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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



lagenous fibers constituting the adventitia. The 

 adventitial layer becomes the major component of 

 the wall of larger veins. 



Another important difference between arteries and 

 veins is in the structural relationships adjacent to the 

 capillary bed. Whereas the arterial channels possess 

 significant muscle terminating in conspicuous pre- 

 capillary muscular elements at the arteriole-capillary 

 junction, minute venules are devoid of muscle. Con- 

 verging capillaries become surrounded with a col- 

 lagenous network to form small venules which mav 

 not acquire a continuous muscular media until 

 diameters of the order of 0.5 mm are reached. It must 

 be clearly recognized on a purely structural basis, 

 therefore, that there is no mechanism at the venous 

 end of the capillaries capable of throttling blood flow- 

 in the way that blood flow may be controlled at the 

 arteriolar end (85). 



In describing these general structural features of 

 veins, reference is specifically being omitted to some 

 of the important variations which are found in the 

 adaptations of specific venous beds to local problems. 

 Bv way of illustration, suffice it to say that in the long 

 veins of the extremities there is the development of a 

 significant component of longitudinally oriented 

 muscle capable of counteracting the gravitational 

 stresses to which these vessels are subjected, while 

 within the cranium venules develop to considerable 

 size without the appearance of any muscular ele- 

 ments (61). 



Vasa Yenarum 



Crucial to an understanding of some aspects of 

 venous function is a recognition of the role of the vasa 

 venarum, which constitute the normal route through 

 which both nutrients and vasoactive substances reach 

 the vein wall. Older literature on this topic has been 

 reviewed by Ramsey (76). There is a dense network 

 of minute vessels in the adventitia of the larger blood 

 vessels which is particularly conspicuous in veins. 

 Although some techniques have failed to reveal a 

 penetration of the capillary plexus into the media, 

 adequate methods have succeeded in demonstrating a 

 profuse capillary bed extending almost to the intima 

 (67). In addition to the capillary plexus, there is clear 

 evidence of an accessory duct system, presumably 

 lymphatic in nature, which is distributed through the 

 adventitia and media. 



It mast be emphasized, however, that the vasa 

 venarum do not penetrate the intimal layer and drain 

 through the local endothelium. Venous drainage from 



the capillary plexus returns to venules running along 

 the superficial layer of the adventitia, and eventually 

 drains into either an entirely different vein or a 

 remote site of the same vein. O'Neill has pointed out 

 that this relationship assures that local obstruction in 

 a venous segment will not block the flow in the vasa 

 venarum, nor can local pockets of high intraluminal 

 pressure induce backflow in the vasa venarum of the 

 venous wall. A similar relationship exists in the 

 arteries. 



Functional confirmation of the anatomical rela- 

 tionships described above has been provided by 

 O'Neill (67). Extensive damage to the intima fol- 

 lowed stripping the tissues surrounding the vein so as 

 to interrupt the vasa venarum, even though blood 

 flow was maintained through the lumen of the vein. 

 This indicates that oxygen and nutrients do not pass 

 in significant amounts from the lumen into the sur- 

 rounding tissue of the vein wall, and that the venous 

 wall is clearly dependent upon the vasa venarum. 

 Comparable evidence may be observed with drugs. 

 Minimal response to vasoactive agents can be demon- 

 strated when the drug flows through the lumen of the 

 veins, while very effective vascular responses result 

 when the drug is applied systemically or topically so 

 that it may reach the media from the adventitial side. 



This anatomical arrangement seriously handicaps 

 the study of functional changes in the vasa venarum. 

 In the case of arteries, the vascular wall is supplied 

 not only by vessels penetrating from the adventitial 

 side, but also by some vasa vasorum interna which 

 penetrate the wall directly from the lumen. Smith 

 (83, 84) has taken advantage of this relationship to use 

 the amount of leakage, in response to internal pres- 

 sure changes, from the surface of an excised arterial 

 segment as a measure of vasa vasorum flow. A similar 

 technique would not be applicable to veins. In the 

 study of diseased veins, a further complication arises 

 in that the anatomical pattern changes qualitatively 

 as well as quantitatively. The early inflammatory 

 phase of vascular disease stimulates a dense invasion 

 of vascular elements into the wall of the vessel with the 

 creation of venous channels that penetrate the intima 

 directly into the lumen of the vessel, creating vasa 

 venarum which have no counterpart in normal veins. 



Innervation 



Veins are copiously supplied with nerves which 

 Thompson (86) demonstrated, in 1893, to be capable 

 of producing constriction of the vein. Bayliss & Star- 

 ling (8) confirmed the existence of neurogenic veno- 



