ANATOMY AND PHYSIOLOGY OF THE VASCULAR WALL 



879 



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fig. 13. Schematic presentation of 

 the behavior of the different tissues in 

 the wall of muscular vessels with differ- 

 ent degrees of extension. Description 

 in the text. [After Wezler & Schliiter 

 (.00).] 



sion curves also become convex to the abscissa. 

 Schliiter & Wezler (80) have described other curves 

 where the first extension curve had an S-shape or was 

 convex to the abscissa. In all these cases the shape of 

 the diagram seems to depend very much on the state 

 of contraction of the smooth muscles. The extension 

 diagram will be concave to the abscissa if the smooth 

 muscles are contracted, and the slope of the curve 

 will be steeper the stronger the contraction. If the 

 smooth muscles are less contracted, the extension 

 curve will show an S-shape or be concave to the 

 abscissa. The whole diagram moves with frequent 

 stretches to larger diameters or volumes, but comes 

 back to the original place if the smooth muscles are 

 stimulated. Thron et al. (90) obtained similar results 

 in vivo. Figure 12 shows plethysmographically ob- 

 tained pressure-volume diagrams of the human hand 

 vessels at different states of contraction of the vascular 

 muscles, due to different temperatures. The extension 

 curve of the constricted vessels (a) is nearly straight 

 and is followed during release by a large hysteresis. 

 The less constricted vessels have less hysteresis with 

 both distention and release convex to the abscissa. 

 This agrees very well with the diagrams of Wezler & 

 Schliiter (100), which were made in vitro (fig. 1 1). 



The diagrams shown in figures 1 1 and 1 2 are in 

 many ways different from the diagrams in figures 6, 

 8, and 10, which were made from the elastic aorta. 

 The most striking differences are: first, the shape of 

 the aortic diagram remains the same whether the 

 smooth muscles are contracted or relaxed; second, 

 the hysteresis of the elastic vessels is smaller than that 

 of the muscular vessels. This indicates that the ar- 

 rangement of the different wall elements must differ in 

 the two types of vessels. The smooth muscles, which 

 play only a minor role in the elastic vessels, take a 

 major one in the stretch curve of muscular vessels. 

 Wezler & Schliiter (100) have designed a model which 

 may give the action of the three wall elements and in 



different contracted states. This model is shown (sim- 

 plified) in figure 13. It is distinguished from the 

 model in figure 9 by the parallel arrangement of the 

 smooth muscles to the other elements. Sections 1 

 through 3 represent, as in figure 9, different stretch 

 phases. The stretch takes place in the vertical direc- 

 tion. The element a represents an elastic fiber, the 

 two elements b are smooth muscles, w-here the in- 

 dividual muscle fibers are in series, and element c is 

 a collagen fiber. Both the elastic and the collagen 

 fibers are wavy (unstressed) in phase 1. At the begin- 

 ning of the stretch, near zero pressure, only the smooth 

 muscles bear the stress. If the muscle fibers are con- 

 tracted the slope of the pressure-volume diagram 

 will be steep in the beginning and concave to the 

 volume abscissa. Since the contracted smooth muscles 

 behave in general like a visco-elastic material, the 

 pressure-volume diagram will show prominent hys- 

 teresis as described in figure o.b. If the muscle fibers 

 are relaxed, the slope of the pressure-volume diagram 

 will be flatter. As extension proceeds, phase 2 will be 

 reached, in which the elastic fibers are straightened. 

 This will be at a higher pressure if the muscles are 

 contracted than if they are relaxed. Finally, the 

 collagen fibers are involved in the stretch (phase 3). 

 If the smooth muscles are in strong contraction, the 

 whole diagram within physiological pressure limits 

 is concave to the volume abscissa. If the contraction 

 is less, the elastic and collagen fibers come into play 

 at lower pressures and the pressure-volume diagram 

 shows an inflexion point and an S-shape. The pres- 

 sure at which the inflexion point is located depends 

 on the intensity of the contraction. If the smooth 

 muscles are relaxed, the vessels will show in the 

 beginning only a plastic elongation without a rise of 

 pressure, but the pressure will increase when the 

 elastic and collagen fibers are involved in the exten- 

 sion. The pressure-volume diagram is, from the very 

 beginning, convex to the abscissa. The collagen fibers 



