i 4 44 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



termined by counting the isotope. Since the spheres 

 were too small to lodge in any arteriovenous vessels 

 except the capillaries, this fraction should represent 

 the proportion of the total blood flow which passed 

 through the capillaries of that tissue. For fasted ileal 

 segments the proportions were mucosa, 38 per cent; 

 submucosa, 8 per cent; muscularis, 22 per cent; and 

 mesentery, 15 per cent. The remaining 17 per cent 

 passed through vessels larger than 12 /x in diameter. 

 The flows in milliliters per minute per gram of tissue 

 were mucosa, 0.42; submucosa, 0.34; muscularis, 

 0.48; and mesentery, 0.69. For fasted jejunal seg- 

 ments, the distribution and flows per gram of tissue 

 were not significantly different, except that the total 

 blood flow was somewhat higher, the difference 

 passing through arteriovenous channels larger than 

 12 xx. 



The other method was employed by Rayner et al. 

 (107) in dogs under morphine analgesia and by 

 Weiner (137) in dogs under pentobarbital anesthesia. 

 Segments of fasted ileum were artifically perfused 

 with blood containing deuterium oxide for short 

 periods of time. On the assumption that the kinetics 

 of distribution of isotopic water is blood-flow limited, 

 the perfusion rate for each tissue was calculated from 

 its isotopic content at the end of the perfusion period. 

 In milliliters per minute per gram of tissue, Rayner 

 et al. obtained the following flows: mucosa, 0.38; 

 submucosa, 0.56; muscularis, 0.66; and mesentery, 

 0.23. Weiner found 0.42, 0.50, 0.51, and 0.16 for 

 the same tissues. 



Comparison of the results obtained with the 

 microsphere and with the D 2 methods in pento- 

 barbital-anesthetized animals shows the major 

 discrepancy to be in the estimation of flow through 

 the mesentery. It seems most likely that the D 2 

 technique underestimates flow through this fatty 

 tissue. The higher submucosal flow obtained with 

 the latter technique may indicate that some D>0 

 exchange occurs across the walls of arteriovenous 

 bridges which abound in that tissue. 



Vessels of Different Sizes 



Many histological studies have demonstrated that 

 the arterioles and venules of the mesenteric organs 

 are connected by vessels varying from true capillaries 

 with a diameter of 10 ju or less to arteriovenous bridges 

 or throughfares with diameters in the range 10 to 

 20 xx to true arteriovenous anastomoses having 

 diameters in excess of 20 xt. The distribution of organ 

 blood flow to these different-sized channels has been 

 investigated in only a very few instances. 



Walder (135) estimated the flow through arterio- 

 venous anastomoses in the artifically perfused human 

 stomach by measuring the perfusion rate before and 

 after presumably blocking all the capillaries with 

 starch granules. On this basis, he concluded that the 

 proportion of the total flow passing through arterio- 

 venous anastomoses was about 5 per cent. 



Lindseth (95) measured the blood flow through 

 arteriovenous vessels of different sizes by injecting 

 small, known numbers of radioactive spheres of 

 different diameters into the arterial supply of the 

 canine intestine. He employed spheres with mean 

 diameters of 12, 20, and 44 /x. Measurement of the 

 number of such spheres which passed through the 

 organ into the venous blood permitted him to calcu- 

 late the partition of the total blood flow among the 

 arteriovenous channels of the three sizes. He found 

 that essentially no 44 xx spheres passed through in 

 either the jejunum or ileum; 3 to 4 per cent of the 

 blood flowed through vessels having diameters be- 

 tween 20 and 44 /x; and 24 per cent in the jejunum 

 and 14 per cent in the ileum passed through vessels 

 1 2 to 20 xx in diameter. Thus, in the fasting canine 

 intestine, the fraction of the total blood flow which 

 passes through true arteriovenous anastomoses 

 (vessels greater than 20 /x in diameter) is very small. 

 However, a quite significant fraction may flow 

 through arteriovenous bridges and hence bypass the 

 capillaries. 



The finding that such a small fraction of the blood 

 passes through channels larger than 20 xx is in 

 agreement with results of investigations by Gordon 

 et al. (58). These workers estimated the size of the 

 largest arteriovenous channels in the intestine of rats 

 and rabbits anesthetized with sodium pentobarbital 

 by determining the minimum pressure required to 

 force mercury, air, or kerosene through the vascula- 

 ture of these organs. They concluded that the intes- 

 tines contain no vessels larger than 25 xx in diameter. 



Several investigators have injected large quantities 

 of glass microspheres into the arteries supplying one 

 of the mesenteric organs and determined the max- 

 imum size of the spheres which passed through. 

 Sherman & Newman (124) did this in the stomach 

 and duodenum of the dog and recovered some 

 spheres as large as 100 to 180 it. Prinzmetal (106) 

 reported spheres of 160 to 37011 in the splenic vein 

 of dogs studied in the same way. Walder (134) made 

 a more complete study and used much smaller 

 quantities of spheres in the artificially perfused human 

 stomach. He found the diameter of the largest 

 spheres to pass through were 1 40 ll and of the mean 



