Urination is controlled by two urinary sphincters, which are muscles acting like valves. The internal sphincter is located where the bladder and urethra join, called the bladder neck. The external sphincter sits below the prostate near the pelvic floor.
These two sphincter muscles act together, and control urine by opening and closing around the bladder neck and urethra. When they get permission from the brain, the sphincters relax to allow the bladder to empty. At the same time, the bladder muscles contract and squeeze urine out of the bladder. When you have finished urinating, the sphincter muscles contract and close while the bladder relaxes.
The pelvic floor muscles also play a part in bladder control. The floor of the pelvis is made up of muscle layers and tissues. The layers stretch like a hammock from the tailbone at the back to the pubic bone in front. Your pelvic floor muscles support your bladder and bowel. The urethra (urine tube) and rectum (back passage) pass through the pelvic floor muscles.
INTERNAL AND EXTERNAL URINARY SPHINCTERS