SPECIAL CAUSES OF EDEMA 291 



Summary. We find that a number of factors may be 

 considered as responsible for edema, some of them being promi- 

 nent in one instance, some in another, but in few cases can we 

 consider one factor alone as the sole cause. In most of the forms 

 of edema, such as those due to renal disease and cardiac disease, 

 it now seems probable that osmotic pressure changes play the 

 most important part ; whereas in inflammatory edema there can 

 be no question that alteration in the capillary walls is the most 

 essential factor. But the mechanical factor of blood pressure 

 cannot be disregarded, although by itself seldom sufficient to cause 

 edema ; associated with other factors it is undoubtedly an 

 important agency, for there are few edemas that are not asso- 

 ciated with increased blood pressure. Hydremia and hydremic 

 plethora may almost be disregarded, except in so far as they 

 may cause altered metabolism in the tissues, injury to vessel- 

 walls, and decreased osmotic pressure within the vessels. 

 Lymphatic obstruction is possibly a factor of some secondary 

 importance if we consider that distended vessels and tense 

 tissues may occlude the lymph capillaries. 



SPECIAL CAUSES OF EDEMA 



We may now consider which of the above factors are at work 

 in bringing about edema under the conditions in which it is 

 usually observed clinically. 



" Cardiac " I$dema. Passive congestion introduces nearly 

 all these factors, for in addition to the increased blood pres- 

 sure there is also an opportunity for changes in the capillary 

 wall, either from stretching and thinning of the cells and cement 

 substances, or from " loss of tone" in the endothelium surround- 

 ing the stomata (Meltzer), or from toxic injury by accumulated 

 products of tissue metabolism. When the stasis is nearly com- 

 plete, or if it is complete for a time and then relieved, the 

 endothelium may be injured through lack of nourishment. As 

 the edematous fluid in passive congestion is usually of a watery 

 type, poor in proteids, the edema is probably less dependent upon 

 capillary permeability than upon other factors, except in the case 

 of acute stasis, when the fluid partakes of the character of the 

 exudates. Undoubtedly the accumulation of crystalloids within 

 the tissues also plays a most important part in this form of 

 edema, Loeb's experiments having shown how greatly osmotic 

 pressure is raised in tissues having deficient oxygen supply. 

 Finally, there is probably more or less obstruction to lymphatic 

 outflow because of the increased pressure upon the lymphatic 



