THE CA USES OF EDEMA 289 



5. Increased Filterability of the Blood Plasma. 



This takes us back to Richard Bright 7 s conception of renal 

 dropsy. He imagined that through the great loss of albumin 

 in the urine the blood became so thinned and watery that it 

 could filter through the vessel-walls, while normal plasma, he 

 thought, was too thick and viscid to do so. The same idea was 

 applied to the edemas of cachexia in cancer, etc., chlorosis, and 

 all forms of edema associated with a decrease in the corpuscular 

 or proteid elements of the blood. With our present knowledge 

 of diffusion of crystalloids and colloids we can readily appreci- 

 ate that a decrease in the blood colloids, such as might occur in 

 these diseases, could not modify the passage of fluids through 

 the capillary walls to any considerable degree. Stewart and 

 Bartels considered that in renal dropsy the increased filterability 

 of the plasma was not due so much to the loss in albumin as to 

 retention of water, which caused an hydremic plethora. But 

 this factor was soon eliminated, for it was found that complete 

 anuria, produced by ligating both ureters, does not cause edema ; 

 and also that to produce an edema by increasing the water of 

 the blood it was necessary to increase it many times as much as 

 it can ever be increased by disease. Simply increasing the 

 proportion of water by removing part of the blood and inject- 

 ing a corresponding amount of salt solution did not cause 

 edema (Cohnheim and Lichtheim). We may, therefore, look 

 upon the hypothesis of increased filterability of the blood as 

 chiefly of historic interest, and not an important factor in the 

 causation of edema. 



6. Disparity of Osmotic Pressure in Favor of the 

 Tissues and I/ymph over the Blood. On a preceding 

 page we have already considered the means by which changes 

 in osmotic pressure in the tissues are brought about, and how 

 they may lead to an accumulation of fluid. The importance of 

 osmotic pressure in causing pathological edema was suggested 

 by Loeb 1 in his studies on the physiological action of ions. 

 He stated that edema occurred when the osmotic pressure was 

 higher in the tissues than it was in the blood and lymph, and 

 the cause was to be sought in conditions that lowered the osmotic 

 pressure of the blood and lymph or raised that of the tissues. 

 This condition he found in the accumulation of metabolic prod- 

 ucts : in the case of muscle, tetanization of a frog's muscle 

 for ten minutes raised the osmotic pressure over one atmosphere; 

 separating a muscle from its blood-supply led to such an increase 

 in osmotic pressure that it took up water from a 4.9 per cent. 



1 Pfliiger's Arch., 1898 (71), 457. 

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