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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



paper) or as the total amount which has appeared 

 in the bile over a period of time — usually several 

 hours and usually expressed as a percentage of the 

 total dose administered. The latter is also often 

 referred to as "recovery" but may be confused with 

 "extraction," a term correctly applied to the per- 

 centage of dye removed from the blood perfusing 

 liver and calculated as the ratio between the arterial- 

 hepatic venous concentration difference and the 

 peripheral arterial or venous concentration. Hepatic 

 extraction must be computed on the assumption that 

 the arterial concentration is a measure of the concen- 

 tration in the blood perfusing the liver by way of both 

 the hepatic artery and portal vein from which ex- 

 traction has occurred to account for the concentra- 

 tion found in the hepatic venous blood. For this 

 reason due allowance must be made for splanchnic 

 circulation time when rapid changes are occurring. 

 Perhaps the most serious confusion has arisen in 

 discussions of extrahepatic removal of BSP. When the 

 plasma level was maintained at a constant level in 

 the dog at about i mg per cent, the extraction of BSP 

 averaged 34 ± (sd) 12 per cent (282), in association 

 with removal rates of from 0.57 to i.g8 mg per min. 

 No more and usually much less than 10 per cent of 

 the amount removed per minute could be ascribed 

 to extrahepatic loss, when direct measurements of 

 hepatic uptake of BSP were made in the anesthetized 

 dog (41, 305). Following hepatectomy, however, the 

 plasma level may fall by 25 or 35 per cent from a level 

 of 1 mg per cent 1 hour after a single intravenous 

 dose, an observation which has been claimed (87, 

 88, 302) to indicate a proportionately large extra- 

 hepatic contribution to removal. The confusion here 

 stems from comparing two fundamentally different 

 removals; one, the percentage of the total removal 

 rate per minute attributable to extrahepatic tissues; 

 the other, the percentage change in plasma concen- 

 tration over the course of 1 hour. If the dog's circu- 

 lating plasma volume following hepatectomy can be 

 taken as 800 ml, then a 30 per cent fall in BSP from a 

 concentration of 1.0 mg per cent to 0.7 mg per cent 

 in the course of 1 hour would entail a total loss of 

 2.4 mg or 0.04 mg of BSP per min, approximately 5 

 per cent of the expected removal of 1 .0 mg per min. 

 This figure does not differ greatly from those ob- 

 tained by direct measurement and it may be con- 

 cluded that extrahepatic loss is negligible under most 

 circumstances. The failure to escape from the vascu- 

 lature may be attributed to the fact that all three 

 dyes are almost completely bound by the plasma 

 proteins (49, 258, 304). Neither rose bengal nor 



indocyanine green enters the urine (258, 304), 

 whereas Bromsulfalein is excreted by the kidney in 

 amounts equaling 0.06 to 2.0 per cent of the total 

 dose (49, 60, 88, 220, 232, 275, 311). Since disap- 

 pearance from the blood depends, therefore, almost 

 exclusively upon the liver, hepatic removal per minute 

 may be computed from the rate of infusion (plus or 

 minus, respectively, the amount removed from or 

 added to the plasma volume, i.e., AP X PV). This 

 conclusion is not vitiated by failure to "recover" 

 more than 60 to 80 per cent of a dose of BSP from 

 the bile nor is indocyanine green necessarily prefer- 

 able because 97.7 per cent of a single dose appears 

 in the bile. The total recovery is a measure of the 

 extent to which other excretory pathways are ac- 

 cessible and of the time allowed for collection. It 

 does not throw light upon the movement into other 

 tissues. 



The incomplete recovery of BSP does suggest, 

 however, that BSP may undergo alteration in the 

 body and that, as a consequence, calculation of 

 hepatic removal may be erroneous. Brauer and his 

 associates (60, 188) have shown that BSP from the bile 

 of the cat, rat, sheep, and chicken can be separated 

 into four fractions by column chromatography having 

 the same absorption spectra. Recent work (45, 93, 

 105, 165, 178, 211) indicates that the chromato- 

 graphic fractions are conjugates of BSP formed in the 

 liver by combination with glutathione at the sulfhy- 

 dryl group (GSH), with the release of bromine. Since 

 GSH is confined to the cells, conjugation must occur 

 intracellularly. In addition to various isomers of 

 BSP-GSH, BSP-cysteinyl-glycine conjugates are 

 formed, presumably by enzymatic hydrolysis of 

 BSP-GSH, since free glutamic acid appears simul- 

 taneously in the bile. There is at present no evidence 

 that conjugation is essential to transfer or storage of 

 BSP (178). Indeed, free BSP appears in the bile and 

 both rose bengal (189) and indocyanine (304) are 

 excreted without any evidence of conjugation. All 

 the biliary BSP conjugates have been found in the 

 blood of man and dog indicating escape from cells. 

 This movement into the blood occurs chiefly within 

 the liver and does not interfere with the calculation 

 of estimated hepatic blood flow (EHBF) since it 

 affects the computation of both hepatic extraction 

 and removal to the same extent and thus cancels out. 

 Enterohepatic circulation (198, 199, 224) of dye is 

 similarly of no concern provided hepatic venous con- 

 centration does not exceed the arterial concentration 

 and provided portal venous blood does not bypass 

 the liver via collateral pathways. In any case, in- 



