CHAPTER XVI 



METABOLISM — DUCTLESS GLANDS 



The nutritive processes in the human body include: (1) the re- 

 ception and digestion of food, followed by absorption of the differ- 

 ent food products, and the distribution of these products to all the 

 cells by the circulating liquids of the body; (2) the absorption 

 of oxygen by the circulating liquids in the lungs, its distribution 

 to all the cells of the body, and its union with the constituents of 

 the cells. We have studied digestion and absorption, and our 

 special problem in this chapter is metabolism. 



Metabolism. — This term includes all the changes that occur in 

 digested food-stuffs from the time of their absorption until their 

 elimination in the excretions. 



Metabolic changes. — The most important chemical changes 

 that occur in metabolism are as follows : — 



(1) The cells take from the blood the substances which they 

 require for repair and growth, and build it up into protoplasm. 

 This involves the conversion of non-living material into the 

 living protoplasm of the cells. 



(2) Oxidation, or the union of oxj^gen with the constituents of 

 the cells, resulting in the release of energy and the breaking down 

 of complex substances into simpler products. 



(3) Some of the simpler products that result from oxidation are 

 acids. These acids must be eliminated or decomposed, as all the 

 digestive fluids of the body, with the exception of gastric juice, are 

 alkaline or neutral, and this condition is essential to nutrition and 

 even to the immediate continuance of life, (a) Some of the w^eaker 

 acids are eliminated in the urine; one, carbonic acid (H2CO3), 

 is a very unstable compound and breaks down to form water 

 (H2O) and carbon dioxide (CO2). (b) When proteins are oxidized, 

 sulphur is set free. This sulphur (S) unites with water (H2O) and 

 ox^^gen (O2) to form sulphuric acid (H2SO4). Sulphuric acid is 

 promptly neutralized by the alkalies present in the tissues, so that 

 the body never contains sulphuric acid, but does contain sul- 



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