The Urinary System 191 



Ammonia is believed to be produced by the kidney from amino acids or urea 

 in the blood. This formation is important in maintaining the acid-base balance of 

 the body. 



Uric acid is a waste product resulting from the metabolism of special proteins 

 known as nucleoproteins. 



Micturition. — Micturition or the emptying of the urinary bladder 

 is, in the adult, under voluntary control. This is a rare example of 

 voluntary control being exercised over an organ controlled by the 

 autonomic nervous system. In the very young individual, this volun- 

 tary control must be learned. 



The desire to micturate occurs when the pressure within the bladder 

 rises due to its being filled with urine. This stimulates certain sensory 

 nerves within the wall of the bladder, and the sensation is transmitted 

 up the cord to the brain, resulting in the individual's being conscious 

 of the filled bladder. 



