162 OSMOTIC PRESSURE 



solution no change is apparent, but if placed in a concen- 

 trated salt solution (hypertonic) water diffuses out of the 

 red cells and they become crenated or shrunken. If the salt 

 solution is very dilute or if water is used (hypotonic) the cells 

 take up water, swell up and burst, the haemoglobin being set 

 free into the plasma. In the case of red corpuscles then, 

 it is understood that the outer layer or envelope of the 

 corpuscle is semi-permeable, and that it will allow water to 

 pass in or out, but not salts. This is not quite true, because 

 the cell envelope has a vital selective power, and is permeable 

 for certain substances. Thus urea in solution can pass 

 through the membrane, and so also can free acids, alkalies, 

 and ammonium salts. With these exceptions, however, the 

 corpuscles are semi-permeable. The osmotic pressure of the 

 blood varies in different animals to a slight extent and so 

 4 normal saline solution ' should be varied for the particular 

 species. In mammals, however, sufficient accuracy is 

 obtained by using a solution containing 0'9% of common salt 

 as being isotonic with the blood. 



The osmotic pressure of the blood varies a little at different 

 times, and reaches a maximum shortly after a full meal. 

 The variation is, however, very slight, for the kidneys at once 

 proceed to correct it, by excreting urine more concentrated 

 or less concentrated, to suit the case. Thus the salt content, 

 and therefore the osmotic pressure, of the urine, varies con- 

 siderably at different times during the day. 



Normal saline solution is the ideal fluid for saline in- 

 jections, for irrigation of the body cavities, the abdomen, the 

 stomach, etc., for the reason that, being isotonic, it does not 

 damage cells. The toxic action of distilled water when drunk 

 is simply due to the fact that, being hypotonic, it is taken up 

 by the cells of the stomach mucous membrane which swell 

 up, die, and may be cast off, so causing catarrh. Water got 

 from melted snow or ice containing no dissolved salts has 

 the same harmful effect. 



When a hypertonic solution of a salt is given by the 

 mouth and absorbed into the blood, or if such a solution is 

 injected directly into the blood stream, the effects on the 

 corpuscles would be to produce crenation and destruction, 

 but for the fact that the tissues are at once drawn upon for 



