PASSAGE OF WATER AND DISSOLVED SUBSTANCES 147 



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 impermeable to dissolved substances. In this case 

 the transference of water from one side to the other depends entirely 

 on the difference of osmotic pressure between the two sides. 



m 



A 



B 



FIG. 24. 



If we suppose two vessels, A and B (Fig. 24) separated by such a 

 membrane, A containing a solution of a and B a solution of /3, water will 

 pass from A to B so long as the osmotic pressure of ft is greater than the 

 osmotic pressure of the solution of a. If B be subjected to a hydrostatic 

 pressure greater than the osmotic difference between the two fluids, 

 water will pass from B to A until the force causing filtration or transu- 

 dation (the hydrostatic pressure) is equal to the force causing absorp- 

 tion into B (the difference of osmotic pressures). Under no circum- 

 stance will there be any transference of salt or dissolved substance 

 between the two sides. Such semi- permeable membranes as this, 

 however, rarely occur in the body over any extent of surface. The 

 external layer of the cell protoplasm may resemble the protoplasmic 

 pellicle of plant cells in possessing this ' semi-permeability ' ; 

 but in nearly all cases where we have a membrane made up of a 

 number of cells, it can be shown to permit the free passage of at 

 any rate a large number of dissolved substances. 



Let us now consider what will occur when the two solutions A and 

 B are separated by a membrane which permits the free passage of 

 salts and water. If the osmotic pressure of B be higher than A at the 

 commencement of the experiment, the force tending to move water 

 from A to B will be equal to this osmotic difference. But there is at 

 the same time set up a diffusion of the dissolved substances from B 



