THE HEPATIC CIRCULATION 



I 40 I 



stant infusion" technique which includes repeated 

 blood sampling, hepatic venous catheterization, and 

 prolonged intravenous infusion. 



The remarkable phagocytic activity of reticuloen- 

 dothelial cells situated in the liver and splanchnic 

 bed early suggested the possibility that particulate 

 substances in colloidal suspension might be removed 

 with sufficient efficiency to permit the development 

 of single injection techniques dog, 272). Various 

 agents in colloidal suspension including carbon, iron, 

 gold, chromium phosphate, polyvinylpyrrolidine, and 

 denatured plasma protein have been studied inten- 

 sively by many workers (33, 109, 1 10, 272, 296, 316). 

 In general it appears that phagocytic removal by 

 the R-E cells within the liver and spleen depends 

 upon particle size, "saturation," splanchnic blood 

 flow, body temperature, and the obscure factors that 

 determine preferential removal (55, 56, 316). Of these, 

 particle size seems to be critical though difficult to 

 define. Large particles (100 A or larger) are taken 

 up more actively than small ones; addition of plasma 

 to the suspension prior to administration appears to 

 enhance removal, possibly by increasing the bulk of 

 small particles with a protein coating like that ob- 

 served directly by Knisely et al. (185) prior to phago- 

 cytosis by KupfTer cells. The majority of studies in 

 intact animals and man have involved the administra- 

 tion of P 32 -labeled chromium phosphate, radioactive 

 gold (Au 198 ) and heat-denatured plasma albumin 

 labeled with I 131 (33, 231, 242, 296). From 80 to 100 

 per cent of a single dose of each of these agents has 

 been shown to accumulate in the liver and spleen 

 and each yields a disappearance curve that can be 

 resolved into two or more simple exponential func- 

 tions. The values in plasma radioactivity do not 

 usually fall to zero but tend to flatten into a straight 

 line on semilogarithmic paper. This phenomenon has 

 been attributed to the very slow removal of small 

 particles which represent only a minute fraction of 

 the dose. In practice the values obtained by extrap- 

 olating the "tail" of the curve back to zero time are 

 subtracted from the initial figures to obtain a disap- 

 pearance curve which is usually a single exponential 

 that can be evaluated simply in terms of the disap- 

 pearance half-time (t\ = ): 



k . 2.303 log C /.5C _ .693 



'2 y 2 



taking any value on the curve as C and l v «. as the 

 time required thereafter for the concentration, 

 plotted semilogarithmically, to fall to half C . The 



value for k, i.e., the fractional clearance, is usually 

 multiplied by the total plasma volume to yield a 

 figure for EHBF. Colloidal chromium phosphate is 

 difficult to prepare with a suitable range of particle 

 size and is now little used. Radioactive gold and 

 iodine are 7-emitters and their disappearance may be 

 followed by external counting. 



Perhaps the most serious difficulty with the single 

 injection techniques lies in choosing the "volume of 

 distribution , ' (Y) to which the hepatic fractional 

 clearance may be referred (108). Although the plasma 

 volume is usually employed, a substantial fraction of 

 the plasma volume within the splanchnic bed from 

 which clearance has occurred cannot be included. 

 The "volume" concerned is presumably one, too, in 

 which admixture is instantaneous and throughout 

 which the same concentration prevails at any mo- 

 ment. Attempts have been made to correct for "mix- 

 ing time" by including a nondiffusible dye, like 

 T-1824, with the test close, but the corrections have 

 proved relatively insignificant and have been 

 deemed unnecessary. Attempts to compute V have 

 proved less successful, especially following trauma or 

 blood loss. Many workers use the volume of dilution 

 calculated from the intercept at zero time obtained 

 by extrapolation of the disappearance curve; others 

 simply report values for k, or use the plasma volume 

 less a fraction held in the splanchnic vessels. Another 

 problem arises from the assumption that hepatic ex- 

 traction is nearly complete. In early studies (296), 

 values of 70 to 90 per cent were reported for col- 

 loidal Au 198 , but more recently reported figures (231) 

 range from 30 to 70 per cent, accounting perhaps for 

 lower values for EHBF. Changes in the composition 

 and properties of gold colloids commercially avail- 

 able may be responsible for this phenomenon. Al- 

 though heat-denatured serum proteins labeled with 

 I 131 appear to be extracted very efficiently and, as an 

 added advantage, are ultimately eliminated by 

 normal metabolic processes, they have found little 

 use in large part because I 131 rose bengal and indo- 

 cyanine green have proved more attractive (19, 183, 

 213). Both I 131 rose bengal and indocyanine green 

 disappear rapidly and exponentially from the blood 

 and, since neither is lost in the urine nor taken up in 

 significant quantity by extrahepatic tissues, both 

 may be used to measure EHBF by the single injection 

 technique. Nevertheless, uncertainty remains re- 

 garding the character of the volume of distribution 

 from which the dyes disappear, the constancy and 

 magnitude of extraction under all circumstances, and 

 the part played by different mechanisms in deter- 



