VOLUME OF BLOOD 



35 



thus 50.2 — 40.6 = 9.6 ml per kg, or 1 1 .5 per cent of 

 the virtual vascular space. The percentage in 

 splenectomized dogs under barbital anesthesia is 

 considerably larger, as might be expected since the 

 excess cells of the spleen cancel out an equivalent 

 volume of excess plasma. In a series of 12 barbitalized 

 dogs, splenectomized for at least 2 weeks in our lab- 

 oratorv, the average \olume of excess plasma was 

 equivalent to ig per cent of the virtual vascular space 

 (Lawson, unpublished data). The values given in 

 table 2 for excess plasma in the intact rhesus monkey 

 are of the same order of magnitude. When excess 

 plasma is calculated from transit-time data, as was 

 done in table 4 for the dog's pulmonary circulation, it 

 is considerably less. 



The percentage excess plasma in the total vascular 

 space may be calculated directly from Hm/H^, since 

 it is equal to (i — H„JH^ X 100. The tissues studied 

 by Gibson and his colleagues (82) and shown in table 

 3 of this chapter were allowed to bleed when they 

 were excised, and contained no grossly large vessels. 

 It is therefore not possible to estimate with any re- 

 liability the fractions of the original vascular spaces 

 which are represented by the data. If the excess 

 plasma is calculated, however, as a fraction of the 

 residual vascular volume, substituting //, for //„ in 

 the calculation, the excess plasma in the kidney sam- 

 ples represents 63 per cent, in the bowel samples 58 

 per cent, and in cardiac and skeletal muscle tissues 45 

 to 47 per cent of the volume. 



Anatomical Limits of the Plasma Compartment 



The mechanisms known or postulated for rendering 

 peripheral segments of the circulatory system poorer 

 in cells than the central circulation are limited in 

 operation to minute vessels. These are the phenomena 

 of plasma skimming as described by Krogh (i 34), and 

 axial cell streaming as demonstrated by Fahraeus 

 (66, 67). Cohnstein & Zuntz (42) in 1888 were the 

 first to comment on the paucity of cells in the smallest 

 vessels, and the cell enrichment which visibly occurs 

 as they dilate. Since their data were obtained by 

 microscopic inspection, they cannot be quantitated. 

 It may be significant for the present purposes, how- 

 ever, that the largest vessels which became recog- 

 nizably cell-poor on constriction, in their observations, 

 had diameters of 30 to 40 fi. It is neither feasible nor 

 profitable to estimate the total volume, in cardio- 

 vascular segments of all diameters, of the layer of 

 stagnant, cell-free plasma. In all segments of large 

 diameter it is an insignificant fraction of the total 



greater 

 greater 



Segment 



Heart 



Systemic arteries 



than 0.3 

 Pulmonary arteries 



than 0.3 

 Systemic veins greater than 0.3 

 Pulmonary veins greater than 



0.3 

 Systemic arteries 0.1-0,3 

 Pulmonary arteries 0.1-0.3 

 Systemic veins 0.1-0.3 

 Pulmonary veins o. i -0.3 



Systemic vessels less than o. i 

 Pulmonary vessels less than o. i 



Totals 



Volume, 

 ml 



140 

 109 



40 



345 

 114 



10 

 28 

 64 

 27 



% of total 



14.0 

 10.9 



4.0 



34-5 



II. 4 



i.ol 



2.81 



6.4 



2 -7] 



I I .2 

 I . I 



12. 9 



12.3 



25.2 



The probable volume distribution in segments of the 

 cardiovascular system. Computed for a 12 kg dog viJith blood 

 volume of 1000 ml, from data of Green (89) and Schleier 

 (206). The numbers designating segments refer to diameters 

 in mm. 



contents. In Fahraeus' studies, a reduction in hemato- 

 crit and in relative viscosity were just measurable in 

 tubes of 0.3 mm diameter, and became marked only 

 in tubes smaller than o.i mm diameter (66, 67). 

 Plasma skimming, as a general phenomenon, appears 

 to be related to axial streaming, and is accordingly 

 limited to vessels of similar small size. Table 5 is an 

 attempt, on the basis of admittedly inadequate data, 

 to formulate for the entire circulatory system the 

 percentage of blood in vessels less than 0.3 mm and o. i 

 mm diameter. In constructing the table I have used 

 Green's expansion of Mall's measurements of the 

 mesenteric vessels (89) and Schleier's presentation of 

 Miller's measurements on the pulmonary vessels 

 (206). I have included an estimate of mean heart 

 volume based on an assumed stroke volume of 25 ml, 

 50 per cent ventricular emptying, and a mean atrial 

 volume equal to 75 per cent of the ventricular volume. 

 All values have been adjusted for a 12-kg dog, with 

 blood volume equal to 1000 ml. It appears unlikely 

 from the table that more than 1 2 per cent of the total 

 circulatory volume is composed of vessels smaller than 

 o. I mm diameter, or 25 per cent of vessels smaller 

 than 0.3 mm diameter. These are somewhat larger 

 estimates than those of Landis cS: Hortenstine (135), 

 who, using Lewis' definition of minute vessels, esti- 

 mated them to comprise ig per cent of the systemic 

 and 2 1 per cent of the pulmonary circuits. If their 

 estimates are corrected to include the 14 per cent of 



