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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



the original length, there could be a reorientation of 

 these elements into the circular plane, leaving a ring 

 with a larger circumference. This indicates, as is 

 probable, that both longitudinal and circular elastic 

 elements are part of a linked network, and therefore 

 not independent. In our studies on isolated rings, 

 when we converted the actual tension-length values 

 to pressures and volume, we used in situ length 

 rather than the actual one. This mathematical step 

 was better than using the actual lengths, but is not 

 necessarily sufficient as a correction if elements 

 previously oriented longitudinally were participating 

 in the circular stretch. 



This is not to say that all the hysteresis phenomenon 

 could be due to a progressive recruitment of longi- 

 tudinal fibers into a circular plane. The loop still 

 present, despite many consecutive stretchings, prob- 

 ably denotes a structural rearrangement of elements 

 under load that were already in the plane of the 

 stretch. It is admittedly strange that such a rear- 

 rangement would have no clear time dimension. 



An occluded, in situ aorta being pulsed by volume 

 injections shows a similar pattern of hysteresis and 

 change in extensibility curve with successive stretches 

 (5). We attempted a quantitative assessment of 

 possible differences between total aortic distensibility, 

 as measured by injections of saline into dead but in 

 situ aortas, and as compiled from stretch data made 

 on rings cut serially from the same vessels (103). The 

 volume required to produce a given rise in pressure 

 was greater than that estimated from the first stretch 

 curves of the isolated rings. It was also a little greater, 

 although perhaps not significantly so, than that 

 expected from the second stretch curves. 



On hindsight, this comparison may not have been 

 so definitive as we supposed. At the time the experi- 

 ments were done, we were not so aware of the large 

 effect of the time interval between successive stretches, 

 of the initial pressure level, and of the peak pressure 

 reached, on the contour and values of the extension 

 curve. 



Selection of Representative Curves 



Historically, an interpretation of vessel wall archi- 

 tecture has been based on the contour of a first, or a 

 second, large and continuous extension curve. It 

 should now be obvious that this contour should not 

 be regarded as characteristic of wall extension during 

 unceasing, repetitive stretches, such as would be 

 present during life. How large the differences between 

 the two curves might be will be elaborated more 



fully later in this paper. Ultimately, a study of vessel 

 wall behavior must be based on pressure and diameter 

 or volume measurements made during life. Such 

 studies will be technically difficult, and interpretation 

 of the records will be difficult, since in life the heart 

 rate and the pulse pressure are continually changing 

 from beat to beat. Work on this problem is in progress 

 in a number of laboratories, and some results have 

 been published (87, 91, 113). Unfortunately, these 

 show some differences, and the adequacy of the 

 various techniques has yet to be firmly established. 

 But taking the data as they now stand, it appears 

 possible that while the amount of hysteresis varies 

 among arteries, it is probably less than that shown 

 by the isolated rings. This may be because the living 

 aorta is being cyclically stretched without pause. 



The suggestion, which needs further corroboration, 

 that the diameter change of a living vessel for a given 

 pulse pressure is less than that given by an isolated 

 ring (61, 78, 87, 91) could indicate that, for some 

 quite unknown reason, a very different distensibility 

 is present in the living vessel. In one study (91) the 

 reported diameter change is so much smaller that our 

 whole concept of the functional character of the vessel, 

 as will be developed in this paper, would have to be 

 changed. We are not satisfied yet that the instrument 

 used could record a change as large as that expected 

 from the ring stretch data. Reconciliation between 

 the various sets of data should not be long delayed. 



Until the properties of the in vivo aorta are known 

 more surely, we must base a description of the factors 

 which might condition the extensibility curve on 

 data taken from isolated rings. If these data should 

 eventually prove quantitatively wrong, we can only 

 hope that the fundamental properties of the vessel 

 would nonetheless be qualitatively the same. First, 

 the nonlinearity of the continuous stretch curve is 

 evidence for an internal architecture more com- 

 plicated than that seen even with rubber or other 

 polymers. In the range of maximal extensibility, the 

 aorta shows more length change per unit tension 

 increase than anv other material of comparable wall 

 thickness. Nature seems to have created a far better 

 volume reservoir than man can duplicate. 



Histological Considerations 



We know from histological evidence that the large 

 vessels have four general types of tissue — endothelial 

 cells (with associated intracellular materials), smooth 

 muscle cells, elastic fibers, and collagenous fibers. 

 Because histology texts tend to emphasize a collection 



