212 THE URINE. 



various diseases of the circulatory apparatus, in certain diseases 

 of the kidneys and liver, etc. Complete anuria may indeed occur. 



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, however, not due to the presence of a 

 free acid, but to acid salts, and in the first instance to acid phos- 

 phates. As the reaction of the blood is alkaline, the question 

 naturally arises : How is it that an acid secretion can be derived 

 from an alkaline fluid? In the case of the gastric juice we have 

 met with a very similar phenomenon, and it was explained that the 

 free acid in that case most likely resulted through a mass-action, on 

 the part of carbonic acid, upon sodium chloride within the oxyntic 

 cells, the hydrochloric acid being then secreted into the lumen of 

 the glandular ducts, while the resulting alkaline carbonate is 

 returned to the blood. Similar conditions probably exist in the 

 kidneys, where, as has been mentioned, the mineral salts are also 

 secreted into the uriniferous tubules through the specific activity of 

 the renal epithelial cells. We may imagine that here also a mass- 

 action on the part of carbonic acid takes place, which in this case, 

 however, is directed toward the alkaline phosphates of the blood, as 

 is shown in the equation : 



Na 2 HPO 4 + H 2 C0 3 = NaH 2 PO 4 + NaHCO s . 



We may then imagine that the resulting alkaline carbonate is 

 returned to the blood, while the acid phosphate appears in the 

 urine. 



The acidity of the urine, however, is primarily due to the 

 character of the diet. In man and the carnivorous animals this 

 is especially rich in albumins, and contains a comparatively small 

 amount of alkaline salts or of organic acids which could be trans- 

 formed into alkaline carbonates in the body. During the process 

 of metabolism, then, the ingested albumins are broken down, and 

 uric acid, hippuric acid, phenaceturic acid, oxalic acid, aromatic oxy- 

 acids, and notably sulphuric acid and phosphoric acid result. These 

 acids, however, are immediately transformed into neutral salts by 

 combining with the available alkaline carbonates which are present 

 in the lymph and the blood. As a consequence the alkalinity of 

 the blood must of necessity diminish. But as such a change would 

 give rise to serious disturbances, and as there is a strong tendency 

 on the part of the body to maintain the composition of the blood, 



