186 The Vertebrate Organ Systems 



tion in the coelom and consists of many hundreds of tubules. Portions 

 of the ducts of the mesonephros are utilized by the adults of the birds, 

 reptiles, and mammals as parts of the genital tract. The metanephros 

 is the functional kidney of the adults of these latter three groups. It 

 is posterior in position in the coelom, and each kidney consists of thou- 

 sands of individual units. The metanephros is unquestionably the most 

 efficient of the kidneys. 



THE FUNCTIONS OF THE URINARY SYSTEM 



The chief function of the urinary system is to aid the circulatory 

 system in maintaining homeostasis. In doing this, it does certain spe- 

 cific tasks which are related to this all-important function. 



1. The Excretion of Waste Materials. Various catabolic 

 products such as urea, creatinine, uric acid, and some salts are removed 

 from the blood by the kidneys. 



2. The Regulation of Blood Composition. Within the blood 

 the concentration of the various compounds and ions is kept at a remark- 

 ably constant level. To a large extent this is made possible by the 

 work of the kidneys in excreting excess amounts of any substance. In- 

 directly, thus, the kidneys are to a great extent responsible for the main- 

 tenance of the normal osmotic pressures of the body fluids. While ex- 

 cess water is excreted by the skin and to a small extent by the lungs, 

 it is really the kidneys which keep the fluid level of the body constant. 

 In case of hemorrhage, or extreme loss of fluids by sweating, the urine 

 becomes more concentrated. 



3. Maintenance of Acid-Base Balance. The concentration of 

 the various ions of the body fluids is regulated by their rates of reabsorp- 

 tion by the kidney tubules. The kidney is able to conserve bases by 

 actually forming ammonia to replace the cation sodium in the body 

 fluids ; thus ammonia rather than sodium compounds may be eliminated. 

 Excess acids are eliminated in this form, while excess bases may be 

 excreted with carbonic acid which is always available from the various 

 metabolic processes. 



THE ANATOMY OF THE MAMMALIAN URINARY SYSTEM 



The principal organs of the mammalian urinary system are the 

 paired kidneys. These somewhat bean-shaped organs are located back 

 of the peritoneum of the dorsal wall of the abdomen. From each kidney, 



