I VII \\(,1 ill SUBSTANCES I Hid 1 1 '.II CAPILLAR"* WALLS 



989 



figure 6.2 net filtration of fluid increased linearly 

 with venous pressures above 10 cm water. A unit 

 rise of venous pressure (1 cm water) increased the 

 filtration rate by .0023 ml per min per 100 ml of 

 forearm tissue when congestion periods of 30 min 

 were used (188) and by 0033 ml per min per 100 ml 

 forearm tissue when congestion periods of 10 min were 

 used (209). As described in section 4, this difference 

 was regarded as the result of increasing interstitial 

 fluid pressure as the volume of filtrate in the tissues 

 increased. 



Brown et al. (22) have more recently studied, by a 

 totally different method, the filtration coefficient for 

 the whole body of man (except the thorax ) during a 

 systemic rise of venous pressure produced by repeated 

 Valsalva maneuvers. Though results varied slightly, 

 depending on the method of calculation, representa- 

 tive filtration coefficients for the whole body were 

 in the first 9.5 min, .0036 ml per min per 100 g body 

 wt per cm rise of venous pressure and, for a total 

 of 29.5 min, .0014. These values can be compared to 

 .0033 and .0023 for the forearm alone. The two sets 



.200 



.160 



080 



.040 



-020* 



fig. 6.2. Rates of filtration measured by pressure plethys- 

 mograph in human forearm during graded elevation of venous 

 pressure for 10-min periods. Plethysmograph temperature, 

 34-35 C. The slope of the line corresponds to a nitration co- 

 efficient (ki) of .0033 ml/min/100 ml forearm tissue per cm 

 H2O increase of venous pressure. [From Landis & Gibbon 

 (209).] 



of figures are similar, presumably because the collec- 

 tive capillary beds of muscle and subcutaneous tissue 

 are large compared to the smaller, though more 

 permeable, capillary beds of liver and intestine. A 

 similar relationship has been found with respect to 

 diffusion (see section 8). The '"whole body" filtration 

 rate appears to decline more rapidly than that of the 

 forearm, owing probably to more rapid return of 

 capillary filtrate by way of the lymphatics, particu- 

 larly during the vigorous respiratory movements 

 required for repeated, brief Valsalva maneuvers. 



Landis & Hortenstine (210) calculated from the 

 forearm filtration figures (188, 209) that a rise of 

 venous pressure, throughout the body, to 10 cm 

 water above normal might, in a man weighing 75 kg, 

 filter as much as 250 ml of fluid from the plasma in the 

 first 10 min. This has proved a fairly good estimate. 

 Brown et al. (22) observed the filtration of 333 ml 

 to 501 ml when systemic venous pressure was ele- 

 vated by 20 cm for 9.5 min. Over 29.5 min an 

 increase of venous pressure by 20 cm water filtered 

 460, 41 7, and 687 ml of fluid, calculated to contain 

 between 1 and 2 g of protein per 100 ml owing, pre- 

 sumably, in part to the very high protein content 

 of capillary filtrate from hepatic and intestinal 

 capillaries. 



The pressure plethysmograph was used by Krogh 

 et al. (188) also to test the effect on filtration rate 

 of changing the osmotic pressure of the plasma 

 proteins. Filtration rates at given venous pressures 

 were measured with the subject recumbent and then 

 at the same venous pressures while the subject stood 

 quietly on a tilt table for 30 min or more. Quiet 

 standing increased the concentration of the circulating 

 plasma proteins by 0.6 to 1.1 g per 100 ml and the 

 protein osmotic pressure of plasma by 33 to 8.7 cm 

 water. At these higher protein osmotic pressures the 

 rate of filtration produced by a given venous pressure 

 was always lower. A unit rise of protein osmotic 

 pressure (1 cm water) was accompanied by a reduc- 

 tion of filtration rate ranging from .0027 to .0045 ml 

 per min per 100 ml forearm tissue. These values were 

 quantitatively similar to the effect produced by 

 elevating venous pressure by 1 cm water, but opposite 

 in sign. Within the limitations of the method these 

 results justified extending the Starling hypothesis to 

 the forearm capillaries of man, and were compatible 

 with the view that the capillaries of the human fore- 

 arm were relatively impermeable to the plasma 

 proteins. 



In the first studies with the pressure plethysmo- 

 graph (188, 209) it was perplexing to find that venous 



