripheral Pressure-Flow Relations
The load on the heart is formed by the hydraulic input impedance of the arterial system. The three main factors contributing to this load are peripheral resistance, which is mainly located in the periphery, (total) arterial compliance, which is mainly located in the large conduit arteries (the ascending aorta accounts for about 50% of the compliance), and (ascending aortic) characteristic impedance which accounts for blood mass and compliance of the proximal aorta (Westerhof et al., 1971). The effect of blood viscosity on these last two factors is so small that the contribution of (Poiseuille) resistance to flow may be disregarded. In other words, the total arterial compliance and the aortic characteristic impedance are mainly determined by the compliant properties of the vessels and the mass of blood. The peripheral bed, however, not only acts as a resistor but consists of a network of compliant vessels. From the viewpoint of an oscillatory load on the heart the compliance of the periphery may not directly play an important role. When the isolated (cat) heart was made to pump into a three element windkessel model of the cat’s arterial tree, where only total arterial compliance of the large vessels was accounted for, the resulting pressure and flow wave forms were close to in-vivo patterns (Elzinga and Westerhof, 1973). However, there exist a number of methods to determine total arterial compliance. Not all of these methods lead to the same value of compliance (Randall et al., 1984; Yin et al.; 1987, Toorop et al., 1987) as discussed by Yin in this volume (Yin et al., 1988). The compliance of the periphery (Morgenstern et al., 1973; Spaan et al., 1981) does play a role in peripheral pressure-flow relations since even small changes in lumen result in rather large changes in resistance. The relationship between compliance (pressure-volume relation) and resistance (pressure-flow relation) in the small vessels is the subject of this review. The pressure-volume relation is not a linear one so that compliance, the slope of the relation, is pressure dependent.
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