The bladder sits in the pelvis, supported by the pelvic floor muscles and surrounding ligaments, holding it in a naturally elevated position. The bladder constantly fills with urine, and can hold 1.5 - 2 cups before giving you the urge to urinate. The muscles around the bladder remain relaxed until you are able to reach a toilet, at which point they contract in coordination with the relaxation of your urethral sphincters (which are otherwise unconsciously contracted) to allow urine to flow out of your body in a steady stream.
Urinary incontinence (UI) can occur for a huge number of reasons, including; if your kidneys produce more urine than normal; your urethra is blocked; your bladder or urethra experience nerve damage; you are psychologically unable to urinate when you feel the urge; urine is constantly present in the urethra stimulating the urge to go; or you have inadvertently trained yourself to empty your bladder when it is not yet full.
A GP will diagnose the type...