824 



HANDBOOK OF PHYSIOLOGY -^ CIRCULATION II 



160- 



140" 



I20 J 



< 



.1 SEC 



fig. g. Pressure and flow values given by Fry et at. (34). 

 Broken line of B, calculated flow values as described in text. 



When it starts to fall, the difference should swing to a 

 negative value. This does not mean that flow down 

 the aorta will then cease. The volume displacement 

 is part of a wave movement, and the pressure differ- 

 ential simply reflects the time lag between the wave's 

 arrival at two points in the tube. Such a continuation 

 of fluid displacement toward the periphery, despite a 

 negative pressure differential, could properly be 

 called an inertial property of the fluid. What all 

 workers are seeking is a complete description of 

 what we mean by a wave, and what factors con- 

 tribute toward its progression through the tube. 



Before leaving the descriptive model, it should be 

 pointed out that no length dimensions were placed on 

 the tube segments that were acting independently. 

 With wall fibers distributed longitudinally as well as 

 circularly, there cannot be such an independence of 

 action of tube segments. A unit of a distensible tube 

 must have a finite length, which, however, has not 

 been defined. This tying of segments to each other 

 must give a distensible tube some of the characteristics 

 of a rigid tube. However, it remains rather incon- 

 ceivable that a whole aorta could act as a single 

 bound entity, and could be given a single lumped 

 resistance value. 



To summarize this section on the behavior of the 

 aorta as a conduit, the initiation of flow through a 



rigid system certainly requires the acceleration of a 

 whole column of fluid, an overcoming of fluid re- 

 sistance for the whole length of the tube, and a phase 

 lag between pressure built up at the generating source 

 (pump) and the flow out the end of the tube. In 

 such a rigid system, resistance factors can certainly 

 be treated as a unit. With a distensible tube, how- 

 ever, depending upon the stiffness of the wall, only 

 a small segment of fluid need be accelerated in any- 

 given unit of time, and the fluid resistance and the 

 phase lag can be relatively small. A model has been 

 presented in which a pressure wave is propagated 

 from a minute segment of such a tube to the next 

 adjacent segment. Of course, the tube is linked lon- 

 gitudinally by extensible fibers, and the length of what 

 is being called a tube segment cannot be defined. 

 But it is not clear that the current trend of treating 

 pressure-flow relations in the aorta as though re- 

 sistance was lumped and as though there were an 

 appreciable phase lag between pressure and flow is 

 helping our understanding. The propagation velocity 

 of the wave must be linked in some way to tube 

 dimensions and to wall distensibility, but no com- 

 pletely satisfactory formula for quantitating this 

 relation appears yet to have been presented. 



THE AORTA AS A BLOOD RESERVOIR 



Changes in Central Pulse Contour During Propagation 



In the description of how fluid displacement 

 through the arterial bed might be calculated from 

 the distensibility values of the various vessels, and 

 the course of pressure change in the ascending aorta, 

 the assumption was made that the central pulse 

 would be propagated intact. This assumption is 

 clearly false. The pulse contour is modified during 

 transmission, this modification resulting perhaps 

 from damping, or from a poor matching between the 

 frequencies of the volume input curve and those set 

 by the clistensibilities and flow resistances of each 

 arterial segment, or from an augmentation of 

 "matched" frequencies, or even from superposition 

 of a wave reflected from the periphery upon the 

 incident wave. It should not be implied that, because 

 pulse contours change, the whole method of calcula- 

 tion of the form of the cardiac ejection curve is 

 invalid. If the contour changes do not appreciably 

 alter the total displacement of fluid out of the as- 

 cending aorta, the total quantitation need not be 

 greatly in error. 



Contour differences between a central and a 



