OSMOSIS. 309 



transudation, until we get equilibrium established at a somewhat higher 

 point, when there is a more dilute fluid in the tissue spaces, and there- 

 fore a higher absorbing force to balance the increased capillary pressure. 

 With diminished capillary pressure there will be an osmotic absorption 

 of salt solution from the extravascular fluid until this becomes richer in 

 proteids, and the difference between its osmotic pressure and that of the 

 intravascular plasma is equal to the diminished capillary pressure. 1 



Here, then, we have the balance of forces necessary to explain the 

 accurate regulation of the quantity of circulating blood according to the 

 conditions under which the animal may be placed, and it seems 

 unnecessary to invoke the aid of vital activity to explain the process. 

 Certain corollaries of this mode of explanation agree well with observed 

 facts of experiment. Thus the more impermeable the capillary the 

 smaller will be the amount of proteid exuded with the lymph. A 

 higher capillary pressure will therefore be needed in its production, and 

 there will be an equally high force tending to its reabsorption. A rise of 

 capillary pressure will only increase the amount of lymph in the 

 extravascular spaces to a certain extent, but will at the same time cause 

 this lymph to be more dilute, so that there will be a corresponding rise 

 in the force tending towards absorption. In consequence of this 

 sequence of events, considerable alterations of capillary pressure may 

 be produced in impermeable capillaries, such as those in the limbs, 

 without causing any appreciable increase in the lymph overflow from the 

 limbs. On the other hand, where the capillaries are very permeable, 

 very little pressure will be required to cause a transudation, since no 

 work is done in the concentration of a proteid solution, and we find as a 

 matter of fact, that capillaries where the pressure is lowest i.e. in 

 the liver are also those which are the most permeable. Here, too, 

 the absorbing force will be insignificant, since there is very little 

 difference in the percentage of albumin between liver blood and liver 

 lymph. 



Moreover, since the pressure on the venous side of the capillaries is 

 considerably less than that on the arterial side, there will be a continual 

 exudation of a very dilute lymph from the arterial capillaries, and a 

 re-absorption of water and salts from this lymph in the venous 

 capillaries. The lymph, therefore, will assume a composition such that 

 the osmotic pressure of its proteids approximates the mean capillary 

 pressure in the part where it is formed. 



This osmotic difference between blood plasma and tissue fluid will 

 not serve to explain the absorption of proteids by the blood vessels nor the 

 absorption of serum from the serous cavities. It is difficult, however, if 

 not impossible, to prove that serum or proteid is absorbed by the blood 

 vessels. In some of my transfusion experiments I have rendered a limb 

 cedematous by means of serum, and in these cases have obtained no 

 evidence at all of absorption by the blood vessels. There is no doubt 

 that serum may be absorbed from the pleural and peritoneal cavities, 

 but the absorption of these fluids is very much slower than the absorp- 

 tion of salt solutions, and is, in fact, so slow that the whole of it can in 

 most cases be effected by the lymphatic channels. A slow absorption of 

 serum from tissue spaces by means of the blood vessels is also physically 

 possible. As the cells of the tissues feed on the proteids of the fluid, 



1 For a fuller discussion of this point, cf. Science Progress, London, 1896, vol. v. 

 p. 151. 



