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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



ignoring the time lag, the expected hysteresis loop, 

 although small in magnitude, was seen. Maintenance 

 of strict identity between the site of measurement of 

 the two variables is difficult at best. If there has been 

 a longitudinal displacement of the aorta, and hence 

 of the circumference recorder, through influences 

 other than the arrival of a volume pulse, a seeming 

 "phase lag" between the two recorders could be 

 produced. 



Curiously enough, with isolated strips of arteries, 

 the time lag is reported in the opposite direction for 

 pressure leads. From this lag is calculated the viscous 

 component of a dynamic modulus (48, 84). 



Summarizing, we can say that despite many 

 studies on the extensibility of the aorta and large 

 vessels, it is still uncertain whether the presented 

 stretch curves may be reflecting to such a great 

 degree the techniques used that they are not readily 

 illustrative of the characteristics of the wall. Work of 

 the future will certainly be concentrated on measure- 

 ments made on living vessels, that will include not 

 only diameter change but changes in length, and 

 perhaps in wall thickness. There are not sufficient 

 data to allow a well-based speculation as to how the 

 in vivo measurements might fit with those obtained 

 from isolated specimens. The question of how muscle 

 contraction might affect tissue extensibility, for the 

 aorta and for the muscular arteries, is yet to be defini- 

 tively answered. Whether an expression of extensi- 

 bility in terms of a modulus is the most satisfactory 

 tool remains questionable. 



ACTION OF THE AORTA AS A CONDUIT 



Pulsatile F/01 



Rigid and Distensible Tubes 



Since the aortic flow is never steady, we can turn 

 immediately to a consideration of pulsatile accelera- 

 tions and decelerations rather than deal further with 

 the classic hydrodynamic equations. As a start, let us 

 visualize a piston pump connected to a rigid pipe of 

 uniform bore, with the piston being driven by a large 

 force. Let us leave the distal end of the pipe open, so 

 that a flow through can be established. Also, let us 

 imagine a valve system so constructed that the barrel 

 of the pump can be filled, during piston withdrawal, 

 from an external reservoir. To start a pump cycle, the 

 first tiny forward movement of the piston will produce 

 a compression of the adjacent fluid. This initial 

 compression will represent a high pressure — one that 

 cannot be recorded, since anv manometer used would 



of necessity have a membrane, the resistance of which 

 toward displacement would be less than that of the 

 fluid or the rigid pipe walls. Once the involved force 

 is sufficient to overcome the factional resistance to 

 fluid displacement, or to overcome the inertia of the 

 fluid column, flow can start. While the time interval 

 between may be short, we can say that there will 

 always be a temporal separation between the creation 

 of the pressure force and flow through the tube. This 

 is commonly spoken of as a phase lag, with pressure 

 leading. The definition of the physical forces and the 

 quantitation of such lags, for both rigid and dis- 

 tensible systems, have occupied the attention of many 

 physically minded workers of late (33, 48, 54, 60, 70, 

 ^5> '39)- I do not consider myself qualified to judge 

 the relative contributions of these papers. 



Now let the piston complete its stroke, and reverse. 

 The pressure in the pump will show a sharp fall, the 

 amount depending upon the speed of inflow from 

 the side reservoir. Since a pressure gradient has been 

 previously constructed in the pipe to produce dis- 

 placement toward the open end, or, if preferred, 

 since fluid has already been accelerated toward this 

 end, flow will continue for a brief interval despite 

 the pressure fall in the pump. Once again, then, we 

 have a phase lag, and the fluid column can be said 

 to have an inertial force. If the pump strokes are 

 repeated at a rapid frequency, the flow per cycle will 

 be related to how well matched the duration of each 

 phase of the pump cycle is to the phase lag, as set by 

 the factional and inertial characteristics of the tube. 

 This principle of matching can be illustrated by 

 another model. Suppose a U-tube mercury manom- 

 eter is made to oscillate by a periodic blowing of 

 air into a side arm on one side of the U-tube. The 

 first buildup in air pressure will displace the mercury, 

 and after this the mercury column will oscillate back 

 and forth, the period being conditioned by the size 

 of the tube and the other components of fluid re- 

 sistance to flow. If the frequency of the air puffs 

 matches that of the mercury column, the excursions 

 will be reinforced. Conversely, if the generating 

 frequency is out of phase with the mercury oscilla- 

 tions, movement of the mercury, or "flow," will be 

 minimal. 



Equations which relate flow to pressure usually 

 express the phase lag in terms of a component of the 

 frequency of the repeated strokes. This is simplest if 

 the pressure buildup by the pump has a sinusoidal 

 form. If the stroke is of a different form, the pressure 

 curve is broken down into terms of a fundamental 

 sine wave and a number of superimposed harmonics. 



