The anatomy of the bladder

Imagine an upside-down pear – that’s the exact shape and size of your empty bladder. It rests on a sturdy hammock of muscles called the pelvic floor. This is your bladder’s best friend, providing support and helping you stay in control.

Your bladder doesn’t stay empty for long, because your kidneys keep filling it up with urine, which inflates it like a balloon.

To do this, the detrusor muscle lining the bladder relaxes, enabling it to triple in size when it’s being asked to hold a lot of pee.

To prevent leaks, your bladder uses two sphincters (fancy name for "rings of muscle").

First up, the internal urethral sphincter guards the neck of your bladder until it reaches 300-500ml (roughly the size of a drink can). At this point, the bladder muscle gets stretched and starts sending “Time to empty me!” messages to the brain. In an overactive bladder, this happens in much lower amounts.

The brain reacts by making the detrusor muscle squeeze and the internal sphincter relax.

This allows urine to flow into the upper part of the urethra – ie, the tube from which you pee.

The insistent urge

When you’re not ready to pee, the bladder holds on. But we all know that horrible feeling when the urge is overwhelming – eventually, crossing your legs doesn’t cut it.

So, you hurriedly find a loo and give your bladder the go-ahead to empty. 

Next, your external urethral sphincter (just above your pelvic floor) obligingly opens and urine streams out. Relief!

It’s all cleverly controlled by your mind: the pelvic floor supports the sphincter, keeping it tightly closed. But bladder weakness can lead to leaks when you cough, sneeze or laugh.

These can be helped a lot by exercises that strengthen the pelvic floor muscles.

This is what your bladder looks like: