[708 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



pulse wave, e.g., initial volume or diastolic pressure; 

 and species differences in distensibility. Thus, one 

 set of values for the pulmonary arterial pulse-wave 

 velocity in the rabbit averages 200 cm per sec (124); 

 this method is based on measurements of pulmonary 

 vascular distensibility and capacity. On the other 

 hand, much lower values (83 cm sec) have been 

 calculated by an alternate approach which involves 

 the registration of phase shifts of a single harmonic 

 component of the pulse wave as it traverses a known 

 distance (140a). Similarly, in the dog, mean velocities 

 have averaged 250 cm per sec in one studv (311) 

 and 400 cm per sec in another (225). Finally, in man, 

 kymographic studies have indicated a velocity of 

 200 cm per sec in the main pulmonary artery and 

 275 cm per sec in the peripheral branches (71). It 

 should be noted that each of these methods has its 

 own peculiar problems: thus, some are troubled by 

 the need for the precise measurement of in vivo 

 distances between points on the pulmonary artery 

 (71, 225); others have to overcome the difhcultv of 

 attempting static measurements under dynamic con- 

 ditions of flow (124). 



Because of the practical limitations and assump- 

 tions involved in the experimental approaches, none 

 of these values seem to offer more than an order of 

 magnitude. However, with a single exception (225), 

 they are consistent with the notion that the pulse- 

 wave velocitv in the distensible pulmonarv arterial 

 tree is somewhat less than in the aorta. 



Pulmonary Circulation Tune 



Measurements of circulation times are in common 

 clinical use for the recognition of heart failure. This 

 use depends on the arrival of a test substance at a 

 chosen site in sufficient concentration to be detected; 

 the value obtained, i.e., the ''appearance time," is 

 related, in a complex way, to the mean circulation 

 time used for the calculation of central blood volume 

 (184, 313). It is clear that the precise value for cir- 

 culation time will be influenced not only by technical 

 peculiarities, e.g., by rate of injection, nature of the 

 test substance, and sensitivity of the detector, but 

 also by physiological events. For example, if the 

 pulmonary circulation lies between the sites of in- 

 jection and sampling, an increase in its blood volume 

 will dilute the test substance excessively and delay 

 its recognition at the test site. It is, therefore, not 

 surprising that values for circulation times from 

 different laboratories are frequently inconsistent. 



One study in the dog (Stewart principle) found the 



total circulation time to be approximately 1 1 sec, 

 and the pulmonary circulation time to average 

 approximately half of the total (184). Other studies 

 indicate that the pulmonary circulation time (pul- 

 monary artery to vein) is somewhat less, i.e., about 

 3 to 4 sec (306, 329). The circulation time for red 



INCREASED PRESSURE DECREASED PRESSURE 



IN L IN T 



fig. 37. Two-chamber model to illustrate the effects of vary- 

 ing pressures around a collapsible rubber tube on its dimen- 

 sions. The tube (a, c, b) runs through one rectangular chamber 

 (T-T) and is exposed for a limited extent (c) to the pressure 

 of the other chamber (L). In .-1 and B, the two ends of the per- 

 fusing system (a, b) are outside of the chambers; in C and D, 

 the entire system is contained within chamber T-T. For the 

 sake of clarity, only A has been lettered; the arrow in each 

 figure indicates the chamber subjected to a change in pressure 

 and the direction of change. A : the pressure in chamber L is 

 greater than atmospheric, the pressure in chamber T-T is 

 atmospheric. The aspect of the tube exposed to L is collapsed. 

 B: the pressure in chamber T-T is less than atmospheric; the 

 pressure in L is atmospheric. The whole length of tube within 

 chamber T-T is increased in diameter; the portion exposed to 

 L is less dilated than the remainder of the tube. C and D : two 

 different but equivalent conditions. As long as the reservoirs 

 are contained within chamber T-T, the same effect is obtained 

 by balancing positive pressure in L against atmospheric pres- 

 sure in T-T (C) or by balancing atmospheric pressure in L 

 against negative pressure in T-T (D). In either case, the result 

 is identical with that illustrated in .4. By analogy, this model 

 suggests that: /) when alveolar pressure \L) is raised (.1), the 

 transmural pressure of adjacent vessels (c) is decreased; 2) the 

 situation of two reservoirs (pulmonary arterial and venous 

 pressures) outside of the pleural cavity (B) corresponds to 

 negative (pleural) pressure inflation as well as to normal 

 respiration; 3) when pleural pressure is decreased {B), the 

 transmural pressures of larger vessels are increased more than 

 those of the capillaries, and ./> the situation of two reservoirs in 

 the pleural cavity (C and D) is a physical identity with positive 

 pressure inflation. [Based on Quincke & Pfeiffer (324).] 



