BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



183 



and, especially, in sweat. The average American diet furnishes 10-15 g. 

 of salt a day, which is more than adequate, except during profuse sweat- 

 ing. An extra gram of salt (above the 5 g. minimum) should be con- 

 sumed for each liter of water intake in excess of 4 1. per day. This may 

 amount to as much as 20-30 g. in extreme cases. So-called heat prostra- 

 tion after hard work in hot weather is often merely the result of un- 

 compensated salt losses and may be prevented by proper attention to 

 salt intake. 



Among the symptoms characterizing potassium deficiency in rats are 

 lethargy, distension of the abdomen, edematous kidneys, discoloration of 

 the skin owing to improper blood supply, failure of growth, and early 

 death. Pathological changes may be detected in the intestines, pan- 

 creas, kidneys, and hearts of such animals. 



The potassium requirement for human beings is unknown, but the 

 element is present in nearly all foods in such large amounts that the 

 daily intake appears to be entirely adequate. 



Calcium and Phosphorus. Both of these elements are essential con- 

 stituents of all living cells. Calcium is present in the animal body in 

 larger amounts than any other mineral element. About 99 per cent of 

 the total is in the bones and teeth, which are made up of approximately 

 one-half moisture and organic matter and one-half inorganic or mineral 

 matter. The latter consists essentially of calcium phosphate together 

 with smaller amounts of calcium carbonate. IMagnesium and other ele- 

 ments are also present in minor amounts. 



The remainder of the calcium exists mostly as Ca++ ions in the body 

 fluids, where it is of fundamental importance for the normal activity of 

 nerves, muscles, and heart, for the clotting of blood, and for maintaining 

 the permeability of cell membranes. Thus the clotting of freshly drawn 

 blood may be prevented, or greatly retarded, by the addition of a 

 reagent, such as sodium oxalate or citrate, which removes the Ca+ + 

 ions by forming an insoluble (oxalate) or nonionized (citrate) product. 

 Coagulation of milk also requires calcium, and the cementing substances 

 which hold cells together in tissues appear to involve this element. 

 Normally, the total calcium content of man's blood ranges from 9 to 11 

 mg. per 100 ml. of plasma. In the young it is slightly higher. About 

 half the total is inorganic Ca++ ions. If the blood calcium falls below 

 certain levels, depending upon the species of animal, tetany (generalized 

 spasmodic muscle contractions) results, and death may follow unless 

 restorative measures are employed. Both vitamin D and the secretion 

 of the parathyroid gland operate in controlling the calcium content of 

 the blood, the former through an increased "net absorption" of food 

 calcium and the latter by mobilization of the calcium in certain labile 

 structures of the skeleton. 



The bulk of the phosphorus of the body, about 80 per cent, is also 



