i 3 88 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



system. Measurements of cardiac output and arterial 

 blood pressure are required to determine whether 

 changes in hepatic hemodynamics are produced by 

 local vasomotor activity or by passive changes in 

 response to alterations in the perfusing pressure. 

 Data on the correlated behavior of other vascular 

 beds aid also in delineating the mechanism of in- 

 tegration and in placing the role of the splanchnic 

 bed in proper perspective. Responses seem to differ 

 between the various vertebrate species either because 

 effective drug dosage levels and the intensities of the 

 various stimuli used are not comparable or because 

 the physiological mechanisms are fundamentally 

 dissimilar. More data are needed to determine which 

 of these alternatives is responsible for many phenom- 

 ena. Meanwhile, interpretation of the behavior of 

 the hepatic circulation in one species (in man for 

 instance), on the basis of the known behavior in 

 another (such as dog) must be made with caution. 

 Methodology is a major stumbling block. Regardless 

 of species, the hepatic circulation is difficult to ap- 

 proach and surgical procedures of some kind are 

 usually necessary. As a result the method of measure- 

 ment may modify or interfere with the response 

 under study. Coniinuous observations over any ex- 

 tended period or repeated examinations at long 

 intervals may be impossible owing to deterioration 

 of the preparation or to the ultimate irreversible 

 damaging effects of mensuration itself. All these dif- 

 ficulties may be laid to the inaccessibility and com- 

 plex arrangement of the hepatic vasculature. 



ANATOMY 



Recent investigations have contributed impor- 

 tantly in characterizing the structural patterns of 

 the hepatic vascular inflow and outflow systems. 

 A variety of techniques has been employed. The 

 injection of plastic masses and colored materials of 

 various kinds into the hepatic artery, portal vein, 

 and hepatic veins has been used with increasing skill 

 and efficacy (97, 120, 148, 205, 214). Careful re- 

 constructions by the wax plate method or by photo- 

 graphic procedures have resulted in a new appraisal 

 of the arrangement of minute hepatic vessels relative 

 to the parenchymal cells. Modern methods of micro- 

 dissection have been less frequently used, but direct 

 observation of the quartz-rod transilluminated liver 

 in living animals has played an important part in 

 providing information on the anatomy and behavior 

 of the sinusoids (185, 270, 299). A large number of 



careful gross dissections of the splanchnic vascular 

 bed has resulted in more reliable statistical data on 

 the various types of arrangements of the hepatic 

 artery and portal vein (120, 145, 214). Although 

 anatomical facts are of vital importance in the inter- 

 pretation of functional data, it must be emphasized 

 that a priori inference regarding functional signifi- 

 cances on the basis of structure alone may be very 

 hazardous. 



The character of the venous and arterial inflow- 

 tracts is particularly susceptible to misinterpretation. 

 The cross section of the hepatic artery is much smaller 

 than that of the portal vein in a ratio of approxi- 

 mately one to five — suggesting that arterial inflow 

 is roughly one-fifth of the portal venous inflow. Since 

 this conclusion has found some justification in the 

 measurement of blood flows, it has served to encour- 

 age further speculation. Cross-sectional area alone is 

 not a good indication of relative flows in the absence 

 of data on pressures and resistances, and it is not 

 surprising, therefore, to find on further study that 

 the only generalization regarding the relationship 

 between arterial and venous inflow, which seems per- 

 missible at present, is that they tend to show a degree 

 of reciprocity. Anatomically the two systems differ 

 greatly. 



The portal venous system drains the vascular beds 

 of the spleen, pancreas, stomach, large and small 

 intestines, and the mesenteries. Each of these beds 

 presents certain unique features that cannot be dis- 

 missed simply because they are not immediately 

 concerned with the hepatic circulation. The volume 

 of blood flowing into the portal vein and the pressure 

 maintained upon the blood in the portal vein are 

 determined to a large extent by the resistances to 

 arterial perfusion within each of these portal units. 

 The dynamics of portal hepatic inflow are therefore 

 bound up intimately with the behavior of extra- 

 hepatic splanchnic circulation. 



The arteries giving rise to the extrahepatic splanch- 

 nic vasculature include a large array of major 

 branches that spring directly from the aorta or from 

 the celiac axis in a rather bewildering variety of 

 patterns recently described in detail by several 

 anatomists. [See (214) for survey.] In general 

 there is an abundance of collateral anastomoses out- 

 side the organs supplied, but exceptions to this tend- 

 ency abound and surgeons must proceed warily in 

 ligating any large branch without prior demon- 

 stration of the area of supply. 



The terminal vessels are equally diverse, ranging 

 from the well-muscled end arterioles (penicilli) in 



