As the bladder slowly fills with urine, the bladder wall muscles relax and expand to accommodate an increasing amount. When it reaches the limit of its expansion, we feel the urge to urinate. The brain instructs the bladder muscle to contract than the sphincter in the urethra relaxes. Thus, the urine can be emptied completely and unhindered. With sufficient fluid intake, the bladder must be emptied 5-7 times a day, and urinating once or twice at night is also considered normal.
The base of the urethra and the bladder is formed by the pelvic floor muscles, which tighten in response to an increase in abdominal pressure (movement, coughing – any activity when the abdominal muscles are tense) and thus help prevent involuntary urine loss. Weakness of these muscles or damage to the structures providing bladder neck suspension leads to involuntary loss of urine and stress incontinence.