DYNAMICS OF PULMONARY CIRCULATION 1 697 



relaxation") (318, 337, 371); c) the pulmonary venous- 

 left atrial segment is less distensible than the systemic 

 venous-right atrial segment (87, 272, 309); d) the 

 successive segments of the pulmonary vascular tree 

 differ considerably in distensibility [the veins and 

 arteries are more distensible than the capillaries ( 1 24, 

 318)]; and e) although the aorta and pulmonary 

 artery are of approximately the same caliber in life, 

 the range of maximum distensibility for the pul- 

 monary artery (10 to 40 mm Hg) is much lower than 

 for the aorta (182). Unfortunately, measurements of 

 pulmonary vascular distensibility in intact animal or 

 man have not yet become practical or reliable (71, 



293)- 



These generalizations about pulmonary vascular 

 distensibility help to explain some physiological 

 features of the pulmonary circulation. For example, 

 the small pulse pressure in the pulmonary artery 

 seems to be a consequence of both the marked distensi- 

 bility of the pulmonary arterial tree, which prevents 

 a considerable rise in pressure as the right ventric- 

 ular stroke volume is ejected, and the low pul- 

 monary vascular resistance, which allows more blood 

 to escape from the pulmonary arterial tree during 

 each systole and causes the pressure to fall earlier 

 during systole (102, 182). The greater distensibility 

 of the pulmonary than the systemic arterial tree also 

 helps to account for the slower velocity of the pulse 

 wave in the pulmonary artery (250 cm/sec) than in 

 the aorta (300 cm/sec). 



The unusual distensibility of the small pulmonary 

 vessels, i.e., of the pulmonary "resistance" vessels 

 affects their hemodynamic behavior. For example, 

 as the pulmonary blood volume is expanded (251, 

 437), small pulmonary vessels share in this increase, 

 leading to an increase in their transmural pressures, 

 passive dilatation of their lumens and a decrease in 

 their resistance to blood flow; since the arterial, 

 capillary and venous portions of the small pulmonary 

 vessels have different capacities and pressure-volume 

 characteristics, the increase in pulmonary blood vol- 

 ume will not be equally apportioned among these 

 vascular segments. Moreover, the distensibility char- 

 acteristics and capacities are such that at low pul- 

 monary vascular volumes and pressures, each in- 

 crement in blood volume will raise the blood pressure 

 less, and passively dilate the vessels more, than at 

 high levels. This hemodynamic behavior is particu- 

 larly relevant to those experiments in which an under- 

 standing of the passive characteristics of the 

 pulmonary vascular tree and of its segments is pre- 

 requisite for interpreting a change in calculated pul- 



monary vascular resistance in terms of pulmonarv 

 vasomotor activity (69, 101). 



Resistance: Pressure-Flow Relations/u/n 



It has been noted above that, for the sake of ex- 

 pediency, flow through the pulmonary circulation is 

 conventionally treated as though it were steady. Ac- 

 cordingly, and by analogy with Ohm's law, the ratio 

 of the drop in mean pressure across the pulmonarv 

 circulation (AP) to the mean blood flow (Q) is used 

 as a measure of pulmonary vascular resistance. This 

 idea of resistance is unambiguous when applied to 

 rigid tubes perfused by a homogeneous fluid flowing 

 in a laminar stream: under these special conditions, 

 the plot of AP against Q, is linear and passes through 

 the origin, i.e., it is predictable and interpretable in 

 physical terms. Complexities are introduced when 

 these concepts are extended to the pulmonary (as 

 well as to the systemic) circulation: the vascular bed 

 is a nonlinear, visco-elastic, frequency-dependent 

 system perfused by a complicated non-Xewtonian 

 fluid; moreover, the flow is pulsatile so that inertial 

 factors, reflected waves, pulse wave velocity, and in- 

 terconversions of energy become relevant considera- 

 tions (156, 277). In such a system, resistance varies 

 with pressure and flow; plots of AP against Q, are not 

 linear and do not pass through the origin (61, 169, 

 175). And, as the result of the many different active 

 and passive influences which may affect the relation- 

 ship between AP and Q, the term "resistance" is 

 bereft of its original physical meaning: instead of 

 representing a fixed attribute of blood vessels, it has 

 assumed physiological meaning as a product of a set 

 of circumstances. 



table i. Representative Values for a Normal Human 

 Subject in the Basal State 



Pulmonary blood flow 6.0 liters/min 



3.1 liters/min/m 2 BSA 



* Calculated from the data in this table: R = (15 — 5)/ 

 (6000/60) = 0.1 R units. t R units express calculated 



resistance as mm Hg/(ml/sec); to convert to C.G.S. units 

 (dynes sec cm -5 ), the value in R units is multiplied by 1328. 



