106 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



figure. Open the pinch-cock and lower the measuring tube until the surface 

 of the water with which the outer cylinder is filled is at the zero point of the 

 graduation. Close the pinch-cock, and raise the measuring tube to ascertain 



if the apparatus is air-tight. Then 

 lower it again. Tilt the bottle so as 

 to upset the urine, and shake well for 

 a minute or so. During this time 

 there is an evolution of gas. Then 

 immerse the bottle in a large beaker 

 containing water of the same tempera- 

 ture as that in the cylinder. After 

 two or three minutes raise the measur- 

 ing tube until the surfaces of the 

 water inside and outside it are at the 

 same level. Read off the amount of 

 gas evolved. This is nitrogen. The 

 carbonic acid resulting from the de- 

 composition of urea has been absorbed 

 by the excess of soda in the bottle. 

 35'4 c.c. of nitrogen are yielded by 

 0"1 gramme of urea. From this the 

 quantity of urea in the 5 c.c. of urine 

 and the percentage of urea can be 

 calculated. If the total urea passed 

 in the twenty-four hours is to be 

 ascertained, the twenty-four hours' 

 urine must be carefully measured and 

 thoroughly mixed. A sample is then 

 taken from the total for analysis ; and 

 then, by a simple sum in proportion, 

 the total amount of urea is ascertained. 

 Sometimes the measuring tubes sup- 

 plied with this apparatus are graduated 

 in divisions corresponding to percent- 

 ages of urea. 



7. Creatinine. — This substance may 

 be detected by adding a little sodium 

 nitro-prusside and caustic soda to the 

 urine. A red colour develops which 



Fig. 38.— Dupr6's urea apparatus. fades on boiling. 



The kidney is a compound tubular gland, the tubules of which it 

 is composed differing much in the character of the epithelium that 

 lines them in various parts of their course. The true secreting part 

 of the kidney is the glandular epithelium that lines the convoluted 

 portions of the tubules ; there is in addition to this what is usually 

 termed the filtering apparatus : tufts of capillary blood vessels called 

 the Malpighian glomeruli are supplied with afferent vessels from the 

 renal artery ; the efferent vessels that leave these have a smaller 

 calibre, and thus there is high pressure in the Malpighian capillaries. 

 Certain constituents of the blood, especially water and salts, pass 



