PRINCIPLES OF TRACER METHODOLOGY 11 



estimation of such physiological entities as the red-hlood-cell volume, 

 plasma volume, lymph volume, and body water. In addition, the vol- 

 umes occupied by other ions of biological interest can be estimated, so 

 that we see values designated as chloride space, bromide space, total 

 exchangeable sodium, and total exchangeable potassium. Various dyes and 

 chemicals have been used for some of these determinations, but the radio- 

 isotope can generally provide a label with more desirable characteristics 

 in regard to completeness of mixing, minimum of metabolic or physical 

 loss from the space to be estimated, and ease of measurement. The gen- 

 eral application to humans has been presented by Edelman et al. (51), 

 blood-volume determinations have been reviewed by Gregersen (52), and 

 red-blood-cell volumes for various species of domestic animals have been 

 reported by Hansard et al. (53). 



Blood-volume measurements are usually based on the labeling of red 

 cells or on the direct labeling of the plasma. The former methods consist 

 in putting the tag into the red cells directly by in vivo synthesis or in vitro 

 exchange. Radioisotopes of iron, phosphorus, chromium, and potassium 

 have been successfully used for this purpose. A measured amount of 

 tagged red cells is then injected into the animal, and after time allowed for 

 mixing, a sample of blood is withdrawn for estimation of the dilution tak- 

 ing place. Albumin tagged with P^^, which is distributed throughout the 

 plasma, is used for the direct measurement of plasma volume similarly 

 to the Evans blue-dye technique. There may be a tendency for the albu- 

 min tracer to equilibrate into areas outside the plasma, specifically into 

 lymph. For valid results the plasma should be sampled before apprecia- 

 ble leakage to the lymph has occurred. Storey et al. (54) have made use 

 of these relationships to estimate lymph space by a procedure with P^'- 

 labeled albumin which does not require a sample of lymph. The equation 

 used with dogs was as follows: 



R*e->'^ = Wl'(V, + 0.67.) (1-3) 



where R* ^ total injected radioactivity 

 e = base of natural logarithms 



k = disappearance constant as determined from several daily 



measurements of plasma after injection 



R'p = radioactivity per milliliter of plasma measured at time I, 



where t is greater than 1 day so as to allow equilibration 



between plasma and lymph 



Vp = plasma volume as determined by measurements within a few 



minutes after injection 

 V L = lymph space 

 This equation was based on the assumptions, experimentally supported, 

 that (a) the tagged albumin disappeared at the same rate from the plasma 



