ANATOMY AND PHYSIOLOGY OF THE VASCULAR WALL 



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fig. 10. Extension-release curves of the 

 thoracic aorta of the human at different ages 

 and with different numbers of stretches. I: 

 14 years old — (a) 4th stretch cycle; (b) 100th 

 stretch cycle. II : 63 years old — (a) 3rd stretch 

 cycle, (4) 15th stretch cycle. [Wagner & 

 Kapal (94).] 



[Bader & Kapal (7)]. The unextensible fibers might 

 be collagen fibers or calcified elastic fibers which are 

 under stress with small extension of the arterial wall 

 [Roach & Burton (76)] or even with no extension at 

 all. 



Elastic vessels, like any tissue, show typical visco- 

 elastic and plastic behavior. An extension-release cycle 

 gives a hysteresis loop which depends in part on the 

 velocity with which the stretch was applied [see 

 Remington (72)]. There is also a shift, on repeated 

 stretching, of the pressure-volume diagram toward 

 greater volume at the initial pressure level, indicating 

 some plasticity. Wagner & Kapal (94) have found 

 with experiments on the human aorta that hysteresis 

 is not only dependent on the stretch velocity, but also 

 on the age of the vessel (fig. 10). It becomes smaller, 

 the older the vessel is. The same effect appears if an 

 aorta is stretched repeatedly. The more frequently 

 the artery is stretched, the smaller is the hysteresis. 

 The hysteresis is greater above the inflexion point of 

 the pressure- volume curve than below [Wagner & 

 Kapal (94)]. 



In large elastic vessels, contraction of smooth 

 muscles does not influence the hysteresis [Remington 

 (72)]. Kapal (42) has shown that the aorta responds 

 to dynamic stresses as would collagen and elastic 

 tissue, but not like smooth muscle. Therefore, it seems 

 that the visco-elastic and plastic behavior of elastic 

 arteries depends mostly on elastic tissue, collagen 

 tissue, and ground substance, but only to a small 

 degree on smooth muscles. This is confirmed by the 



curves of figures 6 and the model in figure 7, where 

 smooth muscles do not affect the mechanical prop- 

 erties of the vessel (see also fig. 5). However, in the 

 more peripheral vessels the increasingly plentiful ring 

 muscles have a correspondingly greater effect on visco- 

 elastic behavior [see Peterson et al. (66), Bergel (12, 

 13)]. The greater elastic incompleteness of collagen 

 fibers, as compared to elastic fibers (see table 1), 

 agrees very well with the larger hysteresis in the upper, 

 collagen-dependent part of the pressure-volume 

 diagram. But with both collagen and elastic tissue, 

 the elastic incompleteness seems to diminish as more 

 stretch cycles are made. The similarity between 

 decrease of the hysteresis with age and with repeated 

 cycles has led to the assumption that, as a result of 

 their elastic incompleteness, the vessels are distended 

 more and more by the pulse pressure during their 

 life, until they reach a stable state, eliminating the 

 visco-elastic and plastic elements [Wagner & Kapal 

 (94)]- 



Vessels of the Muscular Type 



The more peripherally the arteries are located, the 

 higher is the percentage of smooth muscles in the wall 

 (fig. 1). In elastic arteries one cannot distinguish easily 

 between intima, media, and adventitia, whereas in 

 muscular arteries there is a clear separation of these 

 layers. The media consists mostly of smooth muscles, 

 the ring muscles. Between them are collagen and 

 elastic fibers. The elastic membranes, typical for the 



