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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



showed a bowed extensibility curve not unlike that 

 of the aorta or of ligamentum nuchae, and also 

 showed a pronounced hysteresis loop. When f 

 stretched a stocking by the techniques used for an 

 isolated ring of aorta, the dry specimen showed an 

 appreciable rate-dependent element (viscosity) in the 

 tension response to a given strain. When the stocking 

 was wet, this viscous element was relatively reduced, 

 and there was unmasked both a prolonged creep and 

 the "architectural dependency" which is so con- 

 spicuous for the hysteresis behavior of the aorta. 



If the analogy of the stocking is valid, the first 

 part of the stretch curve of the aorta would reflect 

 only a geometrical rearrangement of the net. The 

 resistance to stretch would be a function of the loose- 

 ness of the "weave" and the presence of a lubricant 

 (as in the wet stocking); there also could be a "set" 

 of the net, which could be subject to change with 

 time, with muscular activity, and, very definitely, be 

 influenced by the size of a previous stretch. When, 

 under applied load, the net lost its form, the ex- 

 tensibility would progressively decrease, both because 

 the mechanical advantage of the fibers in resisting 

 the stretch would be increased and because the fibers 

 themselves would now be involved in the extension. 

 If our ideas of the relative extensibilities of the 

 different components is correct, and if they were 

 arranged in the net in series, the elastic fibers, being 

 most extensible, would condition the extension of the 

 whole wall. With more load, these elastic fibers would 

 become stiffer (as they do in ligamentum nuchae), 

 and other components of the net could be increasingly 

 involved. Probably the idea of a parallel outer 

 collagenous jacket should still be retained to con- 

 tribute to the final wall stiffness. 



In an earlier analysis (103) we treated the aorta 

 as though it contained the three tissue types as 

 arranged in parallel. Since muscle had to be able to 

 reduce the vessel diameter below its normal unloaded 

 size, we conceived of the elastic jacket as fitting 

 loosely over the muscle coat. This would mean that 

 muscle alone would be involved in the very first part 

 of the stretch curve, and that only later in the stretch 

 would the elastic fibers start to participate. Such an 

 arrangement seemed amply supported by evidence 

 obtained with stretchings repeated daily, using rings 

 as they were allowed to putrefy. In this process muscle 

 cells lost their integrity first and the unloaded diameter 

 increased while the initial slope of the stretch curve 

 became steeper. Much later, the elastic fibers softened 

 and their continuity became disrupted. Now the 

 unloaded diameter had again increased, and the aorta 



showed a stiffness not unlike that seen at high load 

 levels in the normal state, which was attributed to the 

 collagenous fibers still present. The net model would 

 fit these putrefaction studies equally well, for loss of 

 muscle could partly disrupt the net to give an increase 

 in unloaded diameter and, at the same time, leave the 

 wall less extensible. 



There still remain several features of the visco- 

 elastic behavior of the aortic wall which would not be 

 easily explained on the basis of the net. And the 

 details of net construction are left purposefully vague. 

 The general concept has much in common with the 

 model proposed by Burton (21), except for its de- 

 emphasis of the specific location and role of the 

 muscle fibers themselves. He was much concerned 

 that the muscle be afforded a great mechanical 

 advantage, so that it could always effect a diameter 

 change. Hence he placed these fibers across the plane 

 of a fibrous net, which would protect them from 

 elongation. In muscular tissues it remains uncertain 

 that the contractile ability of muscle fibers is neces- 

 sarily impaired when they are elongated, even by the 

 amount that may normally be developed in an organ 

 such as the urinary bladder. Further, it may be that 

 even in smooth muscle organs the muscle cells are 

 arranged into a somewhat similar net (101, 102). It 

 may be that the muscles in the aortic wall are at- 

 tached to adjacent loops of the net (which would 

 give them more of a parallel arrangement than a 

 series one with the elastic and collagenous fibers), so 

 that they could, by shortening, act to "open the 

 weave," and perhaps increase its "set." This need not 

 mean that the stiffer muscle would now condition the 

 extensibility curve, for we could have, with extension, 

 a warping of the net toward these muscle links. Hence, 

 the internal architecture could be quite different 

 when muscle was contracted than when relaxed, 

 even though the diameter values under a given load 

 might be the same. 



Effects of Active Muscular Contraction on Distensibility 



Whether it is necessary that a model provide 

 muscle with a large mechanical advantage cannot be 

 answered. We are not sure just how effective muscle 

 contraction really is in a vessel under a load equivalent 

 to that of the usual physiological pressure values. 

 Much work is currently being done in which strips 

 of aorta are used as conveniently long tissues to test 

 the effect of drugs, or changed electrolyte environ- 

 ment, on muscle contraction (35, 79, 80, 121). To be 

 useful as a bio-assay material, such strips must be 



