VII. CHEMICAL PATHOLOGY AND PHARMACOLOGY 240 



We hiU'e noted that the formation of insokible sahs of calcium such as 

 oxalate, tartrate, and alkaline phosphates, for I'xample, tends to interfere 

 Avith absorption. Much of this interference with the absorption of calcium 

 can be overcome by inclusion of vitamin 1) in the diet, or by adequate ir- 

 radiation with ultra\'iolet rays of the correct wavelength. Thus, in the 

 absence of vitamin D, less than 20% of ingested calcium is absorbed from 

 the gastrointestinal tract; if there is an adequate vitamin D intake, 50 % to 

 80 % may be absorbed. 



In the case of phosphate, which constitutes the most important anion 

 component of the calcium salts, the situation is rather more complicated, 

 for phosphorus plays an important role in innumerable enzymic processes 

 witliin the cells and is a constituent of many cellular components, e.g., 

 nucleic acid, phosphatides, phosphoproteins, creatine phosphate, hexose 

 phosphates, ATP, i\DP, etc. Phosphorus must be available at all times for 

 basic chemical processes. 



It is therefore not surprising that the absorption of phosphorus from the 

 gut is in large measure independent of vitamin D intake, and that such 

 inefficiency of absorption as is observed in rickets is secondary to the failure 

 of calcium absorption. Moreover, such improvement of phosphorus absorp- 

 tion as is obtained by feeding vitamin D to the ricketic patient is a conse- 

 quence of impro\'ement of calcium absorption.^ The degree of phosphorus 

 absorption can be influenced by the pH of the intestinal tract. Adminis- 

 tration of cations, such as iron, aluminum, or beryllium which form insoluble 

 phosphates Avill interfere with the absorption of phosphate.- 



The level of calcium and phosphorus in the blood is not entirely depend- 

 ent upon the amount of these elements absorl)ed from the gut. Physico- 

 chemical as well as endocrine factors play a part. The relationship between 

 the plasma calcium and inorganic phosphorus concentration is reciprocal, 

 as is that of the ions of any slightly soluble salt in a saturated solution, and, 

 presumably, the maximum concentration of either in the presence of the 

 other determines the magnitude of the solubility product. When the solu- 

 bility product falls below a certain critical value, lime salt deposition 

 becomes irregular, and if this is sufficient to interfere with the normal se- 

 (juences of organic bone growth and mineralization of these tissues, rickets 

 develops. The magnitude of this solubility product in the human varies 

 with age, decreasing as the rate of bone growth diminishes. It can increase 

 to a higher level following fractures, or it may drop to a lower level with 

 infection. Both endocrine and physicochemical factors determine the satura- 



1 R. Nicolaj-sen, Biochcm. ./. 31, 122 (1937). 



^ F. AU)right, and K. C. Rcifoiistein, The l^arathyroid Glands and Metabolic Bone 

 Disease, Williams & Wilkins Co., lialtimore, 194S, p. 38. 



