PROTEIN METABOLISM 805 



by the kidney. A certain small proportion of the nitrogen in the 

 urine is generally turned out in the form of ammonia. This propor- 

 tion is not increased by the administration of ammonium carbonate. 

 If ammonium chloride be given to a starving rabbit it appears in the 

 urine unchanged, and so increases the proportion of ammonia in this 

 fluid. If, however, the ammonium chloride be administered at the 

 same time as the animal is receiving its ordinary vegetable diet there 

 is no increase in the ammonia in the urine, the whole of the ammonium 

 chloride being converted into urea. The factor which determines the 

 proportion of ammonia in the urine is the relative proportion of acids 

 and bases which have to be eliminated from the body. The normal 

 reaction of urine, though acid as regards certain indicators, can be 

 regarded as neutral, since it contains no free hydrogen ions, the 

 ' acidity ' being due to the presence of such substances in solution as 

 acid sodium phosphate. If the fixed alkalies in the food are sufficient 

 to combine with the whole of the acids excreted from the body, then 

 the ammonia will be completely converted into urea and eliminated as 

 such. If, however, a dose of mineral acid be administered to an animal, 

 this must be excreted in combination with a base. If the fixed alkalies 

 available do not suffice for this purpose, the neutralisation of the acid is 

 effected by coupling with ammonia. The ammonia of the urine is there- 

 fore an index to the amount of acids which are excreted. These acids 

 may be introduced directly with the food, as when mineral acids are 

 administered by the mouth, or may be the product of abnormal 

 metabolic processes occurring in the body. Thus under certain 

 circumstances, e.g. in complete carbohydrate starvation, there is a 

 failure in the last stages of the oxidation of fats, and oxy-fatty acids, 

 viz. oxybutyric acid and aceto-acetic acid, are produced in the body 

 in large quantities, but cannot undergo further disintegration. The 

 alkalescence (electrical neutrality) of the fluid media of the body is a 

 necessary condition for the continuance of the life of the cells and 

 especially of the normal processes of oxidation. It is therefore 

 essential for the preservation of life that the acids thus formed and 

 accumulating as a result of the impaired oxidative processes should 

 be neutralised, carried to the kidneys, and excreted by them in 

 combination with some base. When these acids are produced in 

 large quantities the alkalies of the food and of the tissues do not 

 sufiice for their neutralisation. Ammonia, which is a constant inter- 

 mediate stage in the production of urea, is then utilised for this pur- 

 pose and the acids appear in the urine together with the corresponding 

 amount of ammonia. The ammonia therefore of the urine gives 

 valuable information, not as to the total nitrogenous exchanges of 

 the body, but as to the formation of acids in abnormal quantities 



during the processes of metabolism. 



55 



