VOLUME OF BLOOD 



29 



time had to be allowed for both it and the plasma 

 label T-1824 (28). 



It may be assumed that plasma is sequestered along 

 with cells whenever blood stops circulating in these 

 abnormal states, but the estimation of its volume by 

 present methods is of uncertain reliability. Both the 

 T-1824 space and the thiocyanate space have been 

 reported to be reduced by about 30 per cent in rabbits 

 and chickens in deep hypothermia, without demon- 

 strable extravasations of fluid (197). In most studies 

 of other types of shock, however, it has not been 

 possible to demonstrate the sequestration of plasma, 

 presumably because its loss from the circulation is 

 masked by plasma volume replenishment in remain- 

 ing vascular circuits (3, iig, 141). 



DISTRTBLITION OF CELLS AND PLASMA IN 

 THE CIRCULATORY SYSTEM 



The first requirement stated in the introduction is 

 that the value c used in equation i must be equal to 

 the mean concentration of label for the circulatory 

 system as a whole. Even after labels have achieved 

 their final circulatory distribution, they may not have 

 the same concentration throughout the system as in 

 samples of drawn blood. It has long been known that 

 the ratio cells: plasma is not the same in all parts of 

 the circulatory system, the spleen being demon- 

 strablv cell-rich, and many small vessels in other 

 tissues being visibly cell-poor. Since all labels attach 

 themselves either to cell constituents or to plasma 

 constituents, an unequal distribution of cells and 

 plasma in the circulatory system will produce a cor- 

 respondingly uneven distribution of label. 



Despite such inequalities of distribution, if the 

 hematocrit of centrally circulating blood represents 

 the mean hematocrit for the entire circulatory system, 

 a single label, for either cells or plasma, may be used. 

 As long ago as 1874 Malassez (152) recognized that 

 this relationship between the central and the systemic 

 hematocrit would exist only by chance, in the state- 

 ment "La richesse globulaire du sang variant selon 

 les regions de I'arbre circulatoire, rien ne dit que le 

 nombre des globules trouve represente la richesse 

 globulaire moyenne de tout le sang de Teconomie" 

 (152). Although it is now generally recognized that 

 the central hematocrit does not represent the mean 

 in most species, and under most conditions, it has been 

 proposed that a constant relationship exists, so that 

 the mean hematocrit may be derived from the ob- 

 served hematocrit of drawn blood (38, 80, 191, 192). 



It is one of the purposes of this section to examine the 

 validity of this proposal. 



Cell '.Plasma Ratio of Drawn Blood 



All procedures, whether they measure label ac- 

 tivity in whole blood or after blood has been cen- 

 trifugally separated into cell and plasma compart- 

 ments, use the hematocrit directly or indirectly. The 

 hematocrit is usually stated as the percentage volume 

 of packed cells in a sample of blood, but in the present 

 discussion it will be given in its simpler form, as the 

 decimal ratio of cells to total volume. Blood drawn 

 from any definable vascular site with the exception 

 of the splenic pulp and the erythropoietic bone mar- 

 row has essentially the same hematocrit as blood 

 drawn from any other site, including the heart and 

 great vessels. Whenever hematocrits are obtained on 

 drawn blood, accordingly, they represent the hema- 

 tocrit of the central circulation. 



Although centrifugation readily produces plasma 

 which is free of red cells, it is impossible to drive the 

 plasma completely out of the packed cell column in 

 this way (156). The percentage of occluded plasma 

 which remains varies with the centrifugal force ap- 

 plied and with the duration of its application (183), 

 and may vary with different kinds of blood. Gregersen 

 & Schiro (96) in 1937 found that about 4 per cent of 

 the dye T-1824 which had been added to samples of 

 whole blood was missing from the plasma column 

 separated by centrifuging at 1500 g for 30 min. Cen- 

 trifugal hematocrits were not reported. If it is assumed 

 that these were about 0.40, approximately 6 per cent 

 of the packed cell column must have consisted of 

 plasma. Somewhat similar data were obtained by 

 Chapin & Ross (35), who used smaller centrifuge 

 tubes (Wintrobe) subjected to a centrifugal force of 

 1800 g for I hour, and found 8.5 per cent occluded 

 plasma, a value which was checked by observing the 

 dilution of plasma proteins when saline was added. 



It is now apparent that a constant correction can- 

 not be applied to the centrifugal hematocrit, since 

 the percentage of occluded plasma increases with in- 

 creasing hematocrit or with the height to which the 

 centrifuge tubes are filled (37, 61, 114). This is due 

 to a shortening of the effective radius of gyration as 

 the length of the cell column is increased. Relative 

 centrifugal force may be precisely formulated as 



(■2Trn)h 

 RCF = —— g 

 980 



where n is the number of revolutions per second, and 



