PHYSIOLOGY OF AORTA AND MAJOR ARTERIES 



80: 



of these tissues into more or less well-demarcated 

 layers, physiologists have taken what probably is an 

 oversimplified approach to an analysis of wall ex- 

 tensibility, and have considered each of these tissue 

 types to be unconnected and arranged in parallel. 

 But while elastic tissue does appear to be condensed 

 into layers, it also is interspersed between muscle and 

 collagenous fibers. And there seem to be connections 

 between the gross layers themselves, which means 

 that we can hardly consider the influence of any one 

 tissue type alone. Nor are we at all certain about the 

 elastic characteristics of the different tissues, even if 

 they were to behave independently. The extensibility 

 of the endothelial cells is relatively unknown in any 

 quantitative sense. Since they comprise but a very 

 small part of the wall, and since they certainly are 

 not stiff enough that they are torn by even large 

 stretches, they probably are relatively extensible, and 

 their influence can probably be neglected for the 

 present. An estimate of the extensibility of collagenous 

 fibers has been taken from that shown by tendon, 

 although the collagenous fibers of the latter are larger 

 and more densely packed. As opposed to a tissue 

 containing elastic fibers, tendon is quite inextensible 

 and shows a linear stress-strain relation with no 

 discernible visco-elastic properties (21, 52, 67, 97, 

 103, 112). The aortic wall stiffening seen when the 

 pressure rises above about 100 mm Hg has thus been 

 attributed to the resistance to stretch offered by the 

 enclosing jacket of collagenous fibers (21, 103). Aortic 

 walls from which elastic tissue and muscle have been 

 digested show a similar stiff wall (50, 1 1 1 ). This 

 jacket must fit loosely and be considered in parallel 

 to the underlying elastic tissue. Once it starts to 

 participate in the stretch, it will assume the bulk of 

 the applied load. 



On the basis of stretch curves shown by ligamentum 

 nuchae, which is predominantly elastic tissue, elastic 

 fibers are much more extensible than collagenous 

 fibers. Chemically treated aortas which retain only 

 their elastin show an increased extensibility, one not 

 greatly different from that of the whole vessel in the 

 lower stretch range (51, 52, 74, 111). Most workers 

 ascribe a linear stress-strain relation to these fibers, 

 too. Hence my work (97) seems to stand alone in the 

 claim that ligamentum nuchae shows a nonlinear 

 curve not unlike that of the arteries (with a stiffening 

 in the upper tension range despite the absence of any 

 clear collagenous fibrous coat), and also has visco- 

 elastic properties similar to those of the aorta. Since 

 the elastic fibers in both organs seem arranged in a 

 reticulum, and since the visco-elastic properties of 



dog ligamentum nuchae seem clear, the following 

 analysis will be based on a similarity in stretch be- 

 havior of the two tissues. 



Assessment of the extensibility of smooth muscle 

 cells is on most insecure ground. Studies have been 

 made of the stretch properties of muscular organs, 

 such as the bladder or gut, for this purpose (6, 97, 

 101, 102), although the muscle fibers here are en- 

 meshed in a loose weave of collagenous and even some 

 elastic fibers too. If this muscular tissue is subjected 

 to a moderately rapid stretch, its extensibility is only 

 about a third as great as that shown by ligamentum 

 nuchae (97). But because these tissues have such a 

 pronounced time-dependent creep, if one waits for 

 the length to approach a final value under a given 

 load, the total extensibility is greater than that of 

 elastic tissue. This has been the procedure used when 

 elastic moduli for muscular structures have been 

 derived. But it seems unreasonable that the muscle 

 contained in the aortic wall could show such a pro- 

 longed creep. If the muscle cells are coupled to the 

 elastic fibers, creep would be effectively prevented by 

 the resistance these fibers would offer to an elonga- 

 tion. Of course, the greater the amount of muscle 

 involved in the vessel wall, the greater would be the 

 creep. At one extreme, the umbilical artery, which is 

 almost solely muscle, shows a very pronounced stress 

 relaxation under load (122, 141). In most large 

 arterial vessels the relaxation is of more limited 

 degree. Hence the muscle contained would probably 

 be stiffer than the elastic fibers, which would remain 

 the most extensible part of the wall. 



It should be noted that the pulmonary artery shows 

 a difference in distensibility behavior from the aorta. 

 The form of the stretch curve is more akin to that of 

 a large vein (105). The vessel shows more creep than 

 does the aorta (32). The form and the total length 

 change of the stretch curve are different in pulmonary 

 vessels that have been frozen and thawed than when 

 simply kept in Ringer-Tyrode solution (a reflection of 

 the effect of viable muscle?) (32). 



There is histological evidence that, in the aortic 

 wall at least, muscle cells, elastic fibers, and some 

 collagenous fibers are interlinked into a three- 

 dimensional network (11, 109). The elements in this 

 network could be partly in series and partly in 

 parallel. The extensibility of the whole tissue could 

 be a reflection of the form of the net just as well as 

 it could be conditioned by the individual tissues. For 

 example, Bull (20) showed that while a single nylon 

 thread obeyed Hooke's law, and had no visco-elastic 

 behavior, a stocking woven from such a thread 



