THE HEPATIC CIRCULATION 



1425 



as the zero reference planes for the respective sys- 

 tems. In man, the arterial zero reference plane lies 

 at the level of the diaphragm immediately after tilt- 

 ing into the upright position, with moderate change 

 thereafter during vasomotor adjustments (309). 

 Thus the elastic properties of the vasculature are 

 such as to maintain the arterial pressure relatively 

 constant at the level of the diaphragm; pressures at 

 points above, falling, and below, rising, solely by the 

 weight of the column of blood lying between each 

 point and the zero reference level. The imposed blood 

 column does not reach to the level of the uppermost 

 body surface, e.g., the top of the head, because the 

 closed elastic container exerts an attractive force in 

 supporting that portion of the blood above the refer- 

 ence plane. A similar elastic buffering of hydrostatic 

 shifts occurs in the veins. In both dog (86) and man 

 (309) the venous side of the circulation is divided 

 dynamically by cardiac activity into two separate 

 hydrostatic compartments each with a separate 

 zero reference plane. The immediate hydraulic 

 changes with change of position are minimized on 

 both the arterial and venous side and little immediate 

 change in hemodynamics occurs in dog or man. 

 Within a few seconds after assumption of the head-up 

 position, however, the blood pressure does tend to 

 fall and the heart rate to speed. Widespread vaso- 

 constriction quickly checks the decline in arterial 

 pressure. The splanchnic vasculature partakes in this 

 general response, since it has been observed that 

 splanchnic blood flow (EHBF) decreases significantly 

 in man during orthostasis (99). This change is im- 

 paired in hypertensive patients following lumbodorsal 

 sympathectomy (310). Circulating splanchnic blood 

 volume is also reduced in the upright position pre- 

 sumably as a result of reflex alteration in the splanch- 

 nic vascular capacity (42). Further work is needed 

 to define these changes in experimental animals. 



respiration. The intra-abdominal pressure figures 

 prominently in a more active sense as one of the forces 

 involved in determining changes in splanchnic 

 blood flow during respiration. Evaluation of the 

 changes in pressure gradients and flows during the 

 respiratory cycle is complicated by the difficultv of 

 defining precisely what takes place in general terms. 

 The number of factors involved and the variety of 

 combinations possible under different circumstances 

 makes generalization extremely hazardous. Even the 

 dynamics of quiet breathing in recumbency in man 

 can vary from time to time and from person to person, 

 depending upon the individual pattern of abdominal 



and thoracic muscular interplay, fatigue, extent of 

 gastrointestinal and bladder filling, apprehension 

 and the state of consciousness, muscular development, 

 pulmonary or cardiac dysfunction, blood volume, 

 and many other variables. It is to this irregularity 

 that a remarkable diversity of opinion must be at- 

 tributed (64, 132). 



The fact that pressure within the thorax tends to 

 be lower than atmospheric pressure during inspira- 

 tion and somewhat higher during expiration is self- 

 evident; the question that is difficult to answer is how 

 and to what extent this phasic change in pressure is 

 transmitted to the splanchnic vasculature. With 

 descent of the diaphragm during inspiration the vis- 

 cera are forced into the abdominal cavity, the vascu- 

 lar bed subjected to shortening and buckling, and the 

 liver and spleen compressed to some extent as they 

 are thrust out of the thorax. During expiration shifts 

 in the opposite direction must occur. In association 

 with these changes, opposing changes in intra-abdom- 

 inal and thoracic pressure probably occur in such a 

 way as to increase the pressure gradient between 

 abdomen and thorax during inspiration and to re- 

 duce it during expiration. In the main, these in- 

 ferences find support in the experimental record but 

 they must be modified under many conditions. 

 For example, it is possible for contraction of the 

 abdominal muscles during expiration and relaxation 

 during inspiration to counter the usual effect easily 

 and to produce a reversed pattern. Indeed with 

 maximal or laborious breathing in man, this seems 

 to be the rule (73). In the upright position the super- 

 imposed hydrostatic pressure changes also have an 

 effect. As noted above, the intra-abdominal pressure 

 under these circumstances is governed to some extent 

 — modified, of course, by muscular tone and activity — 

 by the hydrostatic forces and the shift in arterial 

 and venous zero reference planes. The weight of the 

 upper abdominal viscera may be supported in part 

 by the retractive forces of the thorax and its contents 

 and, with impeded diaphragmatic excursion, an 

 abdominal component in the respiratory pattern 

 may figure more prominently. The magnitude and 

 direction of thoracico-abdominal pressure shifts 

 may have direct bearing upon blood flow into or 

 out of the splanchnic vasculature or upon the quan- 

 tity of blood held within it at any moment. This is 

 so because collapse of the large draining venous 

 channels may increase resistance to flow more than 

 the rise in the pressure gradient tends to enhance it. 

 This point has been much debated and still remains 

 unsettled. On the one hand, Holt (174) and Duo- 



