What is the bladder?
The bladder is the organ which stores urine produced by the kidneys. It is a muscle in the shape of a bag which can hold around 400 millilitres of urine (Fig. 1a and b).
Most people become aware their bladder is filling when it is half full. In order to empty the bladder, you have to be able to relax. This usually means being in a socially convenient environment, such as a toilet or a private space. The brain will then send a signal to the bladder to start squeezing out the urine.
How often you urinate depends on many factors, but one in particular is how much you drink. Most people urinate less than 8 times during the day, and either not at all or once during the night.