There are two sphincters or muscular valves, that separates the bladder from the urethra. The sphincters must open before the urine can flow into the urethra.

The internal sphincter between bladder and urethra is under involuntary control as it is formed of smooth muscle. The external sphincter is under voluntary control as it is formed of skeletal muscle. When the bladder is full, the parasympathetic nervous system sends impulses to the detrusor muscle of the bladder to contract and the internal sphincter muscle to relax, thus help in the elimination of urine. Whenever a person wants to eliminate urine, the external sphincter can be relaxed by a person’s own will thus, urine is eliminated. Similarly, urination can be prevented or postponed by maintaining the tone of the external sphincter.

Key Points

• The internal sphincter between bladder and urethra has smooth muscle is regulated by the parasympathetic nervous system that relaxes to help in urination.

• The external sphincter between bladder and urethra has skeletal muscle that can be controlled by a person’s own

Key Terms

sphincter: the circular muscle that regulates the opening

bladder: a hollow muscular organ that collects and stores urine

urine: a nitrogenous waste product

parasympathetic: part of the autonomic nervous system

detrusor muscle: smooth muscle found in the wall of the bladder