GENERAL CHARACTERISTICS OF THE URINE. 227 



Specific Gravity. The specific gravity of the total amount 

 passed in twenty-four hours normally varies between 1.015 and 

 1.025. Generally speaking, it increases with the solids, the amount 

 of water remaining the same, and diminishes as the amount of fluid 

 increases while the solids remain constant. Under pathological 

 conditions, however, deviations from this rule are not uncommon. 

 The specific gravity may then fall as low as 1.000 and 1.002, or may 

 be increased to 1.050 and even higher. 



Reaction. The reaction of the twenty-four hours' urine is, in 

 man, normally acid, sometimes amphoteric, and more rarely alka- 

 line. The normal acidity is due in part to acid phosphates and in 

 part to free organic acids. An alkaline urine results when the 

 alkalies exceed the acid equivalents in amount. This may occur 

 under normal conditions, and is then due to a preponderance of 

 monacid over diacid phosphates. An amphoteric urine is the out- 

 come when the acid equivalents of disodic phosphate equal the basic 

 equivalents of the monophosphate, and essentially an accidental 

 event. 



The acidity of the urine is primarily due to the character of the 

 diet. A vegetable diet, owing to the large amount of organic acids 

 which are broken down to alkaline carbonates, tends to produce an 

 alkaline reaction, while an animal diet leads to the secretion of an 

 acid urine ; as in the latter case acid phosphates and certain organic 

 acids predominate, which are not transformed to carbonates (hippuric 

 acid, uric acid, oxalic acid, aromatic oxy -acids, etc.). 



In the herbivorous animals in which a superabundance of alkaline 

 salts is either directly ingested or is formed within the body from 

 salts of organic acids which have been taken with the food, an 

 alkaline urine is thus normally eliminated. Similar conditions at 

 times occur in man, and the elimination of an alkaline urine, the 

 alkalinity being due to fixed alkali, cannot hence be regarded as 

 pathological. During the process of digestion, indeed, when an 

 additional amount of alkaline salts finds its way into the blood in 

 consequence of the formation of hydrochloric acid, an increased 

 alkalinity of the blood would result. This, however, is prevented 

 by the excretion of a urine which, if not alkaline, is at least less 

 acid. 



Generally speaking, the ammonium salts which are formed within 

 the body appear in the urine as urea, but aside from their impor- 

 tance in this respect they represent a reserve of alkali which is capa- 

 ble of preventing an undue diminution in the alkalinity of the blood 

 by vicariously taking the place of the fixed alkalies. This vicarious 

 action is normally also at work, but is then comparatively insig- 

 nificant. If, however, a specially large demand is made upon the 

 alkalies of the body, as when mineral acids are ingested for experi- 

 mental purposes, the vicarious action of the ammonium salts at 

 once enters into play. Unless carried to extremes, the alkalinity 



