530 TRANSUDATION. [BOOK n. 



diffusion or a simple process of filtration, that is to say, can all the 

 phenomena of transudation be explained as simply the results of 

 the one or the other of these physical processes ? 



Diffusion by itself will not account for the results ; for the 

 proteids of the blood-plasma are indiffusible or very nearly so and 

 yet the lymph contains a considerable quantity of these proteids. 

 We have no satisfactory knowledge of the exact composition of 

 lymph as it exists in the lymph-spaces. In the lymph of the 

 larger lymph-trunks the diffusible saline substances are present in 

 about the same proportion, and the indiffusible proteids to about 

 or less than half as much as in blood-serum ; and we may perhaps 

 assume that the lymph in the lymph-spaces contains relatively 

 less proteids but has otherwise the same composition as blood- 

 plasma. Mere diffusion would not give rise to a fluid of such a 

 nature. 



Can we speak of transudation then as a filtration ? The blood 

 is undoubtedly flowing through the capillaries and other small 

 vessels under a certain pressure; we have seen ( 116) that the 

 pressure though variable is roughly speaking about 20 mm. Hg.; 

 and it would be possible to select such a filter or porous partition 

 as would at about this pressure permit the passage of a certain 

 quantity of the inorganic and crystalline constituents of blood- 

 plasma to pass through in company with a relatively smaller 

 quantity of the proteids and a large quantity of the water, the red 

 and white corpuscles being excluded. Such a filtrate would be 

 more or less of the nature of lymph ; and so far we might be 

 justified in speaking of the transudation of lymph as a process of 

 filtration. 



But the question arises, Is the process of transudation one 

 which in all respects conforms to that of filtration ? We may 

 with advantage discuss this important question at some little 

 length. 



Let us first consider what is the pressure with which we have 

 to deal. We may assume that the transudation takes place in 

 the capillaries; we include with the capillaries the minuter veins, 

 and neglect in this respect all other vessels. What factors 

 determine the pressure in a given set of capillaries ? The first 

 most general and most important is the condition of the minute 

 artery (or arteries) feeding the capillaries in question. If the 

 artery is dilated, the pressure in the capillaries will be high or low 

 according to the amount of mean arterial pressure obtaining 

 at the time ; and since this under ordinary circumstances is high, we 

 may say that the widening of an artery means under ordinary 

 circumstances increased capillary pressure in the area fed by that 

 artery. If the artery be constricted, the pressure in the capillaries 

 falls ; part of the pressure which previously was exerted on the 

 capillaries is now used up before it reaches the capillaries, in 

 overcoming the increased resistance offered by the constricted 



