THE HEPATIC CIRCULATION 



1407 



splanchnic blood volume of the dog has been proved 

 to average 79.4 =b 8.9 per cent of the simultaneously 

 determined arterial hematocrit (94). Splenectomy 

 does not significantly affect the value, presumably 

 because the tracer is not dispersed throughout the 

 spleen. Since circulating SBV is computed on the 

 basis of the arterial hematocrit it is evident that the 

 value is overestimated by the extent to which the 

 splanchnic hematocrit differs from the arterial and 

 underestimated by failure to include stagnant splenic 

 blood. Nevertheless, the magnitude of the value 

 indicates at once that the splanchnic reservoir can 

 contribute significantly in systemic circulatory homeo- 

 stasis by mobilizing a large volume of blood to repair 

 deficits in the peripheral circulating volume or by 

 expanding to accommodate an excess that might 

 threaten cardiac stability. 



At present, methodology undoubtedly figures most 

 prominently as a cause for contemporary figures 

 denoting hepatic circulatory variance. Active vascular 

 adjustments must also play an important role in 

 producing the variability observed in measurements 

 of flows, pressures, and volumes at "rest" in view of 

 the abundant evidence of muscular tissue and mus- 

 cular activity in influencing flow and volume. The 

 same fundamental mechanisms are involved in the 

 circulatory changes observed during "acute" re- 

 sponses to various stimuli and stresses. Alterations in 

 vascular dimensions and elastic properties and in 

 hemodynamic patterns arise primarily from the 

 varied interplay of vasoconstriction, closure, or col- 

 lapse of vessels, and rearrangement of vascular path- 

 ways, but numerous additional extraneous factors 

 exert a vital modifying, integrating, and directive 

 influence. Among the latter it is necessary to consider 

 neural mechanisms, humoral agents, and external 

 physical forces that are imposed by abdominal 

 muscular contraction, tissue tension, gravity, respira- 

 tory movements, and the like. 



PRIMARY DETERMINANTS OF HEPATIC BLOOD 

 FLOW AND VOLUME 



Cross Section 



Since blood flow and volume are functions not 

 only of the driving and distending pressures but also 

 of the dimensions of a vasculature, splanchnic vascular 

 anatomy may be considered an immediate de- 

 terminant of hepatic hemodynamics. Structure, as 

 such, however, is not constant in its physiologic 



implications nor is it particularly helpful in indicating 

 the control reference state because death and dissec- 

 tion result in disarrangement of the delicate balances 

 that depend upon tissue turgor and muscle contrac- 

 tion. Nevertheless, anatomic data may help in sug- 

 gesting the points at which resistance to flow should 

 be most marked. Mall's (206) careful measurements 

 ol the dimensions and numbers of vessels within the 

 liver and splanchnic bed can still be used, more than 

 half a century after their publication, as a basis for 

 computing sites of resistance. Assuming that each 

 successive category of vessels gives rise to a new sys- 

 tem of resisting conduits in parallel, the cross section 

 of each conduit progressively diminishing to the level 

 of the capillaries; the frictional resistances to flow 

 at each level may be computed from Poiseuille's 

 law of fluid flow through capillary vessels in parallel 

 as follows: 





-L+-L+J- 

 R, R 2 R 3 



where R T is the total resistance imposed by any 

 category of parallel branches and R u R 2 , R 3 , ■ ■ ■ are 

 the resistances imposed by each constituent branch. 

 Since resistance in each branch varies inversely as 

 the fourth power of its radius (r) and directly as its 

 length (7) and the viscosity (jj) of the perfusate: 



and if the average radius (r) is used : 



R T vl or R r-JF* 



where n is the number of vessels in each category. 

 Changes in the values for viscosity and length con- 

 tribute negligibly to the change in total resistance as 

 the vessels narrow and increase in number. The 

 values presented by Mall for the number of branches 

 and for the average cross section at each level indicate 

 that (i/nr ) reaches a maximum in the smallest 

 arterial branches (or arterioles) in the liver, spleen, 

 stomach, and intestines. It may be inferred, therefore, 

 that arteriolar resistance plays a preponderant role 

 in determining splanchnic and hepatic inflow. Be- 

 yond this point in both the portal venous and hepatic 

 venous systems, values for (i/nr ) fall to very low 

 figures though slight increases do occur at the level 

 of the smallest portal venular branches and the 

 sinusoids. No evidence of a significant postsinusoidal 

 resistance may be adduced from these data, though 



