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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



cannot be extended ad infinitum, but tear at a certain 

 length. 



Smooth muscle is a special case. It can be elongated 

 like a purely plastic material and can behave at any 

 given length like an elastic or visco-elastic material. 

 But it can also recover its original length by contrac- 

 tion. This means that the plastic property of smooth 

 muscle can be neutralized or hindered, leaving only 

 the elastic or visco-elastic elements under stress, as a 

 result of the action of the contractile element. [For 

 further details see Reichel (71)-] 



Endothelium 



The circulatory system is lined almost completely 

 by a single layer of very thin polygonal-shaped cells, 

 the endothelium. This forms a tight, smooth surface 

 on the inside of the vascular wall and serves as a 

 semipermeable membrane for the interchange of 

 materials between blood and tissues. It has a high 

 distensibility. Its ability for regeneration is very good. 

 For instance, 3 weeks after implantation aortic grafts 

 show a smooth continuous lining of endothelial cells, 

 which presumably are built from fibrocytes [Petry & 

 Heberer (67)]. A detailed discussion of the qualities 

 of the endothelium is given in Chapter 29. 



Collagen Tissue 



Collagen tissue is produced by fibroblasts, which are 

 located in all connective tissues. The probable pre- 

 cursors of collagen fibers are the reticular fibers. These 

 are argyrophilic fibers which are found especially in 

 places where collagen fibers are forming, as around 

 aortic grafts. They both show a banded appearance 

 under the electron microscope [see Wassermann (95)]. 

 The collagen fibers consist of a network of long 

 protein chains which are linked by H bonds and ionic 

 bonds. This network is filled with an amorphous 

 substance (mucopolysaccharide). Smaller fibers are 

 glued together to larger fibers by a cement substance 

 which is continuous with the ground substance. 



This structure gives the collagen fibers a very high 

 elastic modulus and also makes them very flexible [see 

 Harkness (36)]. Collagen fibers are nearly 25 times as 

 strong as elastic tissue but 15 times less extensible 

 (table 1). Collagen fibers are found in all vessels, 

 spread over the whole wall. They appear in the un- 

 stretched vessel wall as wavy bundles, but some of 

 them become straightened if the pressure within the 

 vessel is raised above the mean blood pressure [Reuter- 

 wall (74)]. This anatomical design, together with the 



table 1. Elastic, Visco-E/astic and Plastic Behavior 

 of Collagen and Elastic Tissue (102) 



high elastic modulus, enables them to form a "jacket" 

 [Burton (20)], which is put in action only when the 

 fibers are straightened, by increased intraluminal 

 pressure. Thus the collagen fibers are not strained by 

 the normal blood pressure; they serve as a safety factor 

 for the vessels and keep them from ''blowing out" at 

 high pressure. 



Only the collagen fibers increase in number with 

 aging, replacing frayed elastic fibers and degenerated 

 smooth muscle cells [Meyer (58), Kobayashi (46)]. 

 Since the elastic and muscular tissues originally sup- 

 ported the wall tension at normal blood pressures, the 

 replacing collagen fibers must take over this task 

 [Bader & Kapal (7)] with the result that the wall 

 becomes less distensible. This is compensated until 

 the sixth decade of life by enlargement of the diame- 

 ters of the vessels involved [Simon & Meyer (86)]. 

 However, collagen tissue, when overloaded, does show 

 elastic incompleteness, which means that it is to some 

 extent a plastic material (table 1). The fibers do not 

 return to their original length immediately after exten- 

 sion and the residual elongation can be 67 per cent 

 of the total elongation. This plasticity may produce a 

 large hysteresis. It may be that such plastic property 

 can account for the increase in vessel volume seen in 

 aging. 



Ground Substance 



The ground substance has the properties of a col- 

 loid — it is water-insoluble, but can bind water. It 

 consists of the mucopolysaccharides: hyaluronic acid 

 and chondroitin sulfate. It is likely that chondroitin 

 sulfate forms the cement substance which binds 

 collagen fibers together, and hyaluronic acid serves 

 as a lubricating material [see Harkness (36)]. Such 

 a lubricating substance is necessary in the vessel wall, 

 since the fibrous elements of the wall (collagen fibers, 



