VOLUME OF BLOOD 



33 



hemoglobin content was plotted against the arterial 

 hematocrit, it was possible to extrapolate to an arterial 

 hematocrit of loo per cent cells, thus obtaining a value 

 for the total renal vascular volume. From this and the 

 actual renal hemoglobin content at various arterial 

 hematocrit levels, renal blood was estimated to have 

 a mean hematocrit about one-half that of the central 

 circulation. This is of the same order of magnitude 

 as the values shown in table 3. A confirmatory ob- 

 servation, which cannot be correlated quantitatively, 

 is that the last blood to drain from the renal vein after 

 the vessels are clamped is very poor in cells (230). The 

 origin of the extra fluid is obscure, however, since its 

 composition diflfers from that of centrally circulating 

 plasma. 



A totally different procedure consists of injecting 

 cell and plasma labels simultaneously into the blood 

 flowing into a vascular region, and measuring the 

 transit time for each label as the two pass a sampling 

 site in effluent vessels (140). The space traversed by 

 each label is the product of its mean transit time by its 

 volume flow (109, 225, 226). If plasma label traverses 

 the total plasma space, and cell label the total cell 

 space, it should be possible to calculate the two latter 

 volumes by this method. When the procedure is ap- 

 plied to the kidney, however, the plasma space ap- 

 pears much smaller than it does by either of the 

 foregoing methods, the renal hematocrit being only 

 about 10 per cent less than that of the central circula- 

 tion (147, 174)- The diff'erence seems too great to be 

 attributed to the inclusion of large vessel volume in the 

 spaces estimated by the transit-time method. It seems 

 likelv that in a single transit of the renal circuits 

 plasma label does not have time to traverse the total 

 space accessible to it. The reported data fail to show 

 whether plasma label was completely recovered in 

 the effluent blood. When Cr'^' cells and I''" albumin 

 are continuously infused into the renal artery, it has 

 been reported that the albumin transit times can be 

 resolved into two major categories, the shorter one 

 resembling that of the cells, the longer one being 

 about ten times as long (43). 



Data from flow-dilution studies of cell and plasma 

 label are also available for the pulmonary circulation. 

 They agree in showing that tagged cells traverse this 

 system somewhat more rapidly than labeled plasma 

 (fig. I ) and that an equal percentage of the two labels 

 ultimately passes the sampling site (55, 140, 146, 186). 

 Table 4 shows data reported by Rapaport et al. (186) 

 for the pulmonary circulation of the dog, which I have 

 attempted to convert into values for the complete 

 lesser circulation by excluding the portion of the sys- 



From data reported by Rapaport et al. (186). See text for 

 explanation. The column headings are : CV = cell volume 

 in ml, PV = plasma volume in ml, BV = sum of cell and 

 plasma volumes in ml. Ha = observed arterial hematocrit, 

 Hm = mean hematocrit calculated as CV/BV, PVc = plasma 

 volume in ml, calculated as CK X (i - Ha)/Ha, XPV = 

 excess plasma volume in ml calculated as PV — PVc, XPV% 

 = excess plasma volume as percentage of BV. 



temic circulation traversed by the labels, but including 

 the one-half heart volume. The labels in this study 

 were injected into the pulmonary artery, and samples 

 were drawn from a femoral artery. The transit times 

 and calculated spaces thus include values for the aorta 

 and its primary branches. For the table I have esti- 

 mated this portion of the greater circulation to have a 

 volume of 60 ml, which has been converted into vol- 

 umes of cells and plasma, by using the observed 

 arterial hematocrits. It is apparent from the table 

 that the space traversed by plasma label is larger, in 

 relation to the flow of plasma, than is the space 

 traversed by cells in relation to their flow. The excess 

 plasma space is shown in the column headed XPV-in\, 

 and is given in the last column of the table as a per- 

 centage of the total pulmonary blood volume. The 

 next section of this chapter considers the excess plasma 

 space at greater length. The hematocrit of the pul- 

 monary circulation, including one-half the heart, 

 appears by the transit-time method from the data of 

 the table to be about 94 per cent of the arterial hema- 

 tocrit. This is somewhat larger than the hematocrit 

 found for the small vessels of the lungs by tissue analy- 

 sis, after the injected labels have had time to achieve 

 their complete circulatory distribution (see table 3). 



It has not been possible to make comparable transit- 

 time calculations for the greater circulation alone. If 

 label is injected into the aorta and its return to the 



