UTILISATION OF NUTRIENTS 43 



the lungs, and is there given out to the air, only a 

 small proportion being excreted by the skin and 

 intestine. The proteins and amides also give 

 carbon dioxide and water on decomposition, as 

 well as nitrogenous waste products, of which urea is 

 the chief. The latter, together with soluble salts 

 and some of the water, are brought in the blood to 

 the kidneys, and from there pass in the form of 

 urine to the bladder. There are other ways, too, in 

 which water, whether from the food, drink, or the 

 combustion of organic substances in the cells, leaves 

 the body. Some is exhaled as gas from the lungs 

 and skin, whilst the rest is got rid of in fluid form in 

 the urine or faeces. 



The materials taken up by the blood from the 

 alimentary canal are not always split up and 

 oxidised, but when the food supply is liberal 

 they are changed into body tissue or milk. 

 Hereby there arise as a rule many waste pro- 

 ducts, which are capable of further oxidation with 

 the formation of the final products already men- 

 tioned. 



All these changes, which take place from the 

 entrance of the digested compounds of the food 

 into the blood up to the excretion of the final pro- 

 ducts, are grouped under the term " metabolism." 

 Every constituent of the food which can yield heat 

 or energy, or can serve for the production of body 

 tissue, is called a nutrient. 



