EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



99 1 



elusions reached by Burton (31) that vessels of small 

 diameter are relatively indistensible. Under more 

 severe conditions and in other tissues, however, the 

 permeability of the capillary walls may be increased 

 when capillary blood pressures are very high (211) 

 or when blood volume is much increased (131, 331, 

 361, 369). 



In all the regions so far considered, resting average 

 capillary blood pressure is approximately equal to 

 the osmotic pressure of the plasma proteins. In the 

 lung, however, as described in section 2, average 

 capillary pressure is only 5 to 10 mm Hg and there- 

 fore less than half the osmotic pressure of the plasma 

 proteins. Figure 6.4 shows the rate of edema forma- 

 tion in the lungs of dogs plotted against left atrial 

 pressure (132). In contrast to other tissues, net filtra- 

 tion and increase of interstitial fluid volume were not 

 observed until, at atrial pressures of 25 to 30 mm Hg, 

 pulmonary capillary pressure began to exceed the 

 protein osmotic pressure of 25 mm Hg. Thereafter 

 filtration increased linearly with left atrial pressure 

 at a rate of 0.21 g of fluid per hour per mm Hg per g 

 dry wt of lung tissue or 0.065 g per min per mm Hg 

 per 100 g wet lung tissue. The relative ''dryness" of 

 lung tissue which is produced by a low capillary 

 pressure is indicated by the absence of filtration 

 between atrial pressures o and 23 mm Hg. This 

 margin of dryness was reduced to half normal when 

 the plasma protein concentration was decreased by 

 plasmapheresis to an average of 47 per cent of the 

 control protein concentration. Taken together, the 



10 



20 



ATRIAL 



30 



PRESSURE 



40 



m m Hg) 



50 



fig. 6.4. Rate of edema formation (nitration) in lungs of 

 dogs subjected to prolonged elevations of left atrial pressure. 

 Significant nitration did not appear until left atrial pressure 

 exceeded 25 mm Hg, i.e., the osmotic pressure of the plasma 

 proteins. [From Guyton & Lindsey (132).] 



results shown in figures 6.1 to 6.4 permit concluding 

 that in these four regions the net rates of filtration or 

 absorption through the capillary walls depend upon 

 the difference between hydrostatic and osmotic 

 forces acting across the membrane. In view of this 

 evidence the Starling hypothesis of 1896 (345) can 

 fittingly be called now the Starling filtration-absorp- 

 tion principle. 



Progress has gone beyond this qualitative stage, 

 however, because the meaning of k, the filtration 

 coefficient, has been expanded not only by numerical 

 values for a number of capillary beds and membranes 

 (tables 6. 1 and 6.2) but also by more precise definition. 

 Pappenheimer (276) called attention to the fact that 

 the several different "filtration constants," "unit 

 filtration rates," or "filtration coefficients" used by 

 various authors can be related to the equation used 

 by Darcy (62) to describe the viscous flow of fluids 

 through inert porous or fibrous materials, viz. : 



0. 





(6.1) 



where 



Qj = quantity filtered per unit time 



k = specific filtration constant of the porous material or 



membrane 

 A m — area of membrane 

 AP = pressure difference across membrane (in capillaries 



ap = p c - P if - n p , + n ;/ ) 



Ax = path length through membrane (for capillaries 



usually assumed to be 0.3 ju) 

 7j = viscosity of filtrate 



If the area of capillary wall can be measured 

 directly (23, 200, 201, 383) or computed (281, 282) as 

 in table 6.1, the proportionality factor or filtration 

 coefficient consists of k/riAx including the Darcy 

 "specific filtration constant," the thickness of the wall, 

 and viscosity of the fluid. On the other hand, for 

 tissues in which the capillary surface per weight or 

 volume of tissue is not yet known precisely, e.g., in 

 the human forearm, the hind quarters of the rat, and 

 the lung, the proportionality factor for unit tissue 

 weight or volume will consist of kAc/qAx including, 

 in addition, the area of the capillary walls, A c - The 

 term "filtration constant" is certainly inappropriate 

 and should be abandoned for a membrane system as 

 heterogeneous as that in the capillary wall. Filtration 

 coefficient is a preferable term and it is suggested that 

 the symbol k c be used for cases where the area of 

 capillary wall is measured or computed and k t be 

 used for coefficients based on mass or volume of 

 tissue. Newer developments in pore theory have led 



