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A MANUAL OF VETERINARY PHYSIOLOGY 



The Nitrogenous Substances taken up into the blood, either 

 from the disintegration of proteins in the digestive canal or from 

 the metabolism of the tissues, supply the total nitrogen of the 

 urine. A distinction is made between the nitrogen in urine, which 

 results from the breaking down of the food in the intestinal 

 canal and the nitrogen from within — viz., that furnished to the 

 urine by the metabolism of the body tissues — and this is more 

 especially of interest in connection with urea and uric acid. 



The total nitrogen of the urine consists of — 



i. Urea nitrogen. 



2. Uric acid nitrogen. 



3. Ammonia nitrogen. 



4. Kreatinine nitrogen. 



Urea, or carbamide, is the substance in urine which represents 

 the form in which the greater part of the nitrogen is got rid of : 



1 gramme of protein yields 

 J gramme of urea. It is a 

 substance very soluble in 

 water, crystallises readily, 

 the crystals melting on heat- 

 ing, and giving off ammonia. 

 In the urine it exists in a 

 free condition, but is capable 

 of forming salts with acids. 

 These yield typical crystal- 

 line formations, by which 

 they can be identified mi- 

 croscopically, such as octa- 

 hedra, with nitric acid (see 

 Fig. 97). Under the in- 

 fluence of a ferment, the 

 Micrococcus urece, urine, on 

 becomes ammoniacal, due to the breaking up of 

 into ammonium carbonate. This also sometimes 

 occurs in the bladder under pathological conditions. Urea 

 is found in blood in proportions which vary between 4 and 6 

 per 10,000 ; in the urine it exists in the proportion of 300 parts 

 per 10,000. It is only by the constant flushing of the kidneys 

 with blood that this amount of urea is separated, and the size 

 of the stream may be judged by the fact that a dog weighing 

 35 kilogrammes (77 pounds) has 300 kilogrammes (660 pounds) 

 of blood streaming through the kidneys every twenty-four 

 hours. The peculiar selective power of the kidney cells cannot 

 be better exemplified than in the matter of urea. The cells of 



Fig. 97. — Crystals of Nitrate of Urea. 



standing, 

 the urea 



