12 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



and lymph after equilibration, and (h) the lymph-to-blood albumin ratio 

 was 0.6, and therefore after equilibration the lymph-to-blood radioactiv- 

 ity ratio was also 0.6 (note the application of pages 8 to 10). 



Values for total blood volume are now obtainable from the sum of the 

 red-cell volume and plasma volume as determined simultaneously by 

 direct dilution measurement in the animal. In the past, blood volume 

 was usually estimated from the plasma volume and hematocrit (as deter- 

 mined by centrif ugation of venous blood) or from the red-cell volume and 

 hematocrit. It is well known that there are fewer red cells per unit blood 

 volume in the animal as a whole than there are in a sample of blood from 

 a large vein. This introduces a serious systematic error in any quantity 

 calculated from the large-vessel hematocrit. The whole-body hematocrit 

 can be calculated from the independently determined red-cell volume and 

 plasma volume. 



By estimation of radioactivity in organ tissues after equilibration, it is 

 possible to determine the blood content, red-blood-cell content, and 

 plasma content of the tissue. This may be of value in the elimination of 

 perfusion procedures, by allowing calculations from blood and tissue con- 

 centrations to determine what proportion of a substance found in a tissue 

 is there as a result of the blood content of the tissue at the time of analysis. 



Contribution of Two Sources to a Product. There are many biological 

 problems in which it would be of particular value to be able to estimate 

 what proportion of an element or compound in a given product came from 

 one source and what proportion came from another. The labeled- 

 element procedure is perhaps unique in that it makes this type of informa- 

 tion available. The principle may be illustrated by consideration of a 

 plant growing in a soil to which fertilizer labeled with P^- has been added. 

 The specific activities in the plant and fertilizer are determined experi- 

 mentally. It is clear from the dilution of the fertilizer phosphorus with 

 the soil phosphorus that the following relationship holds: 



Specific activity of plant ,, ,^^ rr/ r i i • ^-, a\ 



r, -r- ^. .^ . . ^.,. X 100 = % of phosphorus m (1-4) 



Specific activity of fertilizer p^^^^^ ^j^.^^ ^^,^^ ^^^.-^^^ 



from fertilizer 



The general statement is as follows: The specific activity of the product 

 divided by the specific activity of a labeled contributor substance equals 

 the fraction of the element or compound in the product which was con- 

 tributed by the labeled source. 



An important agronomic problem is concerned with the availability to 

 the plant of fertilizer nutrients added to the soil. Fertilizers labeled with 

 P^^ and other radioisotopes have been widely used for direct measure- 

 ments, as indicated in the above example. Another equally important 

 problem is the evaluation of the availability of a nutrient element in the 



