182 



BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



one part of the cell to the other or through the walls of a cell may be 

 brought about by the presence of these salts. If a high concentration of 

 salts exists in a given locality, water tends to flow in that direction to 

 dilute the solution, while the salts tend to move in the direction of the 

 water, thus equalizing conditions. Similarly, the water-holding capacity 

 of the blood is in part due to this action of its inorganic salts. 



Irritability of muscles and nerves, that is their ability to respond to 

 physiological stimulation in a normal manner, is dependent upon the 

 proper kind and amount of inorganic ions in the body fluids which bathe 

 them. Many enzymes are inactive unless some particular inorganic ion 

 is present. Examples are given below and in Chap. 10. 



Functions of specific elements 



Sodium and Potassium. These elements exist in living tissues almost 

 exclusively in the form of Na+ and K+ ions. They make up the basic 

 portion of several buffer systems which maintain the physiological pH 

 values not only of the blood, as mentioned above, but also of such body 

 fluids as saliva and the pancreatic and intestinal juices. Potassium 

 ions constitute the main base inside the cells of the body, whereas sodium 

 is characteristically more concentrated in the blood plasma and in- 

 terstitial fluids, that is, tissue fluids outside the actual cells. Sodium 

 ions, in fact, make up over 90 per cent of all the cations (positively 

 charged ions) of these fluids (Table 8-3) and thus, together with Cl~ 

 ions, are mainly responsible for their osmotic pressure. 



Table 8-3 

 Approximate electrolyte distribution in human blood plasma * 



Cations Anions 



(Milliequivalents per liter of plasma water) 



* Data from Hawk, Oser, and Summerson. 



Sodium and potassium ions play a vital role in the process by which 

 carbon dioxide is carried by the blood stream from the muscles, where 

 it is produced, to the lungs, where it is eliminated from the body (see 

 p. 187). Both tend also to promote muscle relaxation. 



Common salt, sodium chloride, supplies most of the sodium and 

 chlorine in the diet. The daily salt requirement for normal adult per- 

 sons is in the neighborhood of 5 g., but the exact amount needed depends 

 on the water intake, because salt is carried out of the body in the urine 



