382 ESSENTIALS OF PHYSIOLOGY. 



arches lying in the boundary zone between the cortex and the medulla. 

 From these arches vessels pass outwards in the cortex, and give off 

 short afferent branches, each of which ends in a glomerulus. The 

 blood leaves the capillaries of the glomerulus by an efferent vessel, 

 which is smaller than the afferent, and after a short course breaks up 

 into capillaries round the convoluted tubules ; from these capillaries 

 the blood passes into veins running to the boundary zone, and opening 

 into venous arches parallel with the arterial arches. The veins ulti- 

 mately unite to form the renal vein. Straight branches also arise from 

 the arterial arches to end in capillaries round the tubules in the medulla ; 

 from these the blood passes back to the venous arches. 



THE COMPOSITION OF THE URINE. 



Normal human urine is a clear, yellow fluid, acid in reaction, and 

 containing about 4 per cent, of total solids ; it is free from cells and 

 from protein, except for a small trace of nucleo-protein derived from 

 the bladder and urinary passages. Its specific gravity varies from 

 1015 to 1025, and its daily amount is about 1500 c.c. Its average 

 composition is shown in the following table : 



Total quantity of urine . . . 1500 c.c. 



,, solids . . . . .60 grams. 



urea. . . 33 



,, uric acid ..... 0'75 ,, 



hippuric acid . . . .0-5 



,, sodium chloride . . . 15'0 ,, 



phosphoric acid . . . 2'5 ,, 



,, sulphuric acid .... 2'5 ,, 



,, ammonia . . . . . 0'75 ,, 



creatinine . . . I'O 



,, potassium . . . .^ 



,, calcium . . . . .1 4'0 ,, 



,, magnesium . . . J 



Since the nitrogenous end-products of the metabolism of protein 

 are excreted almost entirely in the urine, its composition largely de- 

 pends upon the quantity of protein food consumed, and on the kata- 

 bolic changes in the tissue proteins. The characters of urine vary, 

 therefore, not only in different individuals, but even in the same 

 individual from day to day, and almost from hour to hour. 



Amount and Specific Gravity. The fluid taken by the mouth leaves 

 the body partly in the urine, and partly through the skin and lungs. 



