38 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



200). The apparent disappearance rate may be 

 exaggerated by dilution with recruited plasma if 

 frequent large samples are drawn (3, 9). The mecha- 

 nisms which clear the plasma of dye are not fully 

 known, and appear to be of several different kinds. 

 Some appear to be specific for dyes as a group. 

 Others are operative for all plasma labels, and involve 

 processes which mix labeled plasma proteins with 

 the total extracellular protein pool, from which 

 they are removed metabolically. 



Azo dyes as a group have long been known to be 

 phagocytized by reticuloendothelial cells (220, 221). 

 In rat and rabbit granules of T-1824 can be seen 

 within liver cells i min after intravenous dye injection 

 (121). In the rabbit, 24 hours after injection, dye 

 granules are visible in macrophages in the kidney, 

 liver, spleen, and lymph nodes (137). The rate of dye 

 disappearance in the rabbit is unusually high, averag- 

 ing about 18 per cent per hour. It is doubled by 

 administration of histamine, and is reduced by 

 antihistaminics or by Thorotrast, the latter pre- 

 sumably producing endothelial blockade (122). It 

 may be significant that the distribution space of 

 T-1824 in the rabbit is about 8 per cent larger than 

 that of radioiodinated serum albumin (244). Cruick- 

 shank & Whitfield (49) reported in 1945 that if cats 

 were previously injected with India ink or with a 

 priming dose of T-1824, the fall in dye concentration 

 during the mixing period when T-1824 was subse- 

 quently injected was reduced and abbreviated, and 

 the value calculated for plasma volume was con- 

 siderably smaller. It was suggested that excessive 

 amounts of dye are phagocytically removed during 

 the mixing period unless the phagocytic cells are 

 already saturated. This observation has not been 

 confirmed in the unanesthetized cat, second dye 

 injections shortly following the first giving approxi- 

 mately the same value for plasma volume (Gregersen, 

 personal communication). Studies on other species 

 have also failed to demonstrate that tissues which 

 might be capable of such excessive unobserved dye 

 removal can be saturated. A constant value for plasma 

 volume is obtained by repeated dye injections in 

 man (27, 31, 190), in the dog (176, 228), and in the 

 rabbit (46). 



Dye appears in bile within i half-hour of its 

 injection in the dog, but the amount excreted in this 

 way during the first few hours accounts for only 2 to 

 7 per cent of that lost from blood (157)- Insignificant 

 amounts are excreted through any channel in the 

 first 6 to 8 hours in the rat, carcass extraction at this 

 time yielding nearly the entire injected dose. Since 



plasma concentration has fallen to about one-third, 

 the remainder of the dye has obviously moved into 

 an extravascular distribution (39). Extravascular 

 accumulations have been demonstrated in cells of 

 the proximal convoluted tubules of the rat's (212) 

 and the dog's (47) kidney. Radioiodine also appears 

 in the dog's tubular cells after radioiodinated serum 

 albumin has been injected, but its accumulation 

 follows quite a different course from that of the dye. 

 Whereas P'" rapidly reaches a plateau of activity in 

 the tubular cells, the dye progressively increases in 

 concentration for a period of 24 hours (47). 



Other known mechanisms for clearing plasma of 

 dye seem to be nonspecific, and to remove other 

 plasma labels as well. Within a few minutes after 

 intravenous injection of dye and radioiodinated 

 albumin, both labels appear in thoracic duct lymph 

 (32, 69, 132, 220, 237). The concentration in lymph 

 remains below that in plasma for several hours (69, 

 237), and since the flow of lymph is small, it seems 

 unlikely that the return of label to blood by way of 

 the lymphatic system could influence the plasma 

 disappearance curve. Of much greater significance 

 is the obvious fact that label must have traversed 

 extra\"ascular spaces before it can appear in lymph. 

 The amount of label lost to these spaces, and the 

 volume of the latter, cannot be computed from any 

 available data (32). Newly formed capillaries appear 

 to be more permeable to dye than older vessels, 

 since dye can be seen outside the youngest capillaries 

 in the rabbit's ear chamber preparation, as soon as 

 30 min after injection (2). Extravascular dye has been 

 demonstrated in the pinna of the human ear within 

 2 hours, by applying sufficient external pressure to 

 render the ear temporarily bloodless for spectro- 

 photometry (44). 



Since excessive removal of dye during the mixing 

 period cannot be dismissed on a priori grounds, proof 

 of the validity of dye measurements of plasma volume 

 has had to rest upon a comparison of dye distribution 

 volume with that of other labels. Gregersen and his 

 colleagues have found the same distribution volume 

 in the dog for T-1824, for three antigens of difTerent 

 molecular size, bovine albumin, bovine globulin, 

 and pneumococcus polysaccharide (90), and for 

 hemoglobin (7). The distribution space of T-1824 '" 

 the turtle is the same as that of dextran with molecular 

 weight 66,000 (213). The dye and radioiodinated 

 albumin are found by the majority of in\estigators 

 to have nearly identical distribution volumes in man 

 (48, 73, 123, 208) and in the dog (73, 210). Technical 

 errors, or an interaction between the two labels 



