440 A MANUAL OF PHYSIOLOGY. 



fore eighty times as great in the hydrochloric as in the acetic acid 

 solution. What we determine by the titration is not the true 

 acidity, but the total amount of hydrogen which can be replaced 

 by metal. The concentration of the hydrogen ions in normal urine 

 is very small, on the average only about 0-003 milligrammes in the 

 litre, or about thirty times as much as is present in the purest distilled 

 water. Urine departs about as much from neutrality in the one 

 direction as blood does in the other. 



Urea, CO(NH 2 ) 2 , is the form in which by far the greater part of 

 the nitrogen is under ordinary conditions discharged from the body. 

 Its amount is as important a measure of protein metabolism as the 

 quantity of carbon dioxide given out by the lungs is of the oxidation 

 of carbonaceous material. Yet a glance at the table on p. 437 

 shows that, when the total protein metabolism is greatly reduced 

 by diminishing the protein in the food, the relative as well as the 

 absolute amount of nitrogen eliminated as urea suffers a great 

 diminution. The relative amount of the other nitrogenous urinary 

 constituents, especially, of the kreatinin, is markedly increased. 

 The significance of this difference is alluded to in speaking of the 

 kreatinin content of urine, and will have to be again considered 

 under Protein Metabolism. Urea is soluble in water and in alcohol, 

 and crystallizes from its solutions in the form of long colourless 

 needles, or four-sided prisms with pyramidal ends. It can be 

 easily prepared from urine. Urea can also be obtained artificially 

 by heating its isomer, ammonium cyanate (NH 4 O CN), to 

 100 C. This reaction is of great historical interest, as it forms 

 the final step in . Wohler's famous synthesis of urea, the first 

 example of a complex product of the activity of living matter 

 being formed from the ordinary materials of the laboratory. 

 Heated in watery solution in a sealed tube to 180 C., urea 

 is entirely split up into carbon dioxide and ammonia, a change 

 which can also be brought about, as already mentioned, by 

 the action of micro-organisms. Nitrous acid, hypochlorous acid, 

 and salts of hypobromous acid carry the decomposition still 

 further, carbon dioxide, nitrogen, and water being the products of 

 their oxidizing action on urea. Thus: CO.2(NH 2 ) + 3NaBrO = 

 3NaBr+2H 2 O + CO 2 +N 2 . This reaction is the basis of the hypo- 

 bromite method of estimating the quantitv of urea in urine (Practical 

 Exercises, p. 480). 



Ammonia. The ammonia in urine is united with acids in the form 

 of salts. Its formation from proteins is determined, as we shall see 

 later on, by the necessity of neutralizing certain acids produced in 

 metabolism e.g., those derived from the sulphur and phosphorus 

 of the proteins, or acids administered experimentally. According 

 to some observers, the percentage amount of the total nitrogen in 

 the urine in the form of ammonia remains the same whether the food 

 be rich or poor in protein (Schittenhelm, etc.), but others state that 

 when the protein is reduced, there is a relative increase in the 

 ammonia-nitrogen (see table on p. 437) (Folin). 



Uric acid (C 6 H 4 N 4 O 8 ) exists in large amount in the urine of birds. 

 The excrement of serpents consists almost entirely of uric acid. In 

 both cases it is mainly in the form of acid ammonium urate. In 

 contrast to urea, uric acid is very insoluble, requiring 1,900 parts of 

 hot, and 15,000 parts of cold, water to dissolve it. In man and 

 mammals the quantity is comparatively small in health, but is 

 increased after a meal, particularly one containing substances rich 



