CHAPTER XL 

 METABOLISM. 



SECTION I. 



THE food-stuffs, after being digested and absorbed into the blood or 

 lymph, are carried to the tissues, in which they pass through a series 

 of complex chemical transformations, the end products of which leave 

 the tissues and are removed from the body by the lungs and kidneys. 

 This series of chemical changes constitutes metabolism, and the 

 metabolic activities of the tissues are of two kinds. On the one hand, 

 the living tissues are constantly undergoing changes whereby a portion 

 of their substance is broken down and removed from the body ; on 

 the other hand, this loss is replaced by the building up of fresh tissue 

 from the nutritive materials supplied in the blood. The former of 

 these processes is called katabolism, and the latter anabolism. 



These changes involve the consumption of a large amount of 

 oxygen, and the evolution of energy in the form of heat and muscular 

 work. The food-stuffs are protein, fat, and carbohydrate, 90 to 94 per 

 cent, of those consumed on an ordinary diet being absorbed into the 

 blood stream, and the remainder being lost in the faeces. The fats 

 and carbohydrates are completely converted into carbonic acid and 

 water, and the proteins partly into carbonic acid and water, the 

 nitrogen being excreted as urea and other incompletely oxidised sub- 

 stances in the urine. The carbonic acid is removed from the body 

 almost entirely through the lungs, and the water by the lungs, 

 kidneys, and skin. The changes undergone by the food-stuffs in. the 

 body may, therefore, be studied in three ways, namely (1) by measur- 

 ing the total amount of heat evolved in their oxidation, (2) by deter- 

 mining the quantity of oxygen required for the carrying out of these 

 oxidations, and (3) by measuring the amount of the end products which 

 are formed. We may also attempt to follow out the series of changes 

 taking place in the individual food-stuffs in the tissues themselves. 



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