Urinary incontinence - How the bladder works

Urinary incontinence

How the bladder works

On average, about 1–2 litres (2–3.5 pints) of urine are produced by the kidneys every day.
Most people do not feel an urge to pass urine until there is about 150–200 mL (about a quarter of a pint) in the bladder.
The normal adult bladder can hold about 500 mL (1 pint) of urine.

The two kidneys (one on each side) make urine from fluids and dissolved waste matter in the blood. Each day, the kidneys empty about 1.5 litres (3 pints) of urine into the bladder. The wall of the bladder is made of stretchy muscle, so that the bladder stretches like a balloon as it fills with urine.

A ring of muscle at the ‘neck’ of the bladder acts like a tap, keeping the urine in. This muscle is strengthened greatly by the muscles and ligaments of the pelvic floor that stretch backwards from the pubic bone to the backbone. Imagine a springy trampoline with tubes passing through it – three tubes in women, but only two in men.

The pelvic floor muscle is like the trampoline.
In women, the three tubes passing through the pelvic floor muscle are the bowel, the vagina and the bladder neck.
In men, the two tubes passing through the pelvic floor muscle are the bowel and the bladder neck.

In women, the bladder and womb lie on top of the pelvic floor. The pelvic floor is stronger in men because they do not have a vagina passing through it.

It is unsurprising that incontinence (leakage of urine) is common. As humans, we walk upright and our bladder system is not really suited for it – it is like holding a pint of urine in a floppy bag that is open at the bottom. If we walked on all fours, this would not be a problem. In fact, without the help of the pelvic floor muscles, the bladder neck would not be able to keep the bladder closed.

When the bladder contains about 250 mL (½ pint) of urine, it sends signals to the brain that you need to urinate soon. When it is convenient to do so, the bladder muscle stops stretching and begins to contract, squeezing the urine out. At the same time, the bladder and pelvic floor muscles relax, allowing the urine to come out. The urine then passes down a tube, called the urethra, to the outside. In women, the urethra is short. In men, the urethra is long; it passes through the prostate gland, and then along the penis to the hole at the end.