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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



forms, though it is usually more prominent in fetal 

 livers (120). 



The liver is a remarkably malleable organ, the 

 adjoining organs molding its surface and determining 

 its shape. Changes in the filling of the stomach in the 

 dog, for example, induce considerable alteration in 

 the configuration of the portion of the liver lying in 

 contact with it. The influence of these deformations 

 on the local hepatic circulation does not seem to 

 have been studied. It is likely that only the more 

 superficial parts of the liver are involved and that 

 resistance to flow is increased in the compressed 

 regions. The liver appears to be equally plastic 

 when interference with outflow, as in congestive 

 heart failure, results in an elevation in vascular dis- 

 tending pressures. Presumably the marked hepatic 

 enlargement involves stretching of lamellae, canalic- 

 uli, and the connective tissue framework (121). 

 To what extent the distortion affects the degree and 

 distribution of resistance to blood flow remains un- 

 determined. Certainly, if the distortion is long main- 

 tained, persistent change in architecture occurs and 

 fibrosis develops. 



METHODOLOGY 



The structure and location of the hepatic vascular 

 bed and its tributaries indicate at once the physical 

 difficulties of quantitative evaluation of hepatic 

 hemodynamics and the variety of measurements 

 required. Among the latter the following appear to 

 be particularly important: a) determination of the 

 minute volumes of blood flowing into and out of the 

 liver, including flows through the various compo- 

 nents of the splanchnic bed; b) measurement of the 

 volume of blood within the hepatic vascular bed and 

 the contributory vessels of the splanchnic viscera; 

 and c) measurement of the blood pressure in the 

 arteries, the hepatic veins, and at the points of junc- 

 tion between the different streams in the portal vein 

 and the sinusoids. Given these data, a complete 

 analysis of the local determinants of flow and integra- 

 tion is possible. Until recently, most hepatic hemo- 

 dynamic parameters have been measurable only by a 

 direct approach invoking considerable traumatic 

 manipulation and interference with normal function. 

 Various indirect methods are now under study in 

 many laboratories which appear to provide a means of 

 measuring local blood pressure and hepatoportal 

 blood flows and volumes in intact animals and man 

 without operation or anesthesia. 



Dirrct Methods 



HEPATIC AND SPLANCHNIC BLOOD FLOWS. Blood flow 



through the hepatic artery, the portal vein, and he- 

 patic veins has been measured directly in experimental 

 animals for many years by a number of devices. The 

 Ludwig stromuhr has been replaced by the thermo- 

 stromuhr and more recently by the rotameter and 

 other types of flowmeter (67). These methods re- 

 quire isolation of the artery or vein for insertion or 

 application of the measuring device. Additional 

 surgery is required to obtain a value for hepatic 

 venous outflow by difference between the flows 

 through the inferior vena cava above and below the 

 entry of the hepatic veins or as the retrograde flow 

 through the inferior vena cava (above the renal 

 veins) after ligation at the level of the diaphragm. 

 Trauma, anesthesia, manipulation, hemorrhage, and 

 loss through collateral channels all contribute to 

 the errors inherent in these procedures. Nevertheless, 

 they possess the great advantage of the direct ap- 

 proach. 



The so-called "collection methods" are equally 

 direct but somewhat easier to use and more accurate, 

 at least with respect to the measurement of hepatic 

 venous outflow and portal venous inflow. Here the 

 outflow is collected, rapidly measured, and then 

 returned to the systemic circulation. Thus, portal 

 venous inflow may be measured as the outflow from 

 the severed splenic vein following splenectomy and 

 temporary occlusion of the portal vein close to the 

 liver (204). Blalock & Mason (35) introduced under 

 local anesthesia a blind brass cannula with lateral 

 openings via the right external jugular vein of the 

 dog, the superior vena cava, and right atrium into 

 the inferior vena cava where balloons affixed to the 

 cannula could be inflated temporarily at points above 

 and below the entry of the hepatic veins during 

 withdrawal of the total venous outflow. More re- 

 cently, Selkurt (264) has measured hepatic venous 

 outflow in dogs by a similar technique after shunting 

 blood from the hind portions of the animal via an 

 external circuit from the femoral veins to an external 

 jugular vein, with ligation of the inferior vena cava 

 below the hepatic veins, and collection of hepatic 

 venous blood from above by a special cannula. 

 Although this method requires general anesthesia 

 and abdominal surgery, inclusion of blood from the 

 lower portion of the vena cava is avoided and a period 

 of complete obstruction of flow from the hind por- 

 tions, with attendant circulatory disturbances, is 

 circumvented. 



