HANDBOOK OF PHYSIOloc.l 



CIRCULATION II 



of protein can range from 60 to 200 g per 24 hours, 

 depending upon conditions. Hence the third, or 

 protein, circulation passing from capillaries to inter- 

 stitial fluid to lymph (fig. 5.2, F-IF-L) involves daily 

 a volume of fluid which approaches the volume of 

 circulating plasma and a mass of protein equivalent 

 to a quarter or more of the mass of the circulating 

 plasma proteins. Cope & Litwin (45a) have recently 

 emphasized the compensatory importance, during re- 

 covery from hemorrhage, of this continuing flow of 

 lymph and its contained protein from the interstitial 

 spaces into the blood stream. 



6. FILTRATION COEFFICIENTS OF 



CAPILLARIES {k c ) J AND OF TISSUES (kt) 



A. Nc 



■jl Capillaries 



Measurements of fluid movement through the 

 capillary wall as a function of hydrostatic and osmotic 

 pressures have been made in single capillaries of 

 amphibian mesenteries (23, 200, 201, 383); in the 

 human forearm (24, 188, 209), in the perfused ex- 

 tremities of frogs (61, 74); of rats (162, 302); of 

 cats and dogs (281, 282); and in lung (132). 



The primary measurements necessary to test the 

 validity of the Starling hypothesis were first obtained 

 by micromanipulation techniques in single capillaries 

 of the frog's mesentery (200, 201) with results shown 



TTp| in vivo= e.6 to II 7cm H2O 

 higher in summer 

 frogs lower in 

 winter frogs 

 (hypoproteinemia) 



E 



ST .04 



5-° 2 



E 



o 04 



.06 

 



ABSORPTION 



CAPILLARY PRESSURE CM WATER 



,.."* Slope -k c = 



filtration coefficient 

 .0056 juVsec/ju 2 / cm H 2 

 A(P c -TT pl ) 



20 25 30 



fig. 6. 1 . Relation between fluid movement through walls 

 of single capillaries of frog's mesentery and capillary blood 

 pressure as determined by micromanipulation methods. Slope 

 of line indicates filtration coefficient \k r ) in n 3 of fluid filtered 

 (or absorbed) /sec /m 2 of capillary wall/cm H2O capillary 

 pressure. Intercept of line with zero axis measures effective 

 osmotic pressure (in vivo) of the plasma protein. [From Landis 

 (200).] 



in figure t>. 1 . When capillary pressure exceeded 12 cm 

 water, fluid passed from the plasma inside the capil- 

 lary to outside the capillary (filtration). When capil- 

 lary pressure was less than 10 cm water, fluid was 

 withdrawn from the e.xtravascular space into the 

 capillary (absorption). At capillary pressures be- 

 tween 9 and 1 3 cm water there were many instances 

 in which little or no movement of fluid occurred. In 

 this range hydrostatic pressure was apparently 

 balanced by the osmotic pressure of the plasma pro- 

 teins. This was taken to be indirect evidence that the 

 walls of the mesenteric capillaries of the frog were 

 relatively impermeable to protein and that, at least 

 in these vessels, 9 to 13 cm water (average 1 1.5 cm) 

 represented the effective osmotic pressure of the 

 plasma proteins in vivo. 



In addition to supporting the filtration-absorption 

 hypothesis of Starling these results also provided the 

 first measure of the permeability of the capillary wall 

 to isotonic fluid. When plotted against capillary pres- 

 sure the rates of fluid movement were directly pro- 

 portional to the difference between the capillary 

 pressure and the effective osmotic pressure of the 

 plasma proteins measured against the capillary wall 

 as a filter. The proportionality constant was com- 

 puted from the slope of the straight line drawn through 

 the observed points by the method oi least squares. 

 This was originally called a "filtration constant," 

 but for reasons given below the term "filtration 

 coefficient" is preferable (276). For normal mesenteric 

 capillaries of the frog the filtration coefficient, k c , 

 derived from 70 observations, averaged .0056 /j :l of 

 fluid per sec per fi 2 of capillary wall per cm water 

 difference between capillary pressure and the osmotic 

 pressure of the plasma proteins. Wind (383) found 

 great variation from capillary to capillary in the 

 toad's mesentery. Collectively, these figures provide a 

 slightly lower average figure, about .0032, during 

 the first 1 5 min after the mesentery was exposed and 

 a somewhat higher average figure, .0084, thereafter. 

 Deviations from these normal filtration coefficients 

 have proved helpful, as will be described below, in 

 measuring the effects of temperature (23), oxygen 

 lack (201), and injury (200) on the filtration-absorp- 

 tion mechanism in the frog's mesenteric capillaries. 



To test the validity of the Starling hypothesis in 

 another tissue, and particularly in man, Krogh et al. 

 (188) studied the movement of fluid through the 

 capillary walls of forearm tissue in a pressure plethys- 

 mograph, by means of which the blood vessels could 

 be collapsed in order to measure small increments of 

 tissue volume produced by filtered fluid. As shown in 



