MINERAL METABOLISM 403 



iiilo ions, each ion is cllcctive antl each conliihules as niiuli lo the 

 osmotic concentration as a molecule ol ghicose or any other iindis- 

 sociated molecule large or small. The intracellular Huid ol a cell con- 

 tains a wide variety of dissolved ionic and molecular materials. The 

 osmotic concentration of the internal fluid is due to all of these solutes 

 and corresponds to one of the sodiinn chloride concentrations listed 

 above. 



If a red cell, for example, from a sheep, is placed in distilled water, 

 Avater passes rapidly by osmosis from the low osmotic concentration 

 outside to the higher concentration inside the cell. This increase in 

 liquid inside causes the cell to swell, disrupts its internal structure, 

 and stretches the membrane to the breaking point. 



On the other hand, a cell placed in a strong salt solution loses water 

 by osmosis to the salt solution and shrinks. Serious shrinkage may 

 permanently injiue or kill the cell. Hence a rather close balance is 

 required between the osmotic concentrations inside and outside cells. 

 When the system is balanced, it is said to be isotonic, and any medium 

 providing such balance is called an isotonic solution. 



The nerve cells of animals are among the tissues that au especially 

 susceptible to abnormal osmotic concentrations. As a result, aj^preci- 

 able changes or even prolonged slight deviations in the osmotic con- 

 centration of the interstitial fluid cannot be tolerated. This require- 

 ment means in turn that the osmotic concentration ol the blood 

 plasma must be very similar to that of the interstitial ihiid and must 

 hold constant. To achieve a degree of constancy i'i the blood, the 

 effect of dietary fluctuations must be eliminated. The kidneys of 

 higher animals can do this over a wide range by excreting practically 

 salt-free urine when little salt is ingested and by excreting very salty 

 urine when the diet is high in salts. Thus the proper functioning of 

 the kidneys assures isotonic body fluids even though the diet varies 

 widely in soluble salts. 



Except for sodium and potassium salts, large quantities of soluble 

 salts are seldom encountered. However, when they are ingested, such 

 materials are rapidly excreted ni soluble form in urine, as in the case 

 of aluminum. Or the excess may be stored in the skeleton, as with 

 calcium. Extra absorbed iron is stored in ferritin. Other elements 

 may be excreted in the feces. Nickel, for example, to some extent 

 jxisses through the intestinal tract without absorption. The remainder 

 is absorbed but largely returns to the intestine for excretion. All 

 these schemes play a part in preserving the balance of electrolytes 

 throughout the body. 



