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HANDBOOK OF PHVSIOLOGY 



CIRCULATION II 



further studies on pressure values alone, without 

 a simultaneous recording of vessel diameter or flow. 



A study of two parameters has often been at- 

 tempted, but not too successfully. It has not proven 

 practical to remove the aorta from the body and 

 insert it into an artificial system where the volume 

 change and the flow through might be measured 

 directly, as from a calibrated stroke of a pump. The 

 problems of coupling this distensible tube to rigid 

 fittings without having an orifice that will severely 

 distort the flow pattern are considerable. No effective 

 means has been devised to occlude completely all 

 exit vessels, including the vasa vasorum, and thus 

 prevent loss of fluid along the length of the vessel. 

 Sometimes a rubber insert has been used (137) but, 

 since most rubber tubes are less extensible than the 

 aorta, this stratagem may have confused matters 

 more than it helped. And we still have no pump which 

 has an ejection pattern like that of the ventricle. 

 Curiously enough, an artificial pump capable of 

 producing a pressure rise similar to that seen with a 

 natural pulse invariably produces turbulence and 

 vibrations of the pressure recorder sufficient to obscure 

 the pulse contour being formed. 



A recording of aortic flow in vivo by a technique 

 which requires vessel cannulation causes enough dis- 

 tortion of the pressure pulse contours that one must 

 be cautious in inferring a direct pertinence to the 

 intact system. Fortunately, several techniques are now 

 in use for the recording of flow (27, 31, 55, 82, 110, 

 120, 131) and the registration of diameter change (88, 

 91, 113) which do not require cutting the vessel. 

 Most flow recorders do require a crimping of the 

 vessel in the region where flow is being measured, 

 which may not be without effect on the flow profile. 

 As yet, the frequency of many such devices usually 

 does not approach that of a good pressure recording 

 system, so that they may not be able to give a faithful 

 picture of rapid change. 



An engineer faced with the problem of designing a 

 conduit system for the most efficient movement of 

 blood would start with some basic equations. First, 

 there would be the Poiseuille formula which states 

 that the pressure fall (for frictional energy dissipation) 

 will be directly related to the flow rate, the fluid 

 viscosity, and the length of pipe, and inversely related 

 to the fourth power of the radius. The last is because 

 adsorptive forces between the fluid and the wall prevent 

 or retard longitudinal movement of the outermost fluid 

 layer. This in turn forces the adjacent shell to shear 

 past it, retarding it with a frictional dissipation of 

 energy, which in turn slows the next shell of fluid, and 



so on to the middle of the pipe. For a given volume 

 flow, the greater the pipe diameter, the less is the 

 total fluid frictional loss. He would also include in his 

 formulas factors relating to the smoothness of the 

 wall and the material of which his conduit will be 

 made, since these condition the size of the boundary 

 layer. He must make corrections if the fluid does not 

 have a constant viscosity at all flow rates, as blood 

 apparently does not (9, 86). He also knows that an 

 equation which applies to laminar flow will not be 

 correct if fluid molecules whorl laterally across the 

 fluid shells, i.e., when the flow becomes turbulent. 

 The frictional cost increases whenever this happens. 

 Finally, when he is required to use pipes of different 

 sizes, he must carefully design the transition areas so 

 that turbulent eddies will not form. Tapered changes 

 in diameter are less conducive to turbulence than 

 abrupt shoulder joints. 



All these formulas, which may be found in texts on 

 hydraulics, are based on the assumption that flow is 

 being maintained steady, and that the conduits have 

 rigid walls. But blood flow is not steady, for it shows 

 several accelerations and decelerations with each 

 pump stroke. A calculation of the energy loss accom- 

 panying such rapid changes in flow rate must be 

 complicated. A whole new set of equations will be 

 required and, to check them, we must be able to 

 measure precisely the amount of acceleration in all 

 parts of the arterial system. Quantitative evaluations 

 of the degree of smoothness or of the absorptive forces 

 for plasma on the endothelial lining cannot be given. 

 Microscopic observation of moving blood in tiny 

 vessels has shown that the red cells congregate in the 

 center of the stream, which is presumptive evidence 

 for a greater axial velocity. Whether we can assume 

 from this that a parabolic flow distribution would be 

 found in the large arterial vessels is open to question. 

 While streamline flow has been described for the 

 aorta (92), there remains some question as to whether 

 a normal flow pattern could be said to have persisted 

 during the measurements, and whether the streaming 

 would apply over the whole cardiac cycle (84). 



The large arteries are not rigid, so that any equa- 

 tion relating energy dissipation to tube radius will be 

 complex. Further, the arterial system seldom has any 

 tubes which continue uninterrupted for an apprecia- 

 ble distance. Each vessel has frequent branchings. 

 With the exception of the ascending aorta (44) and 

 the main pulmonary artery (88), with each branching 

 there is an increase in the aggregate cross-sectional 

 area. These junctions appear smooth and tapered, so 

 that the orifice problem is probably at its simplest. 



