PATTERNS OF THE A-V PATHWAYS 



9>9 



fig. 21. Camera-lucida outline of 

 vessels in the capillary bed of the frog 

 mesentery. [From Zweifach (143)-] 



said to have a vigorous circulation even when tissues 

 were in a resting or anemic state. The bridge was 

 regarded as a muscular capillary and was the central 

 pathway from which the remainder of the capillary 

 vessels were distributed as side channels. 



In 1944, Chambers & Zweifach (20) collaborated 

 on a paper in which the studies were confined to the 

 mesenteric circulation in the dog and the rat. They 

 state that the fundamental architecture is the same 

 for both tissues. The mesoappendix of the rat differs 

 from other parts of the mesentery in its lack of any 

 major vessels coursing from the aorta or to the vena 

 cava, all vessels in the mesoappendix being less than 

 80 to ioo/x in diameter. The description of the capil- 

 lary bed is based primarily on observations in the 

 mesoappendix of the rat, in which the a-v bridge is a 

 prominent structure. The term "metarteriole" (Gr. 

 meta — beyond) is introduced to designate the proxi- 

 mal contractile portion of the central channel. 

 Beyond the metarteriole, muscle cells disappear and 

 the channel continues as the a-v capillary until it 

 joins a venule. Other contractile muscle cells, desig- 

 nated precapillary sphincters (the precursors of the 

 true capillary), were found at the proximal end of the 

 channel but were absent at the venular end. Each 

 central channel and its side branches with their inter- 

 posed true capillaries were said to constitute a struc- 

 tural unit. In a summary in 1946, Chambers & 

 Zweifach (21) state that the basic topography of a 

 predominantly nutritive type of capillary bed is 

 presented as a central channel of which the true capil- 

 laries are side branches. The different portions of the 

 central channel, in sequence, were the metarteriole 

 which exhibits vasomotion and has typical but dis- 

 continuous muscle cells; the proximal portion with 

 atypical muscle cells; the distal portion with no 

 muscle cells; and the nonmuscular venule. The pre- 

 capillaries were described as the proximal muscular 

 portions of the abrupt offshoots of the muscular por- 

 tion of the central channel, acting as sphincters and 



controlling blood flow through capillaries. The true 

 capillaries continue from the piecapillaries and are 

 also direct branches of the distal portion of the central 

 channel and of the nonmuscular venule. 



In 1947, the functional aspect of the structural unit 

 was again emphasized (22). It was pointed out that 

 in some tissues which maintain a constant level of 

 flow volume there is no discernible organization of 

 capillaries, while in tissues such as the muscular sys- 

 tem and the gastrointestinal tract with varying 

 activity the structural unit exists. This vascular 

 pattern allows for great expansion in the number of 

 vessels with an active circulation at one time and 

 restriction of flow to the preferential channel during a 

 period of inactivity. In the decade following the 

 introduction of the preferential channel, new ideas 

 and new terminology were added. For clarification 

 of the structure and function of the terminal vascular 

 bed in the rat mesentery, excerpts from a paper by 

 Chambers & Zweifach (22) follow. "The preferential 

 vessels have been termed thoroughfare or a-v 

 channels. The proximal portion of these channels, 

 termed metarterioles, together with their precapillary 

 sphincteric offshoots, are muscular and spontaneously 

 undergo periodic changes in caliber. This type of 

 movement has been termed vasomotion, a slow inter- 

 mittency of partial relaxation and constriction at 

 intervals of about 30 seconds to 3 minutes. . . . The 

 precapillary sphincteric offshoots lead into an inter- 

 anastomosing system of true capillaries (devoid of 

 muscle elements) which constitutes the bulk of the 

 bed. The capillaries rejoin the distal continuation of 

 the thoroughfare channels through inflowing tribu- 

 taries." The thoroughfare channels, always open, are 

 said to maintain a constant pressure relationship 

 between their arteriolar and venous ends, the flow of 

 blood through them being more rapid than thiough 

 other vessels in the bed. The channel is said to be the 

 site of outward filtration, while inward filtration 

 occurs in the true capillaries. 



