DISPOSAL OF WASTES 



137 



the convex outer surface of the organ (Fig. 113). From there the course 

 of the tuliules is in general two convohited stretches, with a more or less 

 straight-limbed loop between them. The collecting tubules into which 

 they empty converge toward the branches of the ureter, in pyramid- 

 shaped groups. The ureters empty into the bladder, and this discharges 

 through the urethra. 



Excretion by the Kidney. — The elimination of w^aste by the kidney 

 involves two general processes: (1) filtration of a great deal of liquid under 

 pressure from the blood in the glomerulus into the tubule at the renal 

 corpuscle, and (2) resorption of the greater part of this liquid by the 

 uriniferous tubules in the rest of their course. The liquid forced out 

 of the glomerulus, through the inner wall of Bowman's capsule into the 



capsule 



'A^\ illl'i, -^^cc^^^y^ coniaining 



-URETHRA 



Fig. 112. — Excretory system in man. 



renal corpuscles 



pyramid of medulla 

 with collecting 

 tubules 



renal artery 

 renal vein 

 pelvis of kidney 



ureler 



Fig. 113. — Human kidney, bisected. 

 {From Storer, "Ge7ieral Zoology.") 



tubule, consists of water, urea, glucose, amino acids, and the salts of 

 the blood plasma, in about the same proportion as these things exist in the 

 blood. The proteins of the blood, however, are not allowed to pass ; nor 

 are the other colloidal substances, such as the lipids, nor the blood cells. 

 These are all retained in the blood vessels. The amount of fluid thus 

 filtering into the tubule is about 1 per cent of the liquid of the blood 

 passing through the glomerulus. 



Then the resorption of much of this material occurs as the liquid 

 passes along the tubule. The glucose in it is taken back into the blood 

 capillaries, unless there is already too much glucose in the blood. The 

 salts are also partially resorbed, not necessarily in equal fractions, but in 

 proportion to the need of them in the blood. Amino acids return in like 

 manner to the blood; so also does about 99 per cent of the water. What 

 remains in the tubule is therefore a rather concentrated solution of the 

 waste substances, mostly urea and uric acid. This liquid is the urine. 

 About 1500 cc. of it leaves the kidneys daily in an average adult person 



