ANATOMY AND PHYSIOLOGY OF THE VASCULAR WALL 



873 



lium. Then there are veins, which are built to some 

 extent like the arteries. The most striking difference 

 between them is their mounting, for most arteries 

 have a slack connection with the surrounding tissues, 

 whereas the veins are more intimately bound up with 

 the surrounding tissues to make a functional svstem. 



Arteries of the Elastic Type 



The wall of the elastic arteries is characterized by 

 a high percentage of elastic tissue (fig. 1), which may 

 be 40 per cent of the wall in the thoracic aorta, but 

 decreases toward the periphery. The elastic tissue 

 is mostly present as fenestrated membranes — up to 

 50 membranes located one upon the other. There 

 are also star-shaped membranes in the wall of the 

 pulmonary artery [Meyer (59)]. There is a network of 

 elastic fibers between all these membranes. The 

 membranes are connected by smooth muscles, the 

 tension muscles, described by Benninghoff (10, 11). 

 These tension muscles use the elastic membranes as 

 footholds (fig. 3). There are no ring muscles in the 

 thoracic aorta, but they appear in increasing numbers 

 in the more peripheral arteries. Where the ring mus- 

 cles exceed the tension muscles in amount the arteries 

 are called muscular arteries. The collagen fibers are 

 distributed over the entire wall. They lie there in 

 wavy bundles, which become straight if the blood 

 pressure rises over the normal mean value [Reuter- 

 wall (74)]. 



The large amount of elastic tissue and looseness of 

 the collagen fibers give elastic arteries high disten- 

 sibility. For instance, the aorta can be distended to a 

 threefold increase in contained volume over that at 



300 



ZOO 



fig. 6. Pressure-volume diagram of the thoracic aorta of 

 the pig. a: Extension curve made shortly after sacrificing. 

 The aorta was stimulated with epinephrine, b: Extension 

 curve made 8 days later. Smooth muscles were dead. [Bader & 

 Kapal (5).] 



zero pressure. This high distensibility enables an 

 elastic vessel to act as would an air chamber (Wind- 

 kessel). The aorta contributes over 50 per cent to the 

 total vascular air chamber action [Wetterer (98)]. If 

 an elastic artery is stretched, it shows a typical S- 

 shaped pressure-volume diagram, like that in figure 

 6 where "static" stretch curves are given for the 

 thoracic aorta of a pig. [See Chapters 7 and 24 for 

 the explanation of the typical S-shaped pressure- 

 volume diagram of elastic arteries and its relation 

 to the tension-length diagram. [See also Frank (30, 

 31).] A similar S-shaped curve may be obtained from 

 a rubber tube within a nylon tube, where the nylon 

 tube serves as a "jacket" [Bader & Kapal (6)]. In 

 such a pressure-volume diagram the rubber tube is 

 responsible for the curve below the inflexion point 

 and which appears concave to the abscissa; the nylon 

 jacket for the convex part above the inflexion point. 

 This fact, together with the finding of Reuterwall (74) 

 that when elastic tissue becomes straight collagen 

 tissue is still wavy (i.e., still relatively unstretched) 

 and the study of Roach & Burton (75) which in- 

 volved differential digestion of collagen and elastic 

 fibers of the iliac artery, indicates that the part of the 

 pressure-volume diagram from zero pressure to the 

 inflexion point reflects the extension of elastic tissue, 

 whereas the part above the inflexion point is due to 

 the collagen tissue [see Bader & Kapal (7)]. 



The upper curve of figure 6 is derived from a stretch 

 curve made shortly after death, after the aorta was 

 stimulated with epinephrine; the lower curve was 

 made 8 days later when the smooth muscles were dead. 

 Schonenberger & Miiller (83) got similar results on 

 cow aortas with dynamic stretches. Millahn & Jaster 

 (61) stretched pig and cow aortas after relaxing the 

 smooth muscles with acetylcholine, finding that the 

 stretch curve lay below the curve given by the stimu- 

 lated vessel. The pressure-volume diagram can shift 

 to higher or lower pressures depending on the con- 

 tractile state of the smooth muscle, but the shape of 

 the curve never changes. This proves that smooth 

 muscles can increase the wall tension without chang- 

 ing the elastic properties of the vessel, a finding which 

 Benninghoff (10, 11) had proposed as a result of his 

 microscopic studies. Bader & Kapal (5) concluded 

 from their experiments that smooth muscle can be 

 arranged neither in series with the elastic elements 

 nor in parallel. Both arrangements would give, with 

 stimulation, not only a shift of the stretch curve to 

 higher pressures, but also a change of the shape of 

 the curve. 



Since the tension muscles are attached to the elastic 



