THE HEPATIC CIRCULATION 



[ 39' 



flucnce their behavior. They radiate from the portal 

 tracts and converge upon the central veins, produc- 

 ing the appearance in section of hexagonal "lobules" 

 that are centered upon the central vein. This appear- 

 ance is apparently attributable to the degree of filling 

 of the peripheral vessels between the portal tracts, 

 since it has been shown that elevation of hepatic 

 venous pressure or reduction in portal venous pres- 

 sure changes the configuration of the lobule to one 

 centered upon the portal tracts as a result of relative 

 distension of the vessels running between the central 

 veins (121). The walls of the sinusoids are composed 

 of thin endothelial cells, possibly [as Knisely et al. 

 (185) claim] all capable of active phagocystosis, 

 though this question is not yet settled. There is no 

 evidence of muscular tissue and the endothelium is 

 usually closely attached to the parenchymal cells. A 

 narrow perisinusoidal fluid-filled (plasma?) space 

 (Disse) observed on many occasions by light micros- 

 copy has been clearly delineated by the electron 

 microscope (30, 126, i6g). Numerous relatively 

 large fenestrations in the sinusoidal endothelium 

 may permit the plasma to come into direct contact 

 with the hepatic cells. Both luminal and the canalic- 

 ular surfaces of the parenchymal cells are markedly 

 increased by folds and microvilli. The space between 

 the endothelium and the polygonal cells is apparently 

 no greater than 0.5/1 and it may be filled with an 

 amorphous material resembling basement mem- 

 brane rather than plasma. Since the extravascular 

 space is so narrow, the sinusoidal closure must in- 

 volve displacement and apposition of the surrounding 

 cell plates. 



The hepatic venous drainage system begins in the 

 colander-like thin-walled central veins that empty 

 into the muscular sublobular veins. In certain re- 

 spects the central veins appear to be passive sumps 

 not strictly separable from the parenchyma and not 

 unlike a large receiving sinusoid. Opening and 

 closure of sinusoids at the point of entry into the 

 central vein have been described by workers using 

 transillumination techniques (186), but definite 

 structural evidence of muscular sphincters seems to be 

 lacking. In contrast the muscle coats of the "sub- 

 lobular" and other hepatic veins appear to be en- 

 tirely adequate for this purpose. Gibson (143) finds 

 that sinusoids empty only into the central veins al- 

 though Deysach (107) has claimed that sinusoids 

 may occasionally enter the larger hepatic venules 

 directly as "sluice channels" which may bypass the 

 central venous sumps. Gibson believes these vessels 

 are really central veins and he agrees with Deysach 



in viewing the point of passage through the thick 

 muscular wall as a site at which contraction could 

 interfere with flow. In the dog, contraction of the 

 musculature can throw the large as well as the small 

 hepatic veins into corrugated folds that could con- 

 ceivably block outflow completely (287). The extent 

 of this musculature appears to vary greatly in dif- 

 ferent species, but it appears to be weak and rela- 

 tively unimportant in man (132, 143). The dis- 

 tribution of the hepatic veins and their tributaries 

 results in an intimate interdigitation with the system 

 of portal tracts. The finer radicles course at acute 

 angles or perpendicular to the portal tracts from 

 which they are derived. There is no evidence of seg- 

 mentation or lobation as there is in the arrangement 

 of the portal tracts. Except in animals with deeply 

 fissured and lobated livers, the hepatic veins freely 

 cross the "avascular plane" separating the hepatic 

 segments to bind the liver into a single vascular mass. 

 The system empties into the inferior vena cava by 

 three or more terminal branches just below the dia- 

 phragm or within the "caval tunnel" where the in- 

 ferior vena cava is closely applied to or incorporated 

 in the posterior surface of the liver. In certain species 

 (notably the dog and diving mammals) the muscular 

 coat of the hepatic vein becomes more prominent 

 and forms a sphincteric ring at the orifice into the 

 vena cava. The preponderance of the inferior portion 

 of the ring may serve as a "sling" to pull up the lower 

 lip of the opening into a valvelike ridge. 



A lymphatic drainage system runs parallel to the 

 vascular inflow and outflow tracts, to communicate 

 at the hilum and at the junction of the hepatic veins 

 and inferior vena cava with larger trunks that ul- 

 timately carry the lymph through the local lymph 

 nodes to the cisterna chyli or thoracic duct (85). A 

 rich network of lymph vessels lies about and within 

 the walls of the draining vessels and beneath the 

 capsule of the liver, but there is little evidence for 

 lymphatic capillaries within the parenchyma. It is 

 possible that the perisinusoidal space is the terminus a 

 quo of the lymphatics. A more definite point lies in 

 fluid-filled spaces, the so-called spaces of Mall, 

 found in close proximity to the portal tracts, between 

 the limiting plate and the connective tissue making 

 up the bulk of the tract. Arrangements of cells and 

 channels resembling very small lymph nodes are 

 described or pictured at many points within these 

 nets and a rich lymphopoietic layer is found beneath 

 the capsule of the liver in some of the lower verte- 

 brates. Erythropoietic tissue may also occur in adult 



