THE HEPATIC CIRCULATION 



1405 



method yielded a mean value of 1530 ± (sd) 300 ml 

 per min (48), which appears to be fairly representa- 

 tive [and certainly not differing significantly from 

 the figures published by other workers using the 

 same or other methods (33, 78, 231, 242, 273, 296)]. 

 The wide range observed suggests a considerable 

 variation in flow that is also evident (though by no 

 means to the same extent) during the course of a 

 single study in the same subject. For the dog, the 

 values obtained by different workers differ much more 

 significantly (37, 90, 129, 232, 275, 282). To a large 

 extent the disagreement may be ascribed to dif- 

 ferences in preparation, anesthesia, and surgical 

 manipulation. Anesthesia appears to be particularly 

 difficult to control, since it may be associated with a 

 varying degree of hypercapnia with resultant 

 splanchnic vasoconstriction. Light barbiturate anes- 

 thesia appears to produce no change in splanchnic 

 hemodynamics in man so long as the plasma carbon 

 dioxide tension is kept constant (123). Artificial 

 respiration with various mechanical devices pre- 

 disposes to hypercapnia in man and it may be assumed 

 that this is also true of the dog. Hence, it seems reason- 

 able to accept the mean values for EHBF obtained 

 in unanesthetized dogs by Pratt (232), Bollman (37), 

 Fisher (129), and their co-workers of 43.6 ml, 42.5 

 ml, and 45 ml per kg body wt per min, respectively, 

 as the best available estimates. As in man, variance is 

 relatively large (in Fisher's series, for example, the 

 standard deviation was ±9.3 ml/ kg body wt/min) 

 and a similar variation is observed during the course 

 of a single study. The values for EHBF are not cor- 

 related with body size in man as they are in the dog, 

 presumably because the range of variation in body 

 size in man is so much less than in the dog and be- 

 cause a correlation may be obscured by other factors 

 responsible for variance in "resting" EHBF. Dobson 

 & Jones (1 10) have reported mean values for hepatic 

 blood flow (chromic phosphate method) in the un- 

 anesthetized rabbit, rat, mouse (0.74, 1.2, and 1.4 

 ml/ml liver/ min, respectively) in rough agreement 

 with those for man and dog. Similar values have been 

 reported also, in terms of body weight, for sheep and 

 cattle (138, 260). 



Splanchnic J 'oscular Pressures and Resistances 



The figures available for arterial and venous pres- 

 sures and for pressure differentials in different species 

 also indicate close similarities though a definitive and 

 systematic investigation remains to be done. In 

 every series the values range so widely that inter- 



species differences are apparently insignificant (17, ;_\ 

 76, 135, 244, 286, 314). This variation may be ex- 

 plained largely by the technical difficulty of establish- 

 ing strictly comparable "zero reference planes," 

 states of "resting normality," and laboratory condi- 

 tions. Nevertheless, mean arterial pressure may be 

 taken as approximately 100 mm Hg in both man and 

 dog, portal venous pressure as 10 mm Hg, and central 

 (or atrial) venous pressure as o. Since portal and 

 wedged hepatic venous pressure in dog and man 

 behave in the same way and attain the same levels, 

 the value for sinusoidal pressure of 8.5 mm Hg com- 

 puted for the dog by Friedman & Weiner (135) as 

 the midpoint between wedged hepatic and wedged 

 portal venous pressures may be accepted also for 

 man. The pressure gradients therefore are 90 mm 

 Hg between artery and portal vein, 91.5 between 

 hepatic artery and sinusoids, 1.5 between portal 

 vein and sinusoids, and 8.5 between sinusoids and the 

 right heart. 



The resistances that determine these drops in 

 pressure between the arteries and veins can be 

 evaluated only when the distribution of blood flow 

 is known. Exact figures are not available but most 

 workers (though not all) tend to accept the view that 

 hepatic arterial inflow is approximately one-half 

 portal venous inflow (132). If this is the case and if 

 resistances may be computed as the ratio between 

 pressure drop (given above) and flow per second 

 (1530 ml in man and 550 ml in the dog) multiplied 

 by a factor — 1332 — to obtain figures in absolute 

 units (dynes cm -5 sec), the following values for re- 

 sistances within the splanchnic bed would obtain in 

 man and dog (10 kg): 



Arterial sinusoidal (hepatic arterio- 

 lar— R,) 



Arterial portal venous (splanchnic 

 arteriolar — R 2 ) 



Portal venous sinusoidal (portal ven- 

 ular— R 3 ) 



Sinusoidal inferior vena cava (post- 

 sinusoidal — R t ) 



Man Dog 



14,630 48,750 



7200 24,000 



1 20 400 



450 1510 



The interrelationship between resistances is com- 

 plicated by the fact that the splanchnic circulation 

 consists of a combination of resistances both in 

 series and in parallel (fig. 1). The computation of 

 any component requires a precise information re- 

 garding the distribution of total blood flow as well 

 as pressure gradients. Any attempt to infer behavior 

 of a given resistance from values for the pressure 



