!&^Excretio?i 



THE MAIN excretory organs of man and 

 other vertebrates are the kidneys (Fig. 20-1). 

 This pair of large bean-shaped organs is 

 copiously supplied with blood, via the stout 

 renal arteries. Night and day the kidneys 

 work to rid the blood of metabolic wastes. 

 Urea and other excretory wastes, which 

 enter the blood in the different parts of the 

 body, are eliminated as fast as they are 

 formed; and normally these wastes never 

 accumulate to toxic levels in the blood 

 stream. Even with a single kidney, a man 

 may get along quite safely; but if both kid- 

 neys are lost, the individual will then suc- 

 cumb to uremic poisoning within a very few 

 hours. 



But the work of the kidneys involves much 

 more than the elimination of metabolic 

 wastes. The renal organs stabilize and regu- 

 late the composition of the blood. They keep 

 the concentration of water, inorganic salts, 

 glucose, and other essential blood compo- 

 nents at proper levels with an amazing degree 

 of constancy and efficiency. And since the 

 lymph is derived directly from blood plasma, 

 the kidneys also maintain homeostatic con- 

 trol over this intercellular fluid. In short, 

 control of the internal environment of the 



body cells is assumed in large measure by the 

 kidneys. Each day of man's life more than 

 1500 liters of blood are pumped through the 

 renal organs, and from this blood excesses of 

 each component are removed and eliminated 

 in the form of about 1.5 liters of urine. 



EXCRETORY PROCESSES IN LOWER 

 ANIMALS 



Small and relatively simple animals, such 

 as the Protozoa and the Coelenterata, elimi- 

 nate their excretory wastes mainly by diffu- 

 sion. As soon as any metabolic end product 

 is produced in significant quantities in the 

 cells it begins to diffuse from this locus of 

 higher concentration out into the environ- 

 ment, where the concentration is lower. 



Water, however, presents a special problem 

 (p. 133). Water is always an end product of 

 oxidative metabolism, and in fresh-water ani- 

 mals, water cannot diffuse out into the hypo- 

 tonic environment. Consequently small fresh- 

 water animals possess contractile vacuoles, or 

 other mechanisms capable of performing 

 work in forcing water out into the hypo- 

 tonic environment. 



The diffusional escape of excretory wastes 



369 



