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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



One significant anatomical difference noticed in 

 comparing microphotographs of circulatory patterns 

 in the rat mesoappendix with that of other tissues is 

 the absence of paired arteriole and venule in the 

 mesoappendix of the rat. An arteriole emerges singly 

 from its parent vessel, descends into the mesentery, 

 then forms a loop which in its return is joined by other 

 capillary vessels before emptying into the vein which 

 accompanies the artery of origin. In most vascular 

 beds, other than the mesentery, arterioles and venules 

 are found to be adjacent and to branch together until 

 the final ramification which forms the capillary net. 

 The small arterioles, from which the true capillaries 

 arise, form arcades or arcuate patterns with other 

 arterioles rather than continuing as a direct pathway 

 to the venous side. It is not uncommon to see, in the 

 bat wing at least, a short terminal arteriole that 

 quickly joins a collecting venule after giving off one 

 or two branches (see fig. 22). Contrary to the descrip- 

 tion given to preferential channels these terminal 

 arterioles will close down completely or may be devoid 

 of blood when their parent vessel is occluded by con- 

 traction of the circular smooth muscle which invests 

 them. 



According to subsequent papers by Zweifach ( 1 46, 

 147) and Zweifach & Metz (151, 152), in which 

 vascular patterns are compared, it seems that the 

 preferential channel occurs mainly in rat mesentery, 

 outer edge of rat skeletal muscle, and the serosa of 

 the small intestine of the same animal. He states that 

 the preferential channel is unusually prominent in 

 the mesentery but is not a major structural feature of 

 the urinary bladder and the skin. Further reservations 

 as to the ubiquitousness of the preferential channel 

 have appeared as a result of observations in the under 

 surface of the skin, the skeletal muscle, urinary blad- 

 der, several mesenteric structures, and the serosal 

 surface of the small intestine. Zweifach states that "a 

 major variable lies in the structural organization of 



fig. 22. Arteriovenous pathways in the subcutaneous area 

 of the bat wing. 



the different vascular beds, especially the mode of 

 distribution of the capillary system from the arterial 

 vessels. In such tissues as skin and intestinal wall, the 

 majority of capillaries originate as direct offshoots of 

 larger arteries and arterioles. The distal ramifications 

 of the arterioles have relatively few capillary offshoots 

 and usually terminate by interconnecting freely with 

 one another in a series of arcades. This is in direct 

 contrast to the mesentery where the arterial subdi- 

 visions, the metarterioles, serve as the parent stem 

 from which the precapillaries and capillaries branch 

 out." 



Other investigators who have found thoroughfare 

 channels in various tissues include Lutz el al. (82), who 

 confirmed their presence in frog mesentery but failed 

 to identify them in the hamster cheek pouch or retro- 

 lingual membrane of the frog. Baez (6) reports a short 

 arteriole which turns inward to become a draining 

 venule, thus forming a thoroughfare channel in the 

 muscular coat of the small intestine. Staple & Copley 

 (118) describe a thoroughfare channel in the labial 

 marginal gingiva of the mandibular incisor of the 

 hamster. Lee & Holze (77) observed the thorough- 

 fare channel in the human conjunctivae, and Lee & 

 Lee (78) describe the structure in the mesentery of the 

 guinea pig. 



The preferential or thoroughfare channel, either as 

 a structural or functional unit, has not been seen in 

 some areas which have been subjected to extensive 

 studv by various microcirculatory investigators. Nicoll 

 & Webb (88) report that there are no preferential 

 pathways in the subcutaneous tissues of the bat's 

 wing. Clark & Clark (29) do not report them in the 

 rabbit ear. Grafflin & Bagley (55) found no such struc- 

 ture in the frog web and urinary bladder, nor in the 

 human conjunctivae. Later, Grafflin & Corddry (56), 

 reporting a more detailed study on the bulbar con- 

 junctiva of man, described vessels between arterial 

 and venous channels, arteriovenous communications 

 that seem similar to the preferential channel. 



It would seem then that the preferential channel 

 should not be considered as a component of a typical 

 capillary network. Although it is possible to demon- 

 strate a similar anatomical arrangement in terminal 

 vascular beds other than in the mesentery, there is no 

 confirmation of the existence of a preferential channel 

 on a functional basis. It is possible that such a flow 

 pattern is necessary for the relatively avascular mesen- 

 tery, and therefore constitutes a special rather than a 

 typical entity of microcirculation. 



