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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Microcirculation in the Mesentery 



Early descriptions of mesenteric circulation patterns 

 by Zweifach (143) and Chambers & Zweifach (20) 

 dealt primarily with establishing a structural and 

 functional unit, the preferential or thoroughfare 

 channel, which was thought by the authors at that 

 time to be a representative structure of terminal 

 vascular beds. A discussion of the vascular compo- 

 nents of the mesenteric circulation and blood flow 

 through them appeared in 1954 (147)- Zweifach 

 stated that the mesentery represented a simplified 

 vascular structure devoid of ancillary features peculiar 

 to specific organs. In observing normal circulation in 

 the rat mesoappenctix (cecal mesentery), Zweifach 

 found the larger arteries along one side of the mesen- 

 tery to be about one-third as large in diameter as 

 their paired veins. The depth of anesthesia influenced 

 the caliber of these vessels, deep anesthesia causing 

 them to dilate until both vessels had the same diame- 

 ter. Respiratory difficulties caused venous constriction. 

 Terminal arterioles in the mesentery proper had a very 

 rapid flow of blood. Collecting veins had a steady flow 

 of blood with continuous forward flow without cessa- 

 tion or temporary reversal. Capillary circulation, 

 however, showed intermittent flow produced by the 

 contraction of precapillary sphincters, the activity of 

 which was irregular and unpredictable. Preferential 

 channels were found to be unusually prominent in 

 the mesentery. The most important structural compo- 

 nent for regulating capillary blood flow was the pre- 

 capillary sphincter. The precapillary sphincter was 

 found at the junction of all offshoots of the muscu- 

 lar components of the vascular bed. The true capillary 

 network was made up of endothelial tubes with no 

 perivascular muscle cells. In some areas the collecting 

 venules were formed by the joining of several side 

 branches leading from precapillary sphincters. Both 

 terminal arterioles and venules were seen to be inter- 

 connected to form a series of arcades, so extensive in 

 some cases that they completely circumscribed the 

 capillary bed. The metarterioles originated as off- 

 shoots of the arteriolar arcades, extended toward the 

 center of the tissue distributing typical precapillary 

 branches. The arteriolar channels terminated as one 

 or two short capillaries which fed directly into a 

 venous vessel. Zweifach expresses the opinion that the 

 primary mechanisms which readjust circulation 

 through the capillary bed are essentially of a humoral 

 nature. Neurogenic mechanisms, local metabolic fac- 

 tors, and blood-borne substances from organs con- 

 tribute to the local regulation. 



Microcirculation in the Hamster Cheek Pouch 



The use of the hamster cheek pouch for microscopic 

 study of the peripheral circulation was introduced by 

 Fulton et al. (50, 51). 



Although there is no detailed description by these 

 authors of the basic vascular pattern that is seen in 

 this mucous membrane, the literature contains refer- 

 ences to the presence or absence of various vascular 

 structures which will be presented here. 



The cheek pouch is exceedingly vascular compared 

 to rat mesentery or membranes in transparent cham- 

 bers. The pattern differs also from the mesentery in 

 that no preferential channels have been found. A rich 

 network of anastomoses between venous vessels and 

 arteriolar vessels is present. Arterioles, which supply 

 the capillary network, bifurcate progressively into 

 branches of equal significance for the distribution of 

 blood (fig. 5). The arterioles exhibit spontaneous 

 vasomotion (80, 82). Lutz & Fulton (81) state that 

 precapillary sphincters were seen to contract inde- 

 pendently of adjacent smooth muscle in cheek pouch 

 vessels. Intermittent flow from small veins was also 

 seen, but no venous sphincters were identified. 



Lutz & Fulton (81) point out that there is always 

 variation in the demand for blood by the organs, and 

 this variable demand can be satisfied by vasomotor 

 responses without involving the heart or other large 

 structures. The complex anastomosing system of ves- 

 sels in the cheek pouch, for instance, coupled with 

 vasomotion, permits changes in flow. Neither the 

 vessel wall nor the flow are ever quiescent, the most 

 striking feature of the small vessels being their con- 

 stant activity. 



More vein-to-vein than artery-to-artery anasto- 

 moses are seen in the hamster cheek pouch. Venules 

 make up the greatest amount of endothelial surface 

 and contain the greatest proportion of circulating 

 blood at any one time. Lutz and Fulton believe that 

 60 to 70 per cent of the peripheral circulating blood 

 is in the venous vessels. 



Poor & Lutz (97) studied the functional anastomotic 

 vessels in the cheek pouch and reported that artery- 

 to-artery anastomoses were generally one-third to 

 one-half the size of the parent arteriole. These were 

 outnumbered by the vein-to-vein anastomoses. The 

 venous anastomoses were nearly the size of the veins 

 which they connected (fig. 6). 



Microcirculation in Skeletal Muscle 



The description of the distribution of minute vessels 

 in skeletal muscle has not changed to anv marked 



