24 

 The Vertebrate Body (Cont.) 



Excretion, Reproduction, and Regulation 

 in the Frog and Man 



The Excretory System. When we first studied the life processes in 

 amoeba we found that the elimination of the waste products formed in 

 the process of oxidation of food within the cell was a necessary reaction 

 in the life of that animal. However, there were no special structures re- 

 quired, for diffusion of the wastes through the cell membrane into the 

 surrounding water was simple. Excretion is just as important in the 

 chordates, but because of their larger and more complicated bodies it 

 must be done by special organs designed for this purpose. These organs 

 are the kidneys which, in the frog, are two flattened, elongated, reddish 

 brown bodies attached to the dorsal wall of the body cavity. 



The excretory waste is generated in the cells as a part of their meta- 

 bolic activities and diffuses out through the cell membrane into the sur- 

 rounding blood and lymph. It reaches the kidneys through two entirely 

 different blood vessels : the renal arteries, that branch from the dorsal 

 aorta ; and the renal portal vein, that carries venous blood picked up 

 from the posterior region of the body. It is taken away from the kidneys 

 by the renal veins. Within the kidney there are many small capsules 

 containing glomeruli. A glomerulus is a network of capillaries. The 

 blood vessel entering each glomerulus is larger than the vessel leaving 

 it. This creates a pressure within the capillaries and some of the water 

 and minerals of the blood are squeezed out into the surrounding cap- 

 sule. This liquid then flows down little tubules where part of the water 

 and the minerals needed by the body are reabsorbed by the blood. That 

 which is not absorbed continues down the tubules and eventually empties 

 into two ducts on the lateral surface of the kidneys as urine. These 

 ducts are commonly called ureters, since this word is used to refer to 

 the tubes that carry urine from the kidneys in the highest vertebrates, 

 but in the male frog these ducts also carry sperms. Because of this it 

 is somewhat more accurate to refer to these as urogenital, or Wolffian, 

 ducts in the male frog. The urine flows down these ducts and empties 

 into the cloaca. There is a bilobed urinary bladder hanging from the 



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