132 PHYSIOLOGY 



day -810 gramme of urea will pass from A to B through the cylinder of one 

 centimetre in length and one square centimetre in cross-section. The deter- 

 mination of these diffusion coefficients presents many difficulties. The task 

 is, however, rendered easier by the fact, first ascertained by Graham, that 

 diffusion of salts occurs as rapidly through a solid jelly of gelatin or agar-agar 

 as through water. It is therefore possible to make the plug in the diagram 

 solid by the admixture of one of these two substances, and to maintain a con- 

 stant concentration on the two sides of it by the circulation of fluid without 

 affecting the rate of diffusion through the cylinder by setting up accidental 

 currents. 



More important from the physiological point of view than diffusion 

 through fluids is the exchange of fluids (water and dissolved substances), 

 which may take place across membranes. Such processes are of constant oc- 

 currence in all parts of the body and are concerned in such functions as the 

 formation and absorption of lymph, the absorption from and secretion 

 into the intestines, absorption from serous cavities, and so on. In many 

 of these functions we shall have to consider later whether the transference 

 across the membrane is determined solely by the nature and concentration 

 of the fluids on the two sides of it or is effected by the active intervention, 

 involving the expenditure of energy, on the part of living cells forming 

 constituent elements of the membrane itself. It is worth our while, there- 

 fore, to consider at some greater length the purely physical factors which may 

 be concerned in the passage of water and dissolved substances across 

 membranes. 



In the case of fluids containing only one substance in solution, the ex- 

 change across the membrane will be determined entirely by the osmotic 

 pressures. Thus, if two watery solutions, with the same osmotic pressure, 

 are separated by a membrane through which diffusion can take place, no 

 change in volume occurs on either side of the membrane. If the solutions 

 on either side of the membrane are of unequal osmotic pressure, water passes 

 from the side where the pressure is lower to the side where it is higher, and 

 there is a simultaneous passage of the solute from the side of greater to the 

 side of less concentration. 



If, however, the solutions on the two sides contain dissimilar substances, 

 with different diffusion coefficients, the conditions are more complicated, 

 and may tend even to produce a movement of fluid in apparent opposition to 

 the difference of osmotic pressure. Under these circumstances the nature 

 of the membrane itself is all-important. We may therefore shortly consider 

 the various modes in which interchanges may take place across membranes of 

 varying permeability. We shall see that the close analogy which exists 

 between substances in solution and gases, when dealing with ' semi- 

 permeable ' membranes, is also borne out by experiment when used to predict 

 the behaviour of solutions separated by such permeable membranes as 

 occur in the body. 



The simplest case is that in which two fluids are separated by a perfect 

 semi -permeable membrane that permits the passage of water but is absolutely 



