258 INTRINSIC ACTIVITY OF SECRETING CELLS 



physiological effect of iron may be very large, although the uptake 

 is infinitesimal, and the time required for uptake is large. Thus, in 

 an individual weighing, say, 60 kilograms, the weight of blood would 

 be approximately 4 kilograms, that of haemoglobin about 500 grms., 

 and in this the iron would be about 0-4 per cent., or 2 grms. There- 

 fore in a course of iron treatment lasting over some weeks the amount 

 of iron necessary to be taken up in order to produce a marked effect 

 would obviously be so small as to be entirely beyond the bounds of 

 determination under the conditions of experiment. 



Nor does the view of varying permeability of the cell to different 

 dissolved substances, of high permeability for some and low per- 

 meability for others, give any better solution to the real problems 

 of secretion and absorption than that of complete impermeability, 

 for the simple reason that variations in permeability form a passive 

 factor like the variation of a resistance, and hence can at most 

 alter the time relations of the process, and not the end results, 

 and so there can on such a basis be no explanation of the fact that 

 work" is done by the secreting cell in the process, as when a con- 

 stituent dissolved substance is secreted at higher concentration 

 and pressure than in the plasma. Thus if a cell is immersed in 

 a fluid containing any given constituent in solution, it will, if it 

 possesses any degree of permeability whatever for that constituent, 

 become ultimately saturated to the equilibrium point with the 

 constituent, and the point of equilibrium will not vary with the 

 permeability ; the only thing which will vary with the permeability 

 will be the time in which equilibrium is attained. In considering 

 the effects of change in permeability upon the time relationships 

 of absorption and secretion, the factors to be borne in mind are 

 the thickness of the layer through which diffusion has to occur, 

 the difference in concentration of the diffusing dissolved substance 

 or ion at the two surfaces bounding the layer through which diffusion 

 is taking place, and the coefficient of diffusion for the particular 

 substance through the layer. The rate of diffusion, regarded as 

 a purely physical process unaided by the cellular activity (and 

 dependent only upon the difference in osmotic pressure at the two 

 sides of the layer or membrane, the thickness, and the coefficient 

 of permeability or diffusion), may be said to be directly proportional 

 to the difference in osmotic pressure and to the coefficient of per- 

 meability, and inversely proportional to the thickness of the layer 

 or membrane -that is, the length of the absorbing or secreting cell. 



