CHAPTER 26 



The anatomy and physiology of the vascular wall 



HERMANN BADER Physiologisches Inslilut, Universitiit Wurzburg, Wiirzburg, Germany 



CHAPTER CONTENTS 



Elements of the Vascular Wall 



Endothelium 



Collagen Tissue 



Ground Substance 



Elastic Tissue 



Smooth Muscle 

 Different Types of Vessels 



Arteries of the Elastic Type 



Vessels of the Muscular Type 



Capillaries and Arteriovenous Anastomoses 



Veins 

 Nutrition of the Vascular Wall 



Diffusion from the Inside 



Vasa Vasorum 



blood vessels serve as a conducting system for the 

 blood. They carry the blood, forced by the heart, 

 throughout the whole body and back again to the 

 heart. To make this possible there must be a pressure 

 gradient with its highest values in the aorta and its 

 lowest values in the large veins. The circulation there- 

 fore withstands a much higher pressure on the arterial 

 side than on the venous side, a difference which is 

 reflected in the architecture of the wall. 



The stress on the vessel wall is, according to Frank 

 (30), proportional to the blood pressure and the ratio 

 of radius to wall thickness 



<r~ = 



t 



(I) 



[a = stress (force per unit area), p = pressure, r = 

 radius, / = wall thickness]. 



The relationship r/t and the composition of the 

 vessel wall (fig. 1 ) show the level of blood pressure. 

 For example, the aorta has a much smaller ratio of 



r/t than the vena cava, which means that the differ- 

 ence between the wall tensions of the two vessels is less 

 than might be expected, considering the difference 

 between the blood pressures to which they are sub- 

 jected [Burton (20)]. The relationship r/t is smaller in 

 vessels where the principal component is smooth 

 muscle than in vessels where the major component is 

 elastic tissue. For that reason, the smooth muscles, the 

 contractile elements of the wall, need to withstand 

 smaller stress than the elastic tissue, one of the passive 

 elements of the wall. 



The vessels must be tight, so that no blood is lost 

 on the way through the circulatory system. The vessel 

 wall is therefore lined with the endothelium, which 

 serves as a semipermeable membrane for the exchange 

 of material between blood and tissue. 



The arterial side of the circulatory system is not 

 only a simple conducting system, but is also an elastic 

 buffering chamber. The vessels, mainly the aorta, are 

 stretched during systole, and store energy which 

 enables them to force the blood along by elastic recoil 

 during diastole. This ability is given the vessels by 

 their elastic tissue. However, as Frank (3 1 ) has shown, 

 tubing which consists only of distensible material like 

 rubber will "blow out" at a critical pressure. This does 

 not occur in normal blood vessels, since they have a 

 relatively inextensible jacket in addition to the 

 elastic tissue. This jacket is composed of collagen 

 tissue, and is located in all arteries and veins. The 

 collagen tissue bears the stress when the pressure 

 becomes high, protecting the vessel from rupture or 

 blow out. 



Another quality of blood vessels is their ability 

 to regulate the blood supply to different organs, 

 depending on the need. When an organ is active, 

 more blood must be carried to it than when it is at 



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