4 2 HUMAN PHYSIOLOGY 



3. END-PRODUCTS OF METABOLISM 



To this class belong the substances which are formed by 

 the combustion of the energy-yielding substances of the 

 body (proteids, fats, carbohydrates). These products are 

 excreted from the body. 



It is possible and even probable that the end-products of meta- 

 bolism are not formed directly by combustion from the body- 

 substance, but that a series of intermediate substances are formed. 

 But nothing is known concerning these intermediate products; 

 everything that has hitherto been said about them is based on 

 mere assumption. We must therefore be satisfied with enumerat- 

 ing the end-products. 



Among the end-products we may, in the first place, name 

 the already mentioned water and carbon dioxide, the chief 

 combustion products of all organic substances. 



In abnormal conditions, organic substances of carbon, hydrogen 

 and oxygen which can still undergo oxidation are excreted from 

 the body. These are, evidently, products of incomplete combus- 

 tion. Such substances are : 



1. Lactic acid, C 3 H 6 O 3 . There are three lactic acids. 



(a) Ethyline lactic acid, CH 2 OH.CH 2 .COOH, is present 

 in the body in but very small quantities. 



(b} Ethyliden lactic acid, CH 3 .CHOH.COOH. Of these 

 there are two : 



(1) The optically inactive fermentation lactic acid, 

 formed by the lactic-acid fermentation of carbo- 

 hydrates. 



(2) Dextrorotatory sarcoiactic acid, found in the 

 muscles and in urine. 



Sarcoiactic acid is a syrupy fluid, soluble in water, alcohol and 

 ether. It forms a characteristic crystallizable salt with zinc ; this 

 is of use in isolating the sarcoiactic acid. 



Lactic acid is present in the urine if the supply of oxygen is 

 deficient (dyspnoea). 



2. /?-oxybutyric acid, CH 3 .CHOH.CH 2 .COOH, diacetic acid, 

 CH 3 .CO.CH 2 .COOH, and acetone, CH 3 .CO.CH 3 , appear in the 

 urine in certain diseases, especially diabetes. Acetone is present 

 in normal urine in very small quantities. 



3. It may here be mentioned that oxalic acid, COOH.COOH, 



