238 



or indirectly the wastes get into the blood 

 stream and become part of the plasma. 

 Once in the blood, the wastes are carried 

 about until they reach some organ which 

 is specially fitted to take them out of the 

 blood circulation and remove them from 

 the body. Such organs are called excre- 

 tory (ex'cre-tor-y) organs for they have 

 the power of excretion. In each organ 

 special substances are removed from the 

 blood and passed to the outside of the 

 body. The organs which do this are the 

 lungs, the kidneys, and the skin. 



The kidneys — their structure and work. 

 There are two kidneys. Each is a small 

 but extremely important organ. The kid- 

 neys are attached to the abdominal wall 

 behind the small intestine just below the 

 diaphragm. There is one on each side of 

 the spinal column. Each kidney is bean- 

 shaped, about four inches long, and dark 

 red in color. Examination of beef or lamb 

 kidneys will give you a very good idea 

 of man's kidney. See Exercise i. In the 

 outer part of each kidney there are about 

 one million microscopic structures which 

 do the work of filtering the waste sub- 

 stances out of the blood. Each is shaped 

 like a ball with a mass of capillaries coiled 

 around it. From it runs a microscopic 

 tube which, after joining other tubes, 

 ends in the cavity of the kidney. The 

 waste substances which enter the kidney 

 from the blood are largely water, urea, 

 uric acid, and some salts. The solution 

 is called urine. As the urine flows from 

 the million microscopic tubes it fills up 

 the cavity of the kidney. From this cav- 

 ity it flows down the ureters (you-ree'- 

 ters), leading to the bladder. These ure- 

 ters can be easily seen in the kidney you 

 may be studying and in the diagram. 



Houc a Cloifiplex Aimnal Uses Food unit iv 



Adrenol glands 



Opening of ureter 

 into blodder 



Muscular 



wall of bladder 



Fig. 234 The left kidney is cut open to show 

 the cavity where the urine collects. Where does 

 the urine go then? In what organ is it stored? 

 See Figure 23^ to learn niore about the complex 

 outer portion where wastes diffuse out of the 

 blood and collect in brafiched tubes. 



The bladder holds the urine until it is 

 passed from the body. 



While the wastes, for the most part, 

 arc relatively harmless substances in 

 themselves, if they were not removed 

 from the body they would accumulate 

 in the plasma and in the lymph; difi^u- 

 sion out of body cells would be stopped 

 and many of the life processes would 

 thus be interfered with. 



