ANATOMY AND PHYSIOLOGY OF THE VASCULAR WALL 



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fig. 19. Schematic drawing of the vascularization of a 

 middle-sized artery with both its venae comitantes. a: Artery; 

 b: venae comitantes; c: arterial vasa vasorum; d: venous vasa 

 vasorum; e: lymphatic vessel; /.■ capillaries of the arterial 

 sheath; g: capillaries in the stratum longitudinale fibroelasti- 

 cum of the artery = wall capillaries; h: small channel, less 

 than 3 fi thick; i: vascular network in the venous wall; j: 

 capillary network in the venous wall; k: nerve; /.• stratum 

 longitudinale fibroelasticum = adventitia of the artery; 

 m: media of the artery; n: circular sheath of the vessel group; 

 0: conjunctiva of the sheath. [Lang (51).] 



effected by the pressure and concentration gradient 

 across the vascular wall. 



Vasa Vasorum 



The vasa vasorum penetrate the vessel wall to 

 different depths, depending on the thickness of the 

 wall and the type of vessel. The thicker the wall, 

 the greater the part of the wall tissue they supply. 

 In general, veins have greater vascularization than 

 arteries. The vasa vasorum penetrate the aortic wall 

 as far as the inner third of the media (fig. 16). The 

 innermost part of the media and the intima are 

 always free of capillaries. Only arteriosclerotic vessels 

 with a thickened intima show vascularization of the 

 innermost part of the wall [Woerner (101)]. 



The vasa vasorum of the aorta can be classified as 

 vasa vasorum externa and vasa vasorum interna. 

 The vasa vasorum externa originate near the origin 



of arterial branches, such as the intercostal arteries. 

 They soon divide into an outer branch and an inner 

 branch (fig. 17). The outer branch goes into the 

 adventitia and from there sends branches into the 

 wall, whereas the inner branches remain within the 

 wall. Their branching is mostly trichotomous. The 

 vasa vasorum interna originate directly from the 

 lumen (fig. 18) far away from branching vessels, 

 that is, in the aorta on the ventral side [Schonen- 

 berger & Miiller (82)]. They are not very numerous; 

 according to Woerner (101), there are never more 

 than two per square centimeter. They are mostly- 

 found in the proximal part of the aorta and very 

 seldom in the distal part. The vasa vasorum externa 

 and interna anastomose in the aortic wall. The vasa 

 vasorum externa are about 65 to 70 mm in length, 

 the vasa vasorum interna about 30 to 50 mm. 



The more peripherally the arteries are located, 

 the less vascularized is their wall. Figure 19 shows 

 a schematic drawing of an artery with its two venae 

 comitantes from a human shank. The vasa vasorum 

 of the peripheral arteries arise at smaller branches 

 of the artery, similar to the vasa vasorum externa of 

 the aorta. There are no vasa vasorum interna in these 

 arteries. The vasa vasorum never dip into the media. 

 They are located in the stratum longitudinale fibro- 

 elasticum, the innermost part of the adventitia. They 

 form their capillary loops mostly in the longitudinal 

 direction. In addition to these capillary loops there 

 are still smaller vessels. Lang (51) has described two 

 types of such small vessels. The first type is a small, 

 blind-ending channel about 3 mm in length and 

 1 to 3 ju diameter (fig. 19) which is much too small 

 for blood cells to pass. These small channels run into 

 the tip or the venous part of the capillary loop. The 

 second type is a network of small channels of about 

 the same diameter (fig. 20). 



The same vas vasis which supplies the artery also 

 supplies its venae comitantes (fig. 19). In contrast 

 to the case for arteries, the capillaries in the venous 

 wall form a dense network which extends to the media. 

 The segment around the valves is usually not vascular- 

 ized. The venous vasa vasorum do not drain directly 

 into the large vein along which they lie; rather they 

 and their counterparts in the arterial wall drain into a 

 small venous branch [Lang (51)]. 



Schonenberger & Miiller (82) have calculated the 

 drop of pressure in the vasa vasorum externa of the 

 aorta, finding that the capillary pressure within the 

 wall can be sufficiently high only if the origin of the 

 main vas vasis is very near the inner surface of the 

 aortic wall (fig. 17). The intramural capillaries must 

 be quite near to the origin of the vasa vasorum externa 



