EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



96; 



table 2.1. Resting Average Capillary Blood Pressure (P c )* 

 and Osmotic Pressure of Plasma Proteins (Jl p i) 



Average P c 



Arteriolar 

 end 



mm Eg 

 10.6 

 I I .O 



10.7 



22. I 

 28.3 



3'-3 



Venous end 



mm Eg 



7-4 

 7.0 



7-4 

 '2-5 



26.8 



32.O 2of 12.1 



34-3 I !2-2 



3O.6 29.5! 21.9 



- 22 f 



32 



24t 



•5 



Tissue (and Reference 

 for P c ) 



Mesentery (198) 

 Muscle (205) 

 Skin (205) 



Mesentery (202) 



Mesentery (202) 



Intestine (178) 



Skin (203) 

 Skin (92) 

 Skin (89) 

 Skin (225) 



* Direct measurements only. f Summit of the capillary 



loop. 



In the arteriolar end of the frog's mesenteric capillary 

 network pressure averaged 14.5 cm H 2 or 10.6 

 mm Hg; in the venous end about 10 cm H 2 or 

 7 mm Hg. Since the osmotic pressure of the plasma 

 proteins ranged in normal frogs from 5 to 10 mm Hg 

 (41 ), the approximate balance predicated by Starling 

 was present except when starvation, as in winter 

 frogs, reduced the concentration of plasma proteins 

 (23, 41). As shown in table 2.1, capillary blood 

 pressures in frog's muscle and skin were similar to 

 those in the mesentery (205). In the mesenteries of 

 rats and guinea pigs a balance was also found but at 

 a higher level of pressure (202). Pressures were highest 

 in the intestinal capillaries of the cat (178) and in 

 the cutaneous capillaries of man (89, 92, 203, 225), 

 but again in balance with the higher osmotic pressure 

 of the plasma proteins as shown in figure 2.3. Thus 

 in four tissues and in five species the pressures found 

 were generally compatible with Starling's view that, 

 on the average and at resting blood flows, these 

 pressures favor filtration in the arteriolar portion 

 of the capillary network and a balancing absorption 

 in the venous end of the capillary network. But 

 generalizations cannot be extended to tissues with 

 specialized functions. Capillary blood pressures may 

 be higher in kidney and lower in lung. 



Hayman (145) found that glomerular capillary 



pressure in the frog averaged 54 per cent of the 

 simultaneously measured aortic blood pressure. White 

 (377) observed pressures of similar magnitude in 

 Necturus. For mammalian glomeruli direct measure- 

 ments are lacking, but indirect estimates have ranged 

 from two-thirds of arterial pressure by Winton (384) 

 to about 50 per cent of arterial pressure by Gottschalk 

 & Mylle (124). The high rate of glomerular filtration 

 can be explained by these high capillary pressures 

 and the greater effective pore area of the glomerular 

 membranes (278). The mechanism by which 98 

 per cent or more of this filtrate passes back into the 

 blood of the peritubular capillaries cannot be ex- 

 plained so simply. 



Postglomerular or peritubular capillary pressures 

 have been measured directly by Wirz (385) who 

 reported 17.4 ± 2.6 mm Hg for a small series of 

 rats and by Gottschalk & Mylle (124) who found 

 averages of 20.4 and 14.2 mm Hg for large and 

 small peritubular capillaries, respectively, under 

 normal conditions. These pressures increased, how- 

 ever, to very high levels not only during venous 



100 



MAN -Finger tip, heart level 



100 



ARTERIES ] CAPILLARIES 

 ARTERIOLES 



fig. 2.3. Curves comparing gradient of pressure drop 

 (open circles) in four species with the corresponding osmotic 

 pressures (filled circles) of their plasma proteins. [Modified from 

 Landis (207).] 



