EQUILIBRATIONS IN PARTS OF ORGANISMS 153 



expressed as a fraction of the living unit; or Vd/Bq = (AGo X Vs)/ 

 (AG3XB0). 



Volumes of distribution have a wide range when diverse com- 

 ponents are administered (table 5). In general each distribuend 

 has a smaller volume of distribution in man than in dog, and 

 smaller in rabbit than in either. 



Most volumes of distribution have been identified with anatomi- 

 cal ' * compartments ' ' by hypothesis only. A few of the substances 

 commonly administered as indicators of volume are supposed to 

 attain after certain intervals of time virtually equal concentrations 

 in all the ivater of the body (urea, sulfanilamide). Others are 

 thought to become distributed in equal concentrations in the water 

 of extracellular localities (SON', Cr, Br', sucrose, SO*"), with the 

 exception that erythrocytes at least also contain them. Thus by 

 inference it is said that a certain procedure measures something 

 that approximates ''plasma" volume; another, circulating whole 

 ''blood" volume; another "extracellular" volume. In general it 

 is a necessary precaution to specify : Vd of brilliant vital red volume 

 at 0.1 hour after injection by vein ; Vd of sodium thiocyanate vol- 

 ume at 3 hours after injection by peritoneum ; Vd of urea at 2 hours 

 after ingestion by stomach. Perhaps no two methods of measuring 

 volume are likely to agree : either the actual volumes of distribution 

 differ, or some factors in the two procedures are systematic ones. 

 A notion that may be derived is that one procedure is not "su- 

 perior" to another (though the volume it measures be identical 

 with volume found by other methods), but that each procedure 

 measures reproducibly a unique volume of distribution. 



Evidences of regulation now to be mentioned concern either 

 parts of organisms that are volumes of distribution or parts the 

 volumes of which are measured by mechanical or optical methods. 



§ 59. Blood and plasma 



The volumes of "blood" and of "plasma" are peculiarly suited 

 to the establishment of quantitative excesses and deficits ; perhaps 

 this merely means that addition or subtraction is little trouble. 

 Many hypotheses have been erected concerning the lability of those 

 volumes. Choosing increments of volume that directly concern the 

 blood, I here study hemorrhage and transfusion. 



In rabbits, "blood" volumes are ascertained as volumes of dis- 

 tribution of trypan red, divided by hematocrit ratio (fig. 94). The 



