CHAPTER 24 



The physiology of the aorta and major arteries 1 



JOHN W. RE MINGTON Department of Physiology, Medical College of Georgia, Augusta, Georgia 



CHAPTER CONTENTS 



Measurement of Aortic Distensibility 



General Characteristics of the Tension -Length Curve 



The Hysteresis Loop 



Selection of Representative Curves 



Histological Considerations 



Effects of Active Muscular Contraction on Distensibility 



Effects of Aging on Arterial Distensibility 



Expression of Extensibility in Terms of Moduli 



Changes in Length and Wall Thickness of Arteries 

 Action of the Aorta as a Conduit 



Pulsatile Flow in Rigid and Distensible Tubes 



Quantitation of Fluid Displacement and Wall Distensibility 

 Relationships 



Phase Lag and the Harmonics of the Arterial System 



Construction of a Hypothetical Ejection Curve 

 The Aorta as a Blood Reservoir 



Changes in Central Pulse Contour During Propagation 



Resonance and Standing Waves 



Other Factors Which May Alter the Central Pulse Contour 



Calculation of the Stroke Volume From the Central Pressure 

 Pulse 



AT outset, may I say that this article on the function 

 of the aorta and the major arteries makes no pretense 

 of being an authoritative review of the literature, 

 and is not only a generalized treatment but is written 

 with a bias. I came to cardiovascular study via Biol- 

 ogy, in an era when Physics was not such a firmly 

 trothed bride of Physiology- I do not think glibly in 

 terms of abstract formulas, of electrical analogues, or 

 the other erudite devices now so commonly used to 

 clarify the complex problems which underlie pressure 

 wave formation and propagation. I believe that I 

 understand a process only when I can construct some 



1 This manuscript was completed January 15, 1 961 , and its 

 references include only papers I had read in published form at 

 that time. 



sort of a visual image of just how it operates. In many 

 aspects of the subject such a visual model is at present 

 impossible. I can only describe what I have been able 

 to gather about the function of the major arteries, and 

 speculate about what trends future research will 

 follow. 



These large arteries serve two clear functions. First, 

 they comprise a network of conduits through which 

 blood is moved from the centrally located cardiac 

 pump to the various capillary beds. It is important 

 that this transfer be made with a minimal loss of 

 energy. The problem of proper conduit design became 

 more acute when the body form became elongated, 

 instead of remaining spherical. Second, the distensible 

 wall of the vessels allows a temporary storage of blood 

 during the ejection phase of the pump cycle, which 

 allows a buffering of the oscillatory pressure changes. 

 This aspect will be spoken of as the reservoir action of 

 the vessels, admittedly an inadequate label. While 

 the buffering action might serve to protect the small 

 vessels from large pressure changes, it also involves 

 considerable change in the conduit properties. 



We cannot yet form a definitive analysis of the 

 effectiveness of aortic design in meeting these funda- 

 mental requisites. Progress has been handicapped 

 because it has been so difficult to make critical studies 

 on intact vessels. Pressure changes at various points 

 along the arterial system have been measured often 

 over the past 50 years (1, 16, 18, 28-30, 40, 42, 99, 

 116, 132, 134, 135). This has given us knowledge, 

 still far from complete, about the speed of pulse-wave 

 propagation, the contour of the pressure pulse as 

 formed in the upper aorta, and the changes in this 

 form as the pulse moves into the distal aorta and large 

 arteries. Itisdoubtful that any great progress toward an 

 understanding of arterial dynamics will be made by 



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