164 



PHYSIOLOGY FOR BEGINNERS 



of sodium and, in smaller quantity, of potassium. Some salts 

 of calcium and magnesium are also present. Urine is acid in 

 reaction, xlue to the presence of the acid phosphate of sodium. 

 From two to three pints of urine, weighing about fifty ounces, 

 are excreted in twenty-four hours. This contains rather more 

 than one ounce of urea, while the salts and other substances 

 make up about another ounce of solids. 



Urea is a compound having the composition CON.,H 4 ; 60 

 parts by weight of urea contain 28 parts of nitrogen, so that 

 nearly half the weight of urea is nitrogen. In a man taking 

 just as much food and not more food than he ought, the 



FIG. 76. Tubules of the kidney. 

 A, cut lengthwise. B, cut across. 



a, Tubule where secretion goes on ; b, conducting part of tubule ; , nuclei ; c, in B, 

 capillaries cut across. 



amount of nitrogen leaving the body is about equal to the 

 amount taken in, though it may be a little more or a little 

 less according to circumstances. We have taken the amount 

 of nitrogen leaving the body daily to be in round numbers 300 

 grains, and one ounce and a quarter of urea, about the amount 

 excreted in a day, will contain very nearly the whole of this. 

 The small remainder of the nitrogen is excreted by the urine 

 in other substances, of which one of the chief is uric acid. 

 Uric acid occurs only in small amount in the urine of man 

 and of animals which suckle their young, though it is the 

 chief nitrogenous waste product in reptiles and birds ; in these 

 animals urea is absent and uric acid takes its place. 



