99° 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



CC /MIN 



+ 50 



fig. 6.3. Relation of net fluid move- 

 ment in perfused hind leg of cat to 

 difference between the mean hydro- 

 static pressure in the capillaries {pC) 

 and the sum of all pressures opposing 

 filtration (isogravimetric capillary 

 pressure, pCi). The slope of the line 

 corresponds to a filtration coefficient 

 (k t ) of 0.014 ml/min/100 g tissue/ 

 mm Hg pressure difference. [From 

 Pappenheimer & Soto-Rivera (282).] 



ABSORPTION 



FILTRATION 



pressure had to be elevated by 10 to 17 cm water 

 before net nitration could be detected (fig. 6.2). 

 Brown et al. (24) showed later, however, that the 

 regression lines relating filtration and venous pressure 

 passed through zero, provided a) that interstitial 

 fluid was carefully evacuated from the forearm prior 

 to congestion, and b) that a correction was made for 

 the volume of interstitial fluid pressed out of the fore- 

 arm segment during each volume measurement. 



From regression lines such as the one shown in 

 figure 6.2 it is possible to calculate an approximate 

 filtration coefficient (k t ) for forearm tissue if allow- 

 ance is made for the fact that a given elevation of 

 venous pressure produces a somewhat smaller eleva- 

 tion of mean capillary pressure. On the assumption 

 that the latter is 80 per cent of the former, kt for 

 human forearm capillaries becomes approximately 

 .0057 ml per min per 100 g tissue per mm Hg as 

 given in table 6.2. The filtration coefficient for the 

 whole body becomes .0061. 



Among the several perfusion methods that have 

 been used to measure filtration coefficients, the most 

 precise and revealing is the isogravimetric technique 

 developed by Pappenheimer & Soto-Rivera (282) 

 in which filtration and absorption were identified by 

 changes in weight of an isolated limb. Arterial pres- 

 sure, venous pressure, osmotic pressure of the perfus- 

 ing fluid, blood flow, and temperature could be 

 varied at will and their influence on the filtration- 



absorption equilibrium could be measured separately. 

 A detectable effect on fluid movement resulted from 

 a change of venous pressure by 0.5 mm Hg and some- 

 times less, or from a change of arterial pressure by 

 2 to 4 mm Hg. Capillary pressure was 5 to 10 times 

 more sensitive to a change of venous pressure than 

 to a change of arterial pressure. 



Figure 6.3 shows net fluid movement, i.e., filtration 

 or absorption, plotted against the difference in pres- 

 sure across the capillary membranes themselves. 

 Filtration and absorption were proportional to the 

 difference between the calculated mean capillary 

 blood pressure and the isogravimetric capillary pres- 

 sure which is, by definition in this method, the sum of 

 all pressures opposing filtration. In figure 6.3 the 

 slope of the regression line indicates a filtration 

 coefficient of 0105 ml per 100 g tissue per min per 

 mm Hg change of capillary blood pressure. The filtra- 

 tion coefficient was independent of the absolute value 

 of the isogravimetric capillary pressure when this was 

 varied by diluting or concentrating the proteins in the 

 perfusing fluid. Similar methods have been applied 

 recently by Renkin & Zaun (302) to the hind legs of 

 the rat. 



The constancy of filtration coefficients at high and 

 low capillary blood pressures (figs. 6.1, 6.2, 6.3) 

 suggest that under these conditions capillary surface 

 area and capillary porosity are not significantly 

 modified by pressure. This may be related to the con- 



