252 



PRACTICAL PHYSIOLOGY 



of liquid and by certain drugs called diuretics ; it is diminished by 

 excessive sweating or diarrhoea, and by failure of the heart's action. 



Specific Gravity. This is determined by a special form of hydro- 

 metera urinometer graduated so that the zero mark 1,000 - 

 corresponds to distilled water (Fig. 182). 



EXPERIMENT I. Fill a urine testing glass with urine cooled to 

 room temperature, place the urinometer in it, 

 and read off the graduation which is on a level 

 with the surface of the urine. Be careful that 

 the urinometer does not stick to the sides of the 

 vessel. 



The average density varies between 1,015 and 

 1,025, but a highly concentrated urine, e.g. after 

 severe sweating, may reach 1,035, or a very dilute 

 one, e.g. after huge potations, 1,002, and still be 

 healthy. A specific gravity over 1,030, however, 

 usually indicates the presence of sugar or the 

 existence of high fever, and one much below 1,010 

 should raise suspicions of renal trouble. 



1 

 1 



1 



1L 

 JL 

 1 



Reaction. Healthy urine usually reacts acid to 

 litmus. This acidity is due to sodium dihydrogen 

 phosphate, NaH 2 PO 4 , not to free acid. 



EXPERIMENT II. Test the reaction of urine 

 with blue litmus paper and congo red paper. 

 The litmus is turned red, but the congo red is 

 not altered, as it is not affected by the acid salts 

 of any but the strongest acids (see Digestion, p. 

 228). 



The alkaline phosphate, Na 2 HP0 4 , may be pre- 

 sent in urine. It is detected by the addition of 

 calcium chloride to the urine, when a precipitate 

 of calcium phosphate forms if the alkaline phos- 

 phate is present, but not if the acid phosphate 

 alone is present. The amount of alkaline phos- 

 phate may be sufficient to cause the urine to have 

 an amphoteric reaction, turning red litmus blue 

 and blue litmus red, or even to have a definite 

 alkaline reaction. This is often the case during 

 the stage of digestion, when hydrochloric acid is 

 being poured into the stomach, as the removal 

 of hydrochloric acid from the blood leaves an 

 excess of bases. 



Besides the alkaline phosphate, alkaline bicarbonates may be 

 present in urine, causing an effervescence on the addition of acid. 

 This is the case when salts of oxidisable acids (e.g. citric, tartaric, 

 etc.) are being taken by the mouth, and when the diet is an exclu- 

 sively vegetable one. Lastly, an alkaline reaction may be due to 

 ammonia, which is produced by bacterial hydrolysis of urea (see 



FIG. 182. The 

 urinometer. 



