992 HANDBOOK OF PHYSIOLOGY -" CIRCULATION II 



table 6.1. Filtration Coefficients {Hydr adynamic Conductivity) Through Various Membranes* 



* Modified from Renkin & Pappenheimer (301). t Thickness, 0.5 M , r = pore radius. 



Pappenheimer (276) to suggest also that the term 

 "capillary permeability" be reserved for describing 

 the properties of the capillary wall with reference to 

 the diffusion of small molecules. Filtration coeffi- 

 cients, because they deal with flow of fluid through a 

 membrane, would then be a measure of hydraulic or 

 hydrodynamic conductivity of the capillary wall. 



Table 6.1 is taken from the review by Renkin & 

 Pappenheimer (301) with inclusion of some more 

 recent values. It compares filtration coefficients of 

 cell membranes (upper section), of capillary walls 

 (middle section), and of certain artificial membranes 

 (lower section). Cell membranes have smaller filtra- 

 tion coefficients than capillaries, although the differ- 

 ence between the values for the erythrocyte and the 

 mammalian muscle capillary is small. The range of 

 filtration coefficients for capillary walls is very large, 

 amounting to a 200-fold difference in the mammal 

 between muscle capillaries and glomerular capil- 

 laries. The coefficients for artificial membranes, 

 calculated for comparable thickness, are in turn 

 much higher still and, with other evidence, led 

 Pappenheimer et al. (281) to the conclusion that the 

 collective area of the pores involved in the filtration 

 process is only a small fraction of the total capillary 

 surface. Support for this conclusion came from 

 measurements of capillary permeability to small 

 lipid-insoluble molecules, and will be given in sec- 

 tions 8 to 10. 



Table 6.2 compares average filtration coefficients 

 (kt) for extremities of four species. In the forearm of 



table 6.2. Average Filtration Coefficients for Tissues, k t 



* In the text these coefficients are described for a rise oi 

 venous pressure by 1 cm water. To facilitate comparison, 

 values given here have been corrected to 1 mm Hg rise of 

 capillary pressure. It is assumed that \P C = 0.8 AP V . 



man, on the assumption that capillary pressure is 

 increased by 80 per cent of given increases in venous 

 pressure, the average filtration coefficient becomes 

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

 the data of Brown et al. (22) k, for the whole body is 

 .0061. In smaller animals progressively larger filtra- 

 tion coefficients are found. As Renkin & Pappen- 

 heimer suggest (301), this relationship is ideologically 

 fitting because the smaller the mammal, the more 

 active are its metabolic processes and therefore the 

 greater will be the requirement for a more extensive 

 capillary bed (320) and for more rapid exchanges 

 between blood and tissue. To obtain comparable 

 filtration coefficients for other tissues, e.g., liver, 

 intestine, lung, and brain, is far more difficult. Values 

 for lung have been published recently by Guyton & 

 Lindsey (132) and for brain, or perhaps chiefly the 

 arachnoidea, by Coulter (47). 



