CHAPTER 28 



The Urogenital System — 

 Excretion and Reproduction 



Functionally the kidneys have nothing in common with the repro- 

 ductive organs. They are concerned with excretion of wastes and regu- 

 lation oi body fluids; the reproductive organs only with the perpetuation 

 of the species. But the t^vo systems are morphologically interrelated in 

 vertebrates because certain excretory ducts are used for discharging 

 gametes, and it is convenient to treat them together as the urogenital 

 system. First we shall consider the excretory portion of the system, and 

 then relate the reproductive organs to it. 



Although the kidneys come to mind when one thinks of excretion 

 in vertebrates, they do not have a monopoly on the removal of the 

 waste products of metabolism. The gills and lungs, the skin, and to some 

 extent the digestive tract play a role in excretion. Gills eliminate carbon 

 dioxide and some nitrogenous wastes; lungs, carbon dioxide; the skin 

 (especially in amphibians), a certain amount of carbon dioxide and 

 traces of salts and nitrogenous wastes; the digestive tract, bile pigments 

 and certain metal ions. The kidneys remove most of the nitrogenous 

 wastes in the higher vertebrates, but this is not their only function. By 

 removing, or conserving, water, salts, acids, bases and various organic 

 substances, they play a vital role in regulating the composition of the 

 blood and the internal environment of the body. 



238. Evolution of the Kidneys and Their Ducts 



The kidneys of vertebrates are paired organs that lie dorsal to the 

 coelom on each side of the dorsal aorta. All vertebrate kidneys are com- 

 posed of units called kidney tubules, or nephrons, which remove ma- 

 terials from the blood, but the number and arrangement of the nephrons 

 differ in the various groups of vertebrates. Comparative studies have 

 led to the conclusion that each kidney in ancestral vertebrates contained 

 one nephron for each of those body segments that lay between the an- 

 terior and posterior ends of the coelom (Fig. 28.1, A). These nephrons 

 drained into a Wolffian duct which continued posteriorly to the cloaca. 

 Such a kidney may be regarded as a complete kidney, or holonephros, 

 for it extends the entire length of the coelom. A holonephros is found 

 today in the larvae of certain cyclostomes, but not in any adult verte- 

 brate. 



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