242 PKACTICAL PHYSIOLOGY 



in amount till it may equal that of the acid salt when the reaction is 

 amphoteric, or it may even overstep this and cause the urine to react 

 alkaline. This increase of the alkaline salt is established during 

 digestion alkaline tide because of the liberation of chlorine from 

 the chlorides of the blood to produce the hydrochloric acid secreted by 

 the stomach, and, as a consequence of which, bases are liberated and 

 form the alkaline salt of phosphoric acid. 



An alkaline reaction may also be caused by the presence of alkaline 

 carbonates, which cause the urine to effervesce on the addition of a 

 mineral acid. The excretion of these is increased by the administration 

 of the salts of certain organic acids (e.g. citric, tartaric, etc.), the 

 carboxyl radicles of which become oxidised into carbonates in the 

 blood. It is on this account that the urine of herbivorous animals and 

 of vegetarians reacts alkaline. Lastly, an alkaline reaction may be due 

 to ammonia, which is produced by microbal hydration of urea (see 

 Urea, p. 249). On account of this, stale urine always reacts alkaline. 



EXPERIMENT IV. Some urine has been allowed to stand for 

 two days; note that it smells of ammonia, and that the blue stain 

 on litmus disappears if the paper be gently warmed. 



If the newly excreted urine contains a detectable amount of ammonia 1 

 it points to decomposition of urine in the bladder, or to extensive 

 disease of the liver cells. 



Colour. The pale straw colour of healthy urine is due to Urochrome, 

 a pigment which is derived, probably by a process of oxidation, 

 from another pigment called Urobilin, which, however, is present only 

 in traces in healthy urine. In febrile urines, and in the urine of 

 cases in which extravasation of blood into the tissues exists, the 

 urobilin becomes much increased in amount, this suggesting its 

 derivation from haemoglobin, a fact which is borne out by other 

 evidence. Urobilin also exists in the urine as a colourless precursor, 

 called a chromogen, and this may be changed into the pigment by 

 the addition of strong acids. 



A third pigment, called uroerythin, is also present in small amount. 

 It is the colouring matter of pink urate deposits. Its exact chemical 

 relationships are not known. 



Another pigment, which occurs only in the minutest traces in normal 

 urine, but which may be present in considerable amount in pathological 

 urine, is haematoporphyrin. Chemically it is iron-free haematin and can 

 be prepared artificially from haemoglobin (see p. 203). 



Quantitative Composition of Urine. Although it is from the 

 chemical interchange or metabolism in the tissues that most of the 

 I t.c. detectable by the above test. 



