202 EXCRETION BY THE KIDNEYS AND SKIN 



urinary flow by simply drawing water from the tissues into the 

 blood and thus increasing the amount and pressure, or by 

 stimulating the cells of the glomeruli to increased functional 

 activity is a matter as yet undetermined. 



Properties and Composition of Urine. When an ordinary 

 amount of liquid is ingested and when the skin is moderately 

 active the urine, in normal conditions, has a clear reddish amber 

 color and a specific gravity of about 1020. The more fluid 

 ingested the paler will be the color and the lower the specific 

 gravity; the more active the skin the higher will be the color and 

 specific gravity. The urine is diluted in the first case and con- 

 centrated in the second. The fact is, the amount of solids (rep- 

 resented by urea) to be eliminated in 24 hours remains approx- 

 imately the same, and those solids will cause a high or low 

 specific gravity according as little or much water is eliminated 

 with them. The average amount of urine for a day is 2 or 3 

 pints. Normally it has an acid reaction from the presence, not of 

 a free acid, but of acid salts chiefly acid. sodium phosphate. 

 The odor is not disagreeable on ejection, but decomposition 

 soon begins and a characteristic offensive, ammoniacal odor 

 develops. 



The kidney is the most important excretory organ in the body 

 and the large number of urinary constituents is not surprising. 

 The chief organic constituents are urea, uric acid, hippuric acid, 

 xanthin, hypoxanthin, creatinin, phenol, indican, oxalic acid, 

 lactates, etc. The phosphates, nitrates, sodium chloride, and car- 

 bon dioxide are the chief inorganic materials. 



Urea is the most important of the nitrogenous constituents. 

 It contains a large amount of nitrogen. Nearly all of it is 

 removed from the body by the kidneys, and double nephrectomy 

 means death from its retention. Its formation is constant and 

 its removal necessary. Its presence in the blood seems to be 

 the normal stimulus exciting the activity of the cells of the con- 

 voluted tubes. 



