EXCRETION 



The titrable acidity of urine (see p. 25) is chiefly due to the acid (mono- 

 basic) phosphates, such as acid sodium phosphate (NaH 2 PO 4 ), but in 

 an important degree also to organic acids. According to Folin, indeed, 

 the organic acidity may be more than half the total acidity. Normally 

 the acidity diminishes distinctly, or even gives place to alkalinity, 

 during digestion, when the acid of the gastric juice is being secreted. 

 This is sometimes fancifully denominated the alkaline tide. After a 

 fast, as before breakfast, the opposite condition, the acid tide, occurs. 



The acidity varies with the quantity of vegetable food in the diet. 

 The urine of herbivora and vegetarians is alkaline, and is either turbid 

 when passed, or on standing soon becomes turbid from precipitated 

 carbonates and phosphates of earthy bases, while that of carnivora 

 and of fasting herbivora, which are living on their own tissues, is 

 strongly acid and clear. Normal human urine may deposit urates soon 

 after discharge, as they are more soluble in warm than in cold water. 

 They carry down some of the pigment, and therefore usually appear as 

 a pink or brick-red sediment. When urine is allowed to stand after 

 being voided, what is generally described as ' acid fermentation ' occurs. 

 The acidity gradually increases; acid sodium urate is produced from 

 the neutral urate, and comes down in the form of amorphous granules, 

 while the liberated uric acid is deposited often in ' whetstone ' crystals, 

 coloured yellow by the pigment (Fig. 177). Calcium oxalate may also 



Fig. 177. Uric Acid 



Fig. 178. Calcium Oxalate 



be thrown down as ' envelope/ a, b, or less frequently, ' sand-glass ' 

 crystals, c (Fig. 178). If the urine is allowed to stand for a few days, 

 especially in a warm place, or in a place where urine is decomposing, 

 the reaction becomes ultimately strongly alkaline, owing to the forma- 

 tion of ammonium carbonate from urea by the action of micro-organ- 

 isms (Micrococcus urece, Bacterium urees, and others) which reach it 

 from the air, and produce a soluble ferment urease, in whose presence 

 the urea is split up with assumption of water. Thus : 



/NH 2 /O.NH, 



C=0 +2H 2 - C=0 



\NH 2 \O.NH 4 , 



Urea. 



Ammonium carbonate. 



This is a reaction of considerable interest, for the reverse reaction 

 occurs when blood containing ammonium carbonate is circulated 

 through the liver, the ammonium carbonate being converted into urea 

 with loss of water. The enzyme urease is present in higher plants than 

 bacteria and fungi. A solution containing it, conveniently prepared 

 from the soy bean, can be used for the quantitative estimation of urea, 

 and this is probably the easiest and most accurate method. 



tion as a standard. Fehling's solution is employed because it is a blue liquid 

 of a definite depth of tint already prepared in every physiological laboratory. 



