PATTERNS OF THE A-V PATHWAYS 92 1 



Arteriovenous Anastomoses 



A detailed and comprehensive review by Clark (23) 

 in 1938, dealing with arteriovenous anastomoses, 

 obviates the necessity of reporting individual investi- 

 gations up to that time. The discussion here is mostly 

 confined to the results of in vivo studies. 



A direct connection between arteries and veins by 

 passages through which blood is carried without inter- 

 change with extravascular fluids had been described 

 repeatedly since the early 1800's. Such passages were 

 then considered to be rare, occurring as a result of 

 injury or as a developmental anomaly. Their presence 

 in the ear of the living rabbit, as demonstrated by 

 Clark & Clark (29, 30) and Grant (57), established 

 their existence in the normal vascular bed. 



Grant (57) concluded from his observations of reac- 

 tions of these vessels that arteriovenous anastomoses 

 were important in regulating body temperature. 

 Responses to heating the animal indicated that when 

 the body temperature was elevated, dilation of 

 arteriovenous anastomoses permitted a large amount 

 of blood to flow through the ear, thereby increasing 

 heat loss. Constriction of arteriovenous anastomoses 

 occurred when the animal was cooled and thus heat 

 was conserved. This concept was extended in studies 

 of the toes of birds and the fingers and toes of man (58). 



Clark & Clark (30) studied the arteriovenous 

 anastomoses in transparent chambers in the rabbit 

 ear with observations over long periods at high magni- 

 fications. High magnifications made possible the 

 descriptions of structural components. Many arterio- 

 venous anastomoses are present in the ear of the rab- 

 bit, with considerable variations in their arrangement. 

 Some arise directly from the central arterv of the 

 ear, others from secondary or smaller branches, and 

 some form the termination of an artery or arteriole. 

 Most of these, however, arise from small arterial 

 branches. They all empty primarily into larger veins 

 (see fig. 23). 



As to the structure, arteriovenous anastomoses are 

 found to be straight or coiled, with a thick muscular 

 wall on the arterial side and a thinner, funnel-shaped 

 widening on the nonmuscular venous end. Variations 

 from this general pattern include the absence of the 

 funnel-shaped venous end and a continuous muscular 

 wall throughout the entire vessel. The narrow inter- 

 mediate portion has a wall of extra thickness which 

 seems to be the most contractile portion. The venous 

 portion is noncontractile, but the large veins with 

 which the communicating vessel connects often have 

 substantial muscle walls and show definite contrac- 



ART 



fig. 23. Camera-lucida drawing of a plexus of regenerated 

 vessels in the rabbit ear. [From Clark & Clark (34).] 



tility. Most of the cross connections show inside di- 

 ameters of 20 to 40 n during dilatation. Typical 

 anastomoses may be as small as 5 y. in diameter or as 

 large as 40 n- 



Arteriovenous anastomoses are more active than 

 arteries and arterioles and show a greater tendency for 

 independent action. They contract and dilate spon- 

 taneously and periodically, but with a rhythm inde- 

 pendent of either neighboring anastomoses or even of 

 the artery from which they arose. They generally con- 

 tract more rapidly than arteries, both rhythmically 

 and in response to stimuli. 



Clark (23), did not attempt to explain the function 

 of the arteriovenous anastomoses, but felt that it was 

 significant that they occur normally in greatest num- 

 bers at sites most frequently subjected to mechanical 

 and thermal irritations, the kinds of stimuli which 

 produce prolonged dilation of arteries and arterioles. 



From his observations of the frog mesentery, 

 Zweifach (143) describes short arteriolar-venular 

 anastomoses between vessels only slightly larger than 

 capillaries. These short channels effectively divert 

 arterial blood directly into veins. Their caliber changes 

 seem related to the activity of the capillary circulation. 

 When most capillary vessels are open and have active 

 blood flow, the arteriolar-venular anastomoses remain 

 closed, and then open when capillary circulation de- 

 creases. The anastomoses differ in this respect from 

 arteriovenous bridges which maintain a relatively 

 fixed diameter. Arteriolar-venous anastomoses in the 

 mesentery of the mouse were described as short, tor- 



