THE CAUSES OF EDEMA 345 



from its l)l()()(l-supi)ly led to sueli an increase in osmotic pressure tliat it 

 took uj) water from a 4.!) per cent. NaCl solution, wliich lias a j)res- 

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

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

 blood pressure less than ^^o 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 eliminate 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.) Convereely, 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. 



There are some difficulties, however, in applying the influence of 

 osmotic pressure as an explanation of all edemas. For example, in 

 edema of the lungs, as Meltzer points out, what is the force that drives 

 the fluid into the empty air-cells? Equally difficult to explain as the 

 result of osmotic disturbance is the distribution of fluid that is seen 

 in cardiac dropsy. The fluid does not accumulate in the tissues where 

 metabolism is greatest, or where the most oxygen is used ; but rather 

 in the inactive subcutaneous tissues and in the serous cavities. Possi- 

 bly the original transudation does occur in the muscles and solid 

 viscera, and the fluid is then mechanically forced out of them into 

 the surrounding tissue-spaces, later settling according to the laws of 

 gravity or according to the distensibility of the tissues. It is im- 

 portant to take into consideration the fact that demonstrable edema 

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



