2O2 EXCRETION BY THE KIDNEYS AND SKIN 



able as the effect of blood-pressure, is that the amount of 

 urine varies directly as the amount of blood passing through 

 the kidney, independently of the pressure; and these two 

 facts constitute about all that is definitely known concerning 

 the local conditions affecting the amotmt of urine. Whether 

 diuretics increase the 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 ordi- 

 nary 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 concentrated in the second. The fact is, 

 the amount of solids (represented by urea) to be eliminated 

 in 24 hours remains approximately 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 dis- 

 agreeable 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 sur- 

 prising. The chief organic constituents are urea, uric acid, 

 hippuric acid, xanthin, hypoxanthin, creatinin, phenol, indi- 

 can, oxalic acid, lactates, etc. The phosphates, nitrates, 

 sodium chloride, and carbon dioxide are the chief inorganic 

 materials. 



Urea is the most important of the nitrogenous constitu- 

 ents. It contains a large amount of nitrogen. Nearly all 



