The Kidneys^ Most of the solid waste substances from body cells are 

 filtered out of the blood by the kidneys, which are the typical excretory 

 organs of the backboned animals. 



In the human body there are two bean-shaped kidneys, each about as 

 long as the width of the hand. They are located in the back of the ab- 

 dominal cavity, one on each side of the spinal column, slightly lower than 

 the stomach. The kidney is like a gland in structure (see page 169), a mass 

 of tiny tubules, branched and twisted, with a complex network of capillaries. 

 The waste substances diffuse through the walls of the capillaries into the 

 tubules, and the fluid (urine) is gathered by these tubules into a funnel- 

 shaped hollow (see illustration opposite). 



How Do the Kidneys Separate Waste from the Blood? 



The Gland Unit The kidney separates, or filters, organic wastes from 

 the blood by a combination of osmosis and the action of special cells. The 

 separation starts in a tangle of capillaries called a glomerule, embedded in a 

 "capsule" that opens into a long, thin-walled and greatly twisted, or con- 

 voluted, tubule (see illustration, p. 221). 



The process is as follows: 1. Waste substances diffuse into the capsule 

 from the blood in the capillaries of the glomerule. 2. The wastes are carried 

 by the tubule toward the funnel-like pelvis of the kidney, into which all the 

 tubules empty. 3. Much of the water and some of the dissolved substances 

 are reabsorbed from the tubules by the blood in capillaries entangled with 

 the tubule. 4. At the end of the tubule there remains the watery solution 

 called urine. 



Composition of the Urine" The urine is about 96 per cent water. The 

 dissolved substances include inorganic salts and organic substances which 

 result from the breakdown of proteins during metabolism. 



Contents of the Urine 



INORGANIC SALTS 



Sodium chloride 



Sodium 



Potassium 



Calcium 



Magnesium 



as sulfates and phosphates 



ORGANIC SUBSTANCES 



Urea 

 Uric acid 

 Creatinin 

 Coloring matter 



The composition and the concentration of the urine are constantly 

 changing. The proportion of solids and water varies with the activities of 



^See No. 1, 



226. 



2See Nos. 2, 3, and 4, p. 226. 



218 



