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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



tuous vessels that never branched and were not part 

 of the capillary bed. 



Later, similar vessels in the rat mesoappendix were 

 described (20). The connecting passages in that tissue 

 join a metarteriole with a neighboring venule, or an 

 arteriole with a venule. They are muscular for about 

 two-thirds of their length from the arterial end. When 

 such shunts dilate, blood flow ceases in the arterial 

 components distal to the shunts. 



Direct microscopic observations in other tissues of 

 living animals have revealed arteriovenous anastomo- 

 ses. Wakim & Mann (124) carried out microscopic 

 studies on the liver of frogs and various mammals at 

 magnifications up to 600 times, utilizing the quartz 

 rod transillumination technique. They found arterio- 

 venous anastomoses in all animals studied. They saw 

 anastomotic connections between the interlobular 

 branches of the hepatic artery and the portal vein in 

 both amphibian and mammalian livers. Seneviratne 

 (108) observed blood vessels of the livers of frogs, 

 mice, and rats, and described similar anastomoses. 

 For frog liver he described several phenomena. Many 

 short branches from a hepatic artery enter the ac- 

 companying portal vein. Arterioles cross a lobule and 

 enter a portal vein on the other side. Occasionally the 

 arteriole enters a hepatic vein. Small arterial branches 

 pass through the liver and anastomose with subcapsu- 

 lar arteries. In the mouse and rat many types of 

 anastomoses occur between arterial and venous 

 vessels, the commonest being a direct communication 

 by short branches between the hepatic artery and the 

 accompanying portal vein. 



Irwin & MacDonald (64) studied guinea pig livers 

 using the quartz rod technicjue and found the vascular 

 bed to be similar to that described for the liver by 

 Knisely et al. (72). The Knisely group found connec- 

 tions between hepatic arterioles and portal venules 

 which they called arterioportal anastomoses. Bloch 

 (14) described arterioportal anastomoses (APA) as 

 being completely lined smooth-walled tubes that 

 connect hepatic arterioles with portal venules at 

 irregular intervals. The hepatic arteriole winds itself 

 around the portal venule and then sends short 

 branches out to form APA. 



Parpart et al. (93) describe arteriovenous anasto- 

 moses in the spleen of the mouse as seen by micro- 

 scopic observation. There, about one artery in ten 

 makes direct connection with a collecting vein. An 

 arteriole may anastomose with a collecting vein at 

 any point on the vein, lateral connections occurring 

 more frequently than end-to-end anastomoses. When 

 the connection is lateral, the arteriole is perpendicular 



to the venous wall. In the end-to-end anastomosis, the 

 arteriole gradually widens to become a collecting vein. 



Poor & Lutz (97) found no arteriovenous anasto- 

 moses in the hamster cheek pouch. Irwin et al. (66) 

 found arteriovenous shunts in the lungs of guinea 

 pigs and rabbits, although they appeared infrequently. 

 Blood flow through the shunts was unidirectional, 

 going from arteriole to venule. Blood flow through 

 arteriovenous anastomoses in the bulbar conjunctiva 

 has been described by Bloch (15). Weille et al. (132) 

 saw them in the stria vascularis. 



Zweifach (148) has said that there is little doubt 

 that occasional shunts between arteries and veins 

 exist in almost every tissue in the body, but are not a 

 prominent feature of most tissues. He further suggests 

 that pathways, not distinct anatomical shunts, go from 

 arterial to venous systems allowing blood to bypass 

 the capillary network. Communications between ar- 

 terial and venous vessels occur more frequently in 

 terminal vascular beds than in more proximal por- 

 tions. 



It does not seem necessary to assign any highly spe- 

 cialized function to arteriovenous anastomoses, such 

 as heat regulation, although this is still done (39, 98). 

 Folkow (44) is of the firm opinion that arteriovenous 

 anastomoses in the skin are specialized structures pre- 

 dominantly engaged in regulation of heat loss and 

 are regulated by their own constrictor fibers. His 

 evidence, while convincing, is indirect. Van Dobben- 

 Broekema & Dirken (121, 1 22), in a study of the reac- 

 tion of rabbit ear vessels to heating, offer evidence 

 that there is no obvious relationship between the 

 temperature of the ear and the diameter of the 

 arteriovenous anastomoses. Zweifach (148) mentions 

 the possibility that selective vasoconstriction may 

 reduce capillary circulation and cause blood to be 

 shunted through passages which would offer the least 

 resistance to flow from the arterial to the venous side. 



The information derived from the above investiga- 

 tions indicates that terminal vascular beds of most 

 tissues are supplied with short communicating vessels 

 between arterial and venous systems. These arterio- 

 venous connections allow arterial blood to be shunted 

 into the venous system without first passing through a 

 capillary network. As Zweifach (148) has suggested, 

 the shunts may be preferentially in use when vasocon- 

 striction of small arterial vessels beyond the shunts 

 increases resistance to flow. Arterial blood would then 

 be diverted through shunts which afford the path of 

 least resistance. The selective vasoconstriction to which 

 Zweifach refers might result from the response of 

 terminal arterioles or precapillary sphincters to 



