ANATOMY AND PHYSIOLOGY OF THE VASCULAR WALL 



86 9 



elastic fibers, and smooth muscles) must be able to 

 slide past each other with minimal friction during the 

 pulsatile expansion of the vessel. The ground substance 

 is a very viscous material and it probably contributes 

 to the typical visco-elastic behavior of distensible 

 vessels. 



Elastic Tissue 



Elastic tissue is a rubberlike material with high 

 extensibility. It contains the protein elastin without 

 any detectable amount of carbohydrate [Lansing 

 (52)]. In contrast to collagen tissue, it is an extremely 

 insoluble material, and is not influenced by boiling or 

 autoclaving. X-ray diffraction and electron micros- 

 copy do not ordinarily show any internal organization 

 in elastic fibers. It is therefore assumed that the protein 

 fibrils lie without orientation within the fibers [see 

 Lansing (52)]. This disorder gives the elastic tissue its 

 high extensibility. It can be extended to twice its 

 original length, but its tensile strength is ^o to H$0 

 that of collagen tissue (table 1). This explains why it 

 must be protected from excessive elongation and 

 tearing in the vascular wall by the much stronger 

 collagen fibers. 



Elastic tissue forms fenestrated membranes which 

 lie one over the other in elastic vessels. These mem- 

 bra nes serve as footholds for the tension muscles 

 (fig. 3). There is less elastic tissue in the more periph- 

 eral muscular vessels. It is only a very minor com- 

 ponent in the arterioles and precapillary sphincters 

 (fig. 1). Elastic fibers appear in the veins, increasing 

 in amount as they near the heart. They are partly- 

 straight and partly wavy in unstretched vessels. The 

 wavy ones become straight before the collagen fibers 

 straighten out as the pressure rises [Reuterwall (74)]. 

 At ordinary pressures the elastic tissue supports most 

 of the tension in elastic vessels, whereas this task is 

 performed by smooth muscles in the muscular vessels. 



Elastic tissues usually fray with age. This is a normal 

 change which appears in all vessels of old people. 

 Calcification of the fibers is also progressive with age. 

 In addition, the fibers undergo fragmentation, which 

 finally leaves little more than dispersed granular 

 material [see Lansing (52)]. Calcification is especially 

 great in arteriosclerosis. However, Lansing (52) has 

 shown that the percentage of elastin in the vessel wall 

 does not decrease with age, while the calcium content 

 rises in the human aorta from 0.4 per cent in the 

 second decade to 7 per cent in the eighth decade. 

 Frayed and fragmented elastic fibers remaining can- 

 not support the wall tension at normal pressures. This 



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fig. 3. Axillary artery (human). Irradiation of tension 

 muscles in the elastica externa — a: in situ; b: elastica externa 

 artificially lifted off. Muscle endings fasten on the elastic 

 membrane. [Benninghoff (10).] 



task is taken over in old age by collagen fibers, which 

 are under stress with ordinary blood pressure [Bader 

 & Kapal (7)]. Thus, the distensibility of the vessels 

 decreases, but the volume of the aorta increases and 

 its total elastic uptake may remain within normal 

 limits so long as the increase in volume keeps pace with 

 the decrease in distensibility [Kapal & Bader (44), 

 Simon & Meyer (86)]. 



Smooth Muscle 



The smooth muscle cell is an elongated spindle with 

 a single elongated nucleus in the thickest part of the 

 cell. The cells vary very much in size. In the vascular 

 wall they are between 20 and 50 n in length, with their 

 greatest diameter between 5 and 10 n. There are two 

 types of smooth muscles in the vascular wall: "Spann- 

 muskeln'' (tension muscles), which are described in 

 detail by Benninghoff (10, 11), and ring muscles. 



The tension muscles are connected to elastic fibers 

 and membranes, using them as tendons (fig. 3). They 

 can thus raise the tension on the elastic tissue in the 

 vessel wall by contraction (fig. 7) and so affect the 

 blood pressure (see Arteries of the Elastic Type, 



