Final answer:
The involuntary sphincter that keeps the urethra closed when not urinating is the internal urinary sphincter, composed of smooth muscle. This internal sphincter works with the voluntary external sphincter in the micturition process to control urination.
Step-by-step explanation:
The involuntary sphincter that keeps the urethra closed when urine is not being passed is called the internal urinary sphincter. It consists of smooth muscle located at the junction of the bladder and urethra. This sphincter automatically relaxes as the bladder fills, allowing urine to move into the urethra in preparation for voiding. When a person decides to urinate, the internal urethral sphincter relaxes and opens, which in combination with the contraction of the detrusor muscle, allows urine to be expelled from the bladder.
The internal sphincter operates involuntarily and is under autonomic control, while the external urethral sphincter, composed of striated muscle and controlled by the somatic nervous system, requires conscious effort to relax. Both sphincters work together in the process known as micturition, which is the proper term for urination. The micturition reflex can be overridden by voluntary control, a crucial aspect of urinary continence.