454 % A TEXTBOOK OF PHYSIOLOGY 



it may vary greatly. A urine may have a specific gravity below 

 1010 after much drinking, or as high as 1035 after much sweating. 

 An abundant urine of low specific gravity is suggestive of some patho- 

 logical condition, such as diabetes insipidus or chronic renal disease, 

 while the passage of large quantities of pale urine of high specific 

 gravity suggests the presence of sugar in the urine. If, on the other 

 hand, the colour be high, some condition causing loss of water from 

 the body, such as diarrhoea or fever, may be present. The quantity 

 of the solids in the urine per litre may be roughly estimated by 

 taking the specific gravity of the urine at 15 C., and multiplying 

 it by 2-33. 



The osmotic pressure, measured by depression of freezing-point, 

 varies from -0-8 to -2-7C. 



The reaction of the urine, as tested by chemical indicators, is 

 generally acid. To the physical test the urine, like the blood, is 

 neutral (see p. 76). The acidity to Ltinus is due mainly to the acid 

 phosphate of sodium (NaHgPOJ. When, however, urine is passed 

 during the digestion of a meal, it is often amphoteric or alkaline in 

 reaction. This is because the acid phosphate in the blood is con- 

 verted into disodium phosphate (Na,HPO 4 ), owing to the formation of 

 the HC1 of the gastric juice. The reaction of urine may also be alka- 

 line after eating fruits and vegetables containing organic acids (citric, 

 malic, etc.). These are converted in the body into alkaline carbonates, 

 which are excreted in the urine. It is believed by some authorities 

 that the acidity of the urine is in part due to the presence of volatile 

 organic acids in the urine. It is stated that, if urine be distilled, the 

 part which distils off is acid, owing to the presence of such volatile 

 acids. 



For clinical purposes, the total acidity, and indirectly the am- 

 monia, of the urine may be determined by a method in which the 

 acidity is first estimated by titrating with ^ NaHO. Neutralized 

 formalin is then added. The ammonia of the urine combines with 

 this to form the neutral compound urotropine, setting free the 

 acids to which the ammonia is combined. A second estimation 

 with ~Q NaHO gives this acidity, and from it the amount of nitrogen 

 present as ammonia may be calculated by multiplying by 0-0014. 



The Transparency and Colour of the Urine. Freshly voided normal 

 urine is transparent, and possesses a yellowish colour, the exact tint 

 of which fluctuates widely even in health according to the degree of 

 dilution and the reaction of the urine. This colour is due chiefly to 

 the pigment urochrome. Other pigments urobilin, uroerythrin, 

 urorosein also occur in normal urine under various conditions. 



Urochrome. The origin of this pigment is not known. It is 

 probably derived from protein, as it contains nitrogen and sulphur 

 in its molecule. It yields no absorption bands when examined spectro- 

 scopically. 



Urobilin is not present in freshly voided urine, but its chroniogen 

 is urobilinogen. The darkening of urine on standing is due to the 

 conversion of urobilinogen into urobilin. Urobilin, when present, 



