PATIKXyENESIS OF ICDKMA 345 



(). Disparity of Osmotic Pressure in Favor of the Tissues and 

 Lymph over the Blood. — On a proccdiiifi; pa^o wo luiv(! already 

 con:sidercd the incans by which changes in osmotic pressure in the tis- 

 sues 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 J. Loeb''- 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 products: — 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. NaCl solution, which has a pres- 

 sure of over thirty atmospheres. When we consider that in his studies 

 on lung edema Welch was able by ligation of the aorta to raise the 

 blood pressure less than ifo atmosphere, we begin to appreciate how 

 much more powerful are the physico-chemical forces that are at work 

 in the body than is the blood pressure, even of the aorta itself. 



Loeb found that whenever oxidation is impaired in a tissue its 

 osmotic pressure rises, which he ascribed to the accumulation of in- 

 completely oxidized metabolic products, particularly acids, and as a 

 result the muscle takes up water and becomes edematous. On this 

 basis we might explain the edema of venous stagnation as due to ac- 

 cumulation of products of metabolism, partly because of impaired 

 oxidation, partly, perhaps, because of their slow removal in the blood 

 on account of the circulatory disturbance. The so-called "neurotic" 

 edemas may possibly be explained by local increase in metabolic ac- 

 tivity brought about by nervous stimuli, which causes increased forma- 

 tion of substances raising osmotic pressure in the stimulated tissues. 

 In renal edema the retention of water also seems to depend rather on 

 osmotic pressure than on circulatory disturbances or alterations in the 

 vessel-walls, for it has been shown that retention of chlorides, which 

 the diseased kidneys do not eUminate normally, is an important cause 

 of the dropsy in some cases. The chlorides accumulating in the tissues 

 lead to an increased osmotic pressure, which causes the abstraction of 

 water from the blood and its retention in the tissues. (The details of 

 this subject will be considered later.) Conversely, Meltzer and Salant 

 found that salt solution is absorbed from the peritoneal cavity more 

 rapidly in nephrectomized rabbits than in normal rabbits because 

 metabolic products accumulate in the blood and raise its osmotic pres- 

 sure above normal; and it was observed by Fleisher and L. Loeb^^ 

 that the rate of absorption of fluid from the peritoneal cavity is in- 

 creased when the osmotic pressure of the blood is raised. 



32 Pfluger's Arch., 1898 (71), 457. 

 " Jour. Exper. Med., 1910 (12), 510. 



