PATTERNS OF THE A-V PATHWAYS 



907 



fig. 11. Photomicrograph from the anterior wall of rat 

 ileum. (Courtesy of Dr. Silvio Baez.) 



between adjacent layers of muscle. The parent vessel 

 either divides in two, or arches and becomes a venule 

 which isjoined by venules from neighboring capillary 

 nets before entering a submucosal vein. Such an 

 arrangement, whereby a metarteriole leaves the sub- 

 mucosa and enters the smooth muscle coat where it 

 gives off a capillary network and then returns to the 

 submucosa as a venule, constitutes, according to 

 Baez, a distinctly organized terminal vascular unit. 



The minute vessels of this unit are both muscular 

 and nonmuscular, the muscular component being the 

 centrally located metarteriole and the precapillary 

 vessels which branch from it and in turn give rise to 

 the nonmuscular capillary network. The muscular 

 vessels and their parent metarterioles are considered 

 by Baez to be the most highly reactive of the mural 

 vasculature, a fact demonstrated by vasoactive drugs 

 and varying intraluminal pressures. The capillary 

 bed of the muscular coat of the gut, which is served 

 by these metarterioles, shows periodic changes in 

 blood flow; the changes being independent of flow 



through the arterial plexus of the submucosa. The 

 muscular coat may be devoid of circulation while 

 blood continues to flow through arteries of the sub- 

 mucosa and mucosa. 



In some instances the parent vessel may begin as a 

 short arteriole, rather than a metarteriole, which in 

 turn then gives rise to several metarterioles when it 

 reaches the muscular coat. The pattern of distribution 

 is then the same as described above; the metarteriole 

 forms a central channel, and turns inward to become 

 a venule or breaks up into two or three capillaries. 



The mucosal artery continues toward the muscu- 

 laris mucosa after having given off the vessels just 

 described which go to the outer muscular coat. One 

 or two short vessels are now seen to branch from the 

 mucosal artery. The short vessels subdivide into 

 several capillaries which reunite as a venule and 

 empty into a submucous vein. Deeper in the sub- 

 mucosa all mucosal arteries anastomose with a sim- 

 ilar mucosal artery and give rise to one or two 

 branches which in turn subdivide to form capillary 

 nets. The mucosal artery terminates by penetrating 

 the base of a villus. 



Baez was unable to find any arteriovenous anasto- 

 moses in the submucosa of the jejunum or ileum. It 

 was possible to follow all the arteries and arterioles of 

 the submucous plexus to their finest ramifications 

 without observing any short cuts from the arterial 

 to the venous side. This was also true in vessels in the 

 muscular coat. He does elaborate, however, on the 

 direct connection between arterioles and venules at 

 the bases of villi. While the arterial component does 

 deliver arterial blood directly to the venule which 

 drains the villus, it cannot be called a true arterio- 

 venous anastomosis in that the arteriole gives off 

 branches to neighboring structures. The location of 

 these vessels is the same as the location of vessels 

 described by an earlier investigator (117) as arterio- 

 venous anastomoses. Baez points out that in an in- 

 jected and fixed preparation the capillary offshoots 

 might be closed, giving the appearance of a true 

 arteriovenous anastomosis. 



Two or three venules from adjacent villi were seen 

 to converge to form a mucosal venule. The mucosal 

 venule also was joined by an arterial capillary which 

 originated as a branch of the nearest mucosal artery. 

 The small vein thus formed then emerged into the 

 submucosa where it was joined by other veins of 

 similar origin to form an intricate anastomosing 

 arcade. These submucous arcades were further en- 

 larged by venules from the outer muscular coat. The 

 flow of blood through the venules of the muscular 



