How the Bladder Works

The bladder is an expanding muscular sack, which collects urine after it has been filtered through the kidneys. It is located in the lower abdomen, below the stomach and behind the pubic bone.

The kidneys filter out any waste from all of the food and liquids we consume, and all liquid waste – between 2 and 3 pints a day, depending on how much has been consumed – is collected in the bladder. Healthy people usually go to the toilet between 4 and 8 times in a 24 hour period, although this depends upon each individual’s bladder capacity. Most people have a bladder capacity of between 400 and 600ml, and find they will expel about 500ml of urine each time they visit the toilet.

The Urge to Urinate

Once the bladder is about half full, sensor receptors in the walls of the bladder send signals to the spinal cord, via the pelvic nerves. This triggers a reflex action, known as the micturition reflex, when the muscle within the bladder wall contracts, creating the urge to urinate. Once the brain is aware of this urge, it instructs the various muscles of the bladder to contract or relax, and by doing so, allow urine to escape in a controlled manner.

The correct functioning of the bladder depends upon a number of different muscles and sensors playing their part. The bladder wall incorporates something called the detrusor muscle, which is normally relaxed to allow urine to enter and become stored in the bladder. When told to do so by the brain, it contracts to push urine out of the bladder. Urine must then pass through the sphincter, at the opening of the urethra, which relaxes to allow urine to escape. The pelvic floor muscles, which are located below the bladder and connected to the pubic bone, must also be relaxed in order to pass urine.