1426 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



marco & Rimini (117) believe that reduction in 

 venous cross section at the level of the diaphragm 

 and within the superior vena cava will impede return 

 of blood to the heart during inspiration. Thus, radio- 

 micrographic studies in the rat indicate that hepatic 

 outflow may be more rapid during expiration (58). 

 This thesis has been vigorously disputed, on the other 

 hand, by the group with Wiggers in Cleveland which 

 has included Opdyke (222), Alexander (3), and 

 Brecher (64). These workers have shown that, 

 though venous collapse may increase resistance with 

 an increased pressure drop at the diaphragm, there 

 is nonetheless an inspiratory increase in return, 

 provided the tendency to venous collapse is not ex- 

 aggerated by marked changes in the gradient, and 

 provided alteration in the timing of phasic changes 

 does not predispose to "depletion" of the venous 

 chamber with resultant collapse. Both groups are 

 probably correct, however, in view of the manner 

 in which the response may be altered by extraneous 

 factors, such as respiratory rate, position, and body 

 size. During inspiration under ordinary circumstances 

 the blood held in the hepatic venous tree and the 

 inferior caval system flows out somewhat more 

 rapidly than inflow so that venous return is initially 

 augmented — and splanchnic outflow increased. The 

 accompanying compression of outflow channels and 

 the rise in intra-abdominal pressure would operate 

 to reduce inflow so that net flow might change very 

 little, rise or fall, depending upon the duration and 

 frequency of the inspiration phase. Direct observation 

 as well as measurement of flow bears out this con- 

 clusion, at least for the dog. The increased outflow 

 resistance may actually conduce to portal venous 

 pooling (3), again depending upon the interplay of 

 all the other factors concerned. It may actually be 

 rather difficult to define splanchnic flow and volume 

 under these circumstances because the '"depleting" 

 phase of inspiration may be followed by filling during 

 expiration not only from the arterial side but also by 

 retrograde flow from the right atrium into the hepatic 

 venous chamber as angiographic studies have shown. 

 The effect of anatomic and dimensional differences 

 requires further study. 



exercise. The effect of exercise must be determined 

 to a large extent by the manner in which it affects 

 respiratory activity, intra-abdominal pressure, and 

 gas exchange as well as by release of various vaso- 

 active agents. In quietly resting human subjects in 

 recumbency, exercise (alternate leg raising) induces a 

 significant reduction in both hepatic blood flow and 



splanchnic blood volume presumably as a result of 

 vasoconstrictive activity (298). Reallocation of 

 splanchnic blood volume seems to occur quickly 

 and may indeed play a role in the maintenance of 

 cardiac output prior to the establishment of a new 

 equilibrium. When blood pressure rises, the fall 

 in splanchnic blood flow may not occur in spite of 

 vasoconstriction. This phenomenon has been ob- 

 served in dogs (superior mesenteric arterial flow- 

 measured by thermostromuhr) exercised on a tread- 

 mill (172). Possibly there was a similar absence of 

 change in EHBF, in the face of an increment in he- 

 patic temperature, in three normal human subjects 

 studied by Graf (152). The Bromsulfalein clearance is 

 of questionable value in evaluating the effect of exer- 

 cise, since BSP extraction tends to increase in associa- 

 tion with a rise in hepatic arteriovenous oxygen 

 difference (34, 200). Barcroft and his associates (20) 

 found that exercise (running) caused a significant 

 splenic contraction in dogs and cats, which tended to 

 persist in proportion to the duration and severity of 

 exertion. A definite pattern of response of splanchnic 

 arteriolar and venous constriction can be made out 

 but, in view of the varied and opposing forces that 

 are brought into play during exertion, a diversity 

 of responses is probably the rule in normal life. 



HEPATIC CIRCULATORY INTEGRATION 

 AND DYSFUNCTION 



Hepatosystemic Interrelationships 



The participation of the hepatic and splanchnic 

 circulation in general systemic reactions is usually 

 diflicult to detect and to delineate. The changes ob- 

 served during exercise, assumption of the upright 

 position, and respiration have been noted above 

 because they entail a direct effect upon the intra- 

 abdominal vasculature. In addition, any tendency 

 for cardiac output or arterial pressure to fall or to 

 rise is associated with concomitant changes in hepatic 

 blood flow and splanchnic blood volume. In the 

 main, these adjustments appear to provide for con- 

 tinued perfusion of the liver without undue inter- 

 ference with corrective responses elsewhere in the 

 body to restore the status quo ante. Owing to the com- 

 plexities of the splanchnic circuitry, however, the 

 precise mechanisms of local adjustments are usually 

 obscure. Little or no information is available regard- 

 ing minor shifts. More is known about adjustments in 

 such extreme disorders as circulatory collapse and 

 congestive heart failure. Unfortunately, the need for 



