242 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



pressure of blood serum is ' = 5,000 mm. of mercury approxi- 



1*87 



mately, or a pressure of nearly 7 atmospheres. 



The osmotic pressure of solutions may also be compared by observing 

 their effect on red corpuscles, or on vegetable cells such as those in Trades- 

 cantia. If the solution is hypertonic, i.e. has a greater osmotic pressure 

 than the cell contents, the protoplasm shrinks and loses water, or, if red 

 corpuscles are used", they become crenated. If the solution is liypotonic, e.g. 

 has a smaller osmotic pressure than the material within the cell-wall, no 

 shrinking of the protoplasm in the vegetable cell occurs, and if red corpuscles 

 are used they swell and liberate their pigment. Isotonic solutions produce 

 neither of these effects, because they have the same molecular concentration 

 and osmotic pressure as the material within the cell- wall. 



Physiological Applications. It will at once be seen how important all these 

 considerations are from the physiological standpoint. In the body we have 

 aqueous solutions of various substances separated from one another by 

 membranes. Thus we have the endothelial walls of the capillaries separating 

 the blood from the lymph ; we have the epithelial walls of the kidney tubules 

 separating the blood and lymph from the urine ; we have similar epithelium 

 in all secreting glands ; and we have the wall of the alimentary canal 

 separating the digested food from the blood-vessels and lacteals. In such 

 important problems, then, as lymph-formation, the formation of urine and 

 other excretions and secretions, and absorption of food, we have to take into 

 account the laws which regulate the movements both of water and of sub- 

 stances which are held in solution by the water. In the body osmosis is not 

 the only force at work, but we have also to consider nitration : that is, the 

 forcible passage of materials through membranes, due to differences of 

 mechanical pressure. Further complicating these two processes we have to 

 take into account another force : namely, the secretory or selective activity of 

 the living cells of which the membranes in question are composed. This is 

 sometimes called by the name vital action, which is an unsatisfactory and 

 unscientific expression. The laws which regulate filtration, imbibition, and 

 osmosis are fairly well known and can be experimentally verified. But we 

 have undoubtedly some other force, or some other manifestation of force, in 

 the case of living membranes. It probably is some physical or chemical 

 property of living matter which has not yet been brought into line with the 

 known chemical and physical forces which operate in the inorganic world. 

 We cannot deny its existence, for it sometimes operates so as to neutralise 

 the known forces of osmosis and filtration. 



The more one studies the question of lymph-formation, the more con- 

 vinced one becomes that mere osmosis and filtration will not explain it entirely. 

 The basis of the action is no doubt physical, but the living cells do not 

 behave like the dead membranes of a dialyser ; they have a selective action, 

 picking out some substances and passing them through to the lymph, while 

 they reject others. 



The question of gaseous interchanges in the lungs has been another 

 battlefield of a similar kind. Some maintain that all can be explained 

 by the laws of diffusion of gases ; others assert that the action is wholly 



