SECRETION OF THE URINE. 393 



etable diet renders man's urine alkaline, and it is said to become 

 less acid after meals. In the herbivorous mammalia the urine is 

 normally alkaline so long as their digestion is going on, but when 

 they are deprived of food for some time it becomes acid, showing 

 that the alkalinity depends upon their diet. 



The specific gravity of urine varies greatly at different times, 

 commonly, however, ranging between the figures 1015-1020. 

 After copious drinking it may go as low as 1003, and after pro- 

 longed abstinence from liquids, or very active sweating, it may 

 attain 1040. 



The quantity of urine secreted is also very variable, that pro- 

 duced by an adult usually amounting to about 2 pints per diem 

 (1000-1500 c.c.). The amount is increased by- 1 - (1) elevation of 

 the general blood-pressure, or the pressure in the renal vessels 

 from any cause whatever ; (2) contraction of the cutaneous vessels 

 from cold; (3) copious drinking; (4) excess of nitrogenous diet; 

 (5) the presence of soluble matter in the blood, such as sugar, 

 salt, etc. ; and (6) the presence of urea, as well as various medi- 

 caments, has a special action on the renal secretion, greatly in- 

 creasing the amount of urine passed. 



Although the quantity of urine differs so much under different 

 circumstances, the amount of solids excreted by the kidneys in 

 the twenty-four hours remains pretty much the same, being on an 

 average over 1? ounces (50 grammes) for an adult man. 



From this it is obvious that the height of the specific gravity 

 must vary inversely with the amount secreted, so that the more 

 scanty the urine the higher we expect to find the percentage of 

 solids. 



SECRETION OF THE URINE. 



We have just seen that the arterial twig, or afferent vessel, 

 which enters the capsule of Malpighi, breaks up into a set of 

 capillary loops, which are only covered by a single layer of ex- 

 tremely thin epithelial cells separating them from the lumen of 

 the urinary tubule, and that the pressure in the vessels of the 

 glomerulus is habitually higher than that in most capillaries, and 



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