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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Aorta 





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Varta Cava 



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fig. I . Variety of admixture of the four tissues in the wall 

 of different blood vessels. The figures under the name of the 

 vessel are the diameter of the lumen and below it the thickness 

 of the wall. [Burton (20).] 



rest. This is best done, according to the law of Poi- 

 seuille, by changing the radius of the small supplying 

 vessel by relaxation of the smooth muscles in the 

 arterial wall. 



Any living organ must be nourished. All but the 

 smallest blood vessels have their own circulatory- 

 system, the vasa vasorum, which supplies blood to the 

 vessels from the outside. In addition, simple diffusion 

 from the inside transports nutrients and oxygen to the 

 inner avascular layer of the blood vessel. This outward 

 fluid shift may be aided by the radial pressure gradient 

 through the vessel wall. 



The purpose of this chapter is to discuss and to 

 interpret these qualities and functions of the vascular 

 wall and to explain the performance of the various 

 wall elements in the different types of vessels. 



ELEMENTS OF THE VASCULAR WALL 



The terms used in this article are, for the most 

 part, those denned by Landowne & Stacy (50). Here 

 we will consider some of these terms in detail. Collagen 

 tissue, elastic tissue, and smooth muscle have three 

 qualitites in common, which appear differently. These 

 qualities are elasticity, visco-elasticity, and plasticity. 



Elasticity is that property of a material which 



determines its tendency, when stressed, to return to 

 its unstressed geometrical configuration without loss 

 of energy. If a material is completely elastic, all energy 

 applied to it by an external straining force can be 

 recovered as mechanical energy. Figure 2a shows an 

 extension release cycle of such a perfectly elastic 

 material, illustrated by a spring. Any given length 

 has its particular tension. The extension curve and 

 release curve are the same. It can be linear, as in 

 figure 2(7, or convex or concave to the abscissa, de- 

 pending on the material stretched. Tension-length 

 diagrams of organic materials usually show a curve 

 which is convex to the abscissa. A perfectly elastic 



c 

 o 

 l/l 



c 



length 



fig. 2. Behavior with stretch of different materials. Tension 

 and length are taken as arbitrary, a: Elastic material, demon- 

 strated by a spring. Extension and release curve are the same. 

 b: Visco-elastic material demonstrated by a spring, which has 

 a brake disc on the top and which moves in a liquid. Extension 

 and release curves inscribe a hysteresis loop, the size of which 

 depends on the velocity of the extension and release. Outer 

 curve: fast stretch; inner curve: slow stretch. An infinitely slow 

 stretch gives the same curve as a. c: Viscous or plastic material 

 demonstrated with a brake disc, which moves in a liquid. The 

 material keeps every length to which it was brought by an 

 external force. The force depends on the velocity of the exten- 

 sion. Upper curve: fast stretch; lower curve: slow stretch. In each 

 the rate of viscous deformation is constant. 



