EXCRETION OF WASTE PRODUCTS 181 



regulating mechanism for the composition of the blood. 

 Increase in acidity of the blood will cause a greater increase 

 in the acidity of the urine and the converse. We must refer 

 once more to the difference between the true acidity or hydro- 

 gen ion concentration and the titratible acidity. The body 

 possesses regulating mechanisms, hence we find that weak 

 acids or bases are often formed to neutralise excess of alkali 

 or acid respectively. 



Sometimes the urine has the composition of a stabilising 

 solution containing phosphates. Thus if a drop of such a 

 urine is placed on red litmus paper it neutralises the acid, so 

 that a purple spot is produced which appears blue against the 

 red of the paper. If a drop of the same urine is placed on 

 blue litmus paper the alkali is neutralised and the purple colour 

 of the neutral litmus appears red in contrast to the blue 

 background. Such urine is called amphoteric, as it apparently 

 turns blue litmus red and red litmus blue. 



THE NITROGENOUS CONSTITUENTS OF URINE 



The total nitrogen of urine is important as it is used as an 

 indication of the total amount of protein destroyed. Protein 

 contains on the average sixteen per cent, of nitrogen, hence 

 the total nitrogen multiplied by 6-25 gives the weight of protein 

 destroyed. It is customary to subdivide the total nitrogen 

 into various fractions and to estimate the percentage of the 

 various nitrogenous constituents to the total nitrogen. 



TABLE XXXV 



Showing the distribution of Nitrogen and Sulphur on different 



Diets* 



Urea forms the major portion of the nitrogenous excretion. 

 It is mainly an indication of the amount of protein in the 

 diet. The protein has been traced, during digestion, into the 

 * O. Folin, Amer. Journ. Physiol., 1905, vol. 13, p. 117. 



