108 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



the pigment proper. In certain diseased conditions the amount of 

 urobilin is considerably increased. 



The most abundant urinary pigment is a yellow one called 

 urochrovie. It shows no absorption bands. It is probably an oxi- 

 dation product of urobilin (Eiva, A. E. Garrod). (See Lesson XXVI.) 



Reaction. — The reaction of normal urine is acid. This is not due 

 to free acid, as the uric and hippuric acids in the urine are combined 

 as urates and hippurates respectively. The acidity is due to acid salts, 

 especially acid sodium phosphate. Under certain circumstances the 

 urine becomes less acid and even alkahne ; the most important of 

 these are as follows : — 



1. During digestion. Here there is a formation of free acid in 

 the stomach, and a corresponding liberation of bases in the blood 



Pig. 



-Urinometer floating in urine 

 in a testing glass. 



Fia. 40. — Crystals of urea : a, four-sided 

 prisms ; 6, indefinite crystals, such 

 as are usually formed from alcohol 

 solutions. 



which passing into the urine diminish its acidity, or even render it 

 alkaline. This is called the alkaline tide ; the opposite condition, tJie 

 acid tide, occurs after a fast, for instance, before breakfast. 



2. In herbivorous animals and vegetarians. The food here con- 

 tains excess of alkaline salts of acids like tartaric, citric, malic, &c. 

 These acids are oxidised into carbonates, which passing into the urine 

 give it an alkaline reaction. \ 



Specific Gravity. — This should be taken in a sample of the twenty- 

 four hours' urine with a good urinometer (see fig. 39). 



The specific gravity varies inversely as the quantity of urine 

 passed under normal conditions from 1015 to 1025. A specific 

 gravity below 1010 should excite suspicion of hydruria; one over 

 1030 of a febrile condition, or diabetes, a disease in which it may rise 



