844 PHYSIOLOGY OF DIGESTION AND SECRETION. 



diuresis, when a similar amount injected directly into the blood 

 may have no effect. These suggestive experimental facts indicate 

 the possibility that the secretion of the kidney may be controlled 

 in a more specific way than has been supposed heretofore. 



The Composition of Urine. The urine of man is a yellowish 

 liquid that varies greatly in depth of color. It has an average 

 specific gravity of 1.020 and usually an acid reaction to litmus 

 paper. This acid reaction is attributed generally to the presence 

 of acid phosphates, particularly acid sodium phosphate (NaH 2 P0 4 ), 

 but, according to Folin, it is due partially to organic acids. As 

 determined by physicochemical methods, normal human urine 

 shows on the average a true acidity equivalent to a concentration 

 of hydrogen ions of 10~ 6 or 0.000001 grams per liter. In place of the 

 actual concentration of the hydrogen ions it is customary to use the 

 logarithm of the figure expressing the concentration. These logar- 

 ithms are all negative, but are written without the minus sign as 

 the hydrogen exponent. Thus: 



HYDROGEN ION CONCENTRATION. HYDROGEN EXPONENT. 



1 X 10- 1 or 0.1 1 



2 X 10- 3 or 0.002 2.7 (+ 0.3 3) 



5 X 10-' or 0.000005 5.3 (+ 0.7 6) 



In this nomenclature the acidity of the urine varies between limits, 



+ 

 expressed by the figures 4.82 and 7.45 (H concentrations, 0.000016 



and 0.00000008).* In comparison with these figures it may be 

 recalled that a neutral reaction, such as is shown by water, is 



expressed by the exponent 7, while blood inclines to the alkaline 



+ 



side, exponent of 7.4 (H concentration, 0.4 X 10" 7 ). One of the 

 conditions influencing the reaction of the urine is the character 

 of the food. On an animal diet the acidity is increased; on a 

 vegetable diet it is diminished and may reach the neutral point 

 or the degree of alkalinity exhibited by the blood. In the latter 

 case the urine may give a blue reaction to litmus paper, owing to 

 the alkaline salts present, but, as has been explained (p. 416), this 

 reaction does not demonstrate a true alkalinity, that is, an excess of 

 OH ions over the +H ions. Distinct alkalinity in this sense 

 may be shown when the urine has undergone bacterial fermenta- 

 tion with the production of ammonia. The general explanation of 

 the effect of food that has been suggested (Drechsel) is that upon 

 an animal diet more acids are formed (from the oxidation of the 

 sulphur and phosphorus of the proteins) than in the case of the 

 vegetable foods in which the alkaline salts of the vegetable acids 

 give rise on oxidation in the body to alkaline carbonates. The 



* Henderson and Palmer, "Journal of Biological Chemistry," 13, 394, 1912, 



