THE HEPATIC CIRCULATION 



'393 



Changes in local blood flow and velocity may be 

 detected by instruments recently developed upon the 

 principle of the "thermostromuhr." Grayson (155) 

 and his associates have used a tiny copper-constantan 

 thermocouple and heating wire implanted in the 

 liver for this purpose. The measured loss of heat to 

 the tissues appears to be a linear function of tissue 

 thermal conductivity and blood flow. The first of 

 these variables may be determined as a constant for 

 each liver after cessation of circulation; changes in the 

 second can then be computed in percentage terms 

 from changes in conductivity. The unit may be left 

 in place indefinitely and measurements made as 

 desired after healing of the wound through which the 

 leads emerge. Movement of blood can be assessed 

 only in a collar of tissue approximately 0.5 cm long 

 and 0.3 cm in diameter within the immediate vicin- 

 itv of the embedded thermocouple. Although total 

 flow cannot be measured, the instruments provide a 

 satisfactory means of following either acute or chronic 

 adjustments in small animals. A somewhat similar 

 device has been introduced by Grabner & Neumayr 

 (151 ) for the purpose of estimating blood flow through 

 a hepatic vein. A tiny thermistor affixed to the tip 

 of a Cournand catheter is heated several degrees 

 above the temperature of the blood after insertion 

 into an hepatic vein. Any change in temperature of 

 the element is directly related to a change in the 

 velocity of the blood flow in the immediate vicinity 

 of the ''pickup," or to the actual volume of flow if 

 the calibre of the vein is constant. Movements of 

 the catheter tip with respiration, reversal in the 

 direction of flow, scar formation, proximity of large 

 vessels, and changes in hepatic blood temperatures 

 may jeopardize the validity and usefulness of the 

 method, but it does possess the advantage of detecting 

 rapid and transient alterations. 



The transillumination method (185, 270, 299), 

 discussed above in connection with the delineation 

 of the finer anatomy of the liver, has proved valuable 

 in defining the character of blood flow through the 

 terminal radicles of the hepatic artery and portal 

 vein. The technique invokes careful exposure of the 

 liver in anesthetized or pithed animals with as little 

 trauma and blood loss as possible. A quartz rod, 

 provided with a conduit through which warm Ring- 

 er's solution may bathe the tissue examined, must 

 be inserted under the edge of the liver. The liver 

 edge transilluminated by light conducted through 

 the quartz rod may then be examined microscopi- 

 cally at high magnification. Respiratory movements 

 of the liver can be prevented in the anesthetized ani- 



mals by the introduction of 100 per cent oxygen 

 through a catheter placed in the trachea. Fluores- 

 cence microscopy and transillumination with ultra- 

 violet light following injection of fluorescent materials 

 permit somewhat better visualization of the blood 

 stream within the sinusoids and of the movement of 

 the materials from the blood into the parenchymal 

 cells and bile canaliculi (153). Dyes and particulate 

 substances have been used similarly to follow flow in 

 visible light. The conditions under which observations 

 must be made are obviously unphysiologic and limit 

 the extent to which generalizations may be adduced. 

 Local extraneous factors such as changes in tissue 

 tension, the direct effects of immobilizing and han- 

 dling the liver, the influence of foreign materials and 

 fluids within the abdominal cavity, as well as the 

 effects of anesthesia, prolonged immobilization of 

 the body in an abnormal position, and the limited 

 area available to study combine to make interpreta- 

 tions most uncertain. When considered in relation 

 to information obtained by other methods, however, 

 studies of the transilluminated liver may be most 

 helpful and revealing. 



Studies of the perfusability of the isolated liver 

 have also contributed to the knowledge of the he- 

 patic circulation, though here again interpretation in 

 terms of the intact animal and the circulatory system 

 as a whole must be made with caution. The prepara- 

 tion of the liver in investigations of this kind has 

 varied widely. At one extreme the liver is handled 

 with greatest care to avoid prolonged interruption 

 of blood flow either by perfusing the liver in situ or 

 by rapid transfer from the living animal to the per- 

 fusion apparatus where it is maintained under con- 

 ditions as closely as possible approximating those 

 in situ (10, 25, 61, 83, 277). The contributions of the 

 arterial and venous inflow to total outflow, the 

 character of intrahepatic adjustments, and the re- 

 sponse to an array of controlled pressure-flow states 

 impossible to impose in the intact animal may be 

 precisely evaluated in the isolated perfused liver. 

 Techniques have steadily improved with the de- 

 velopment of more effective anticoagulation, oxy- 

 genation, and surgery. At the other extreme, the 

 liver is removed at a varying time after death and 

 perfused with different foreign substances, ranging 

 from saline to kerosene (1 1 1 ). Intrahepatic resistances 

 and the interplay of the inflow systems at different 

 pressures have been evaluated by this means. Kero- 

 sene oil has been used by Dock ( 1 1 1 ) because it is 

 confined to the vascular channels and does not dif- 

 fuse, as saline solutions do, into extravascular tissues 



