474 EXCRETION 



gramme 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 carbon- 

 aceous material. Yet a glance at the table on p. 471 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 1 80 C, urea is entirely split up into carbon dioxide and am- 

 monia, 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 farther, 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 hypobromite method of estimating the 

 quantity of urea in urine (Practical Exercises, p. 512). 



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. 471) (Folin). 



Uric acid (C 5 H 4 N 4 O 3 ) 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 con- 

 trast 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 containing material (e.g., thymus gland) rich in nucleins, 

 from the nucleic acid of which purin bodies are derived, or sub- 

 stances containing purin bases in the free state e.g., hypoxanthin 

 in meat. In mammals the amount of uric acid excreted depends 

 little, if at all, upon the quantity of protein in the food, but a great deal 

 upon the quantity of purin bodies, whether free or combined. When 

 nitrogenous food is omitted altogether, the absolute quantity of uric 

 acid is diminished, but the proportion of the total nitrogen of the urine 

 eliminated ts uric acid is increased, since the ' endogenous ' uric acid 

 (p. 585) still continues to be formed and excreted. 



