The urine is held in the bladder until it can be expelled from the body. It passes through the urethra which is a tube leading from the bladder to the outside of the body by urination. The detrusor muscle is a layer of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. When the bladder is stretched this signals the parasympathetic nervous system to contract the detrusor muscle. It encourages the bladder to expel the urine through the urethra when this happens. For the urine to exit the bladder two different sphincters must be opened. The internal sphincter which is autonomically controlled and the voluntarily controlled external sphincter must be opened.
The lining of the urinary bladder consists of transitional epithelium so it does not produce mucous. The wall of the urinary bladder form rugae which are a series of ridges produced by folding the wall of an organ. The urinary bladder usually holds between 300-350 ml. of urine (converts to approximately 10- 12 fl. oz.). As urine volume increases the rugae thin out signaling to the brain by the nerves travelling along synapses to the parasympathetic nervous system that it is ready to release the urine held within it through a process called urination. This triggers the parasympathetic nervous system to open the internal sphincter that allows the bladder to release the urine to outside the body through the urethra and the voluntarily opened external sphincter. These nerves are underdeveloped in infants and must be developed in the toddler before control of the urinary bladder can be established through toilet training.