396 MANUAL OF PHYSIOLOGY. 



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) elevation of 

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

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

 sels 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 

 medicaments, has a special action on the renal secretion, greatly 

 increasing 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 H 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 constantly greater than that of the second capillary network 

 around the convoluted tubules. 



The general arrangement of these vessels, and the high pres- 

 sure in the glomerulus, give the impression that it is simply a 

 filtering apparatus by means of which the fluid parts of the blood 

 pass into the urinary tubules. This view seems supported by the 

 fact that the quantity of urine secreted bears a direct proportion 

 to the blood pressure in the minute renal vessels, whether the 

 change in pressure depends on local vascular mechanisms or on 

 changes in the general blood pressure. 



