THE HEPATIC CIRCULATION 



1413 



laries foi-27.5 per cent, and the veins for the remainder 

 or 69.2 per cent. Assuming that the gastric, colic, 

 and pancreatic vessels hold no more than twice the 

 amount in the mesenteric vessels, and omitting the 

 spleen, the splanchnic bed in Mall's "dog" held a 

 total of 153 ml or 28.5 per cent of the blood volume 

 of an animal weighing 7 kg (taking the liver weight 

 and blood volume as 2.5 per cent and 7.7 per cent, 

 respectively, of body weight). Since the total values 

 compare favorably with those yielded by other 

 methods, the figures indicating distribution of volumes 

 may be regarded as equally valid in pointing to a 

 predominance of the veins in determining the volume 

 of blood contained within the liver and the splanchnic 

 bed at rest. As noted above, the veins of the splanchnic 

 bed are generously supplied with muscle and it may 

 be surmised that their capacity is subject to change 

 by venomotor activity. 



The obvious constriction or dilatation of veins — 

 including those of the splanchnic vasculatures — in 

 response to chilling, tapping, warming, or various 

 injurious manipulations indicates clearly the ability 

 of the venous musculature to alter the calibre and 

 length of the veins (132). The mechanisms by which 

 venous smooth muscle effects these changes, the 

 integration and function of circular, spiral, and longi- 

 tudinal fibers in different veins, and the patterns of 

 contraction and relaxation are most obscure. Zweifach 

 (317) and others (132) have reported spontaneous 

 "intermittent activity" not only in the arteries and 

 arterioles of the mesenteries but also in the small 

 venules, with cycles of alternate filling and emptying 

 that appear to be irregular, unpredictable, and 

 independent of the innervation. Similar fluctuations 

 have been observed by Knisely and his associates 

 (185) at the level of the central veins and sinusoids in 

 the liver, presumably secondary to activity of the 

 well-muscled sublobular veins. The mass, configura- 

 tion, and extent of the hepatic venous musculature 

 ranges widely among species and is apparently 

 capable of a corresponding range of constrictive 

 action, from complete sphincteric throttling at in- 

 numerable points throughout the total drainage net 

 to a modest reduction in capacity. Unfortunately, 

 quantitative data are lacking and even the qualita- 

 tive studies are so incomplete and fragmentary that 

 it is impossible at present to assess the pattern and 

 extent of change at different levels in a variety of 

 species. 



Spontaneous vasomotion appears to be randomly 

 distributed, involved in strictly local shifts in volume 

 but not in sweeping changes that move blood between 



major units of the cardiovascular system. Studies 

 based upon measurements of circulating splanchnic 

 blood volume (regional dilution of I '"-labeled HSA) 

 indicate that large changes in SBV may occur in the 

 course of normal circulatory adjustments. In man, for 

 example, both tilting into the upright position and 

 exercise in recumbency have been found to induce 

 splanchnic vasoconstriction with a fall in hepatic 

 blood flow and splanchnic blood volume (42). It is 

 not yet clear if a fall in distending pressure secondary 

 to a more marked increase in the gastrointestinal and 

 splenic inflow resistance than in hepatic venous out- 

 flow resistance, or if an active reduction in venous 

 capacity is responsible. The fact that splanchnic 

 denervation interferes with the response to tilting 

 suggests that venoconstriction may be essential. Even 

 if capacity is affected by venomotor activity, however, 

 the extent of filling still depends upon the level of 

 distending pressure and upon the manner in which 

 distensibility is altered by "stretch" itself. 



The arrangement of collagenous tissue in the ad- 

 ventitia, of muscle in the media, and of elastic tissue 

 in the inner layers of vessels appears to result in an 

 elastic behavior resembling that of three springs in 

 parallel, the weakest representing the elastic tissue; 

 the intermediate, muscle; and the stiffest, collagen. 

 Interconnections and viscous changes in muscle and 

 elastic tissue complicate the effort to devise a truly 

 representative model (31, 241) [see also Chapters 

 24 and 26 of this volume]. The stretch or volume- 

 pressure response curve yielded by isolated vessels 

 proves to be concave to the pressure axis at low pres- 

 sures, linear over an intermediate range, and finally 

 convex at high pressures, suggesting that vascular 

 distensibility is dominated initially by muscle, then, 

 by elastic tissue and, finally, by collagen and fibrous 

 tissue, as stretching occurs. Such a sigmoid curve has 

 been obtained for canine splanchnic veins under a 

 variety of conditions in situ (4, 7). The inflections 

 occur at quite different pressures than they do in the 

 aorta in conformity with the differences in structure. 

 The concavity to the pressure axis and flattening 

 occur at a much lower pressure (at about 40 cm saline 

 as opposed to 140 mm Hg for the aorta) indicating 

 dominance of fibrous tissue in the vein. A more 

 marked increase in splanchnic venous distensibility 

 is evident at physiologic pressures during the vaso- 

 constrictive action of catecholamines, presumably 

 because smooth muscle contributes more importantly 

 under these circumstances. At lower pressures (below 

 15 cm saline), or when constriction results in a very 

 low venous cross section, distensibility seems to de- 



