Pelvis and bladder. Coloured magnetic resonance imaging (MRI) scan of the human bladder (blue) and pelvis. The ball-and-socket hip joints (red) can be seen at centre left and right. The pelvic bones (red) appear in the upper centre, surrounding the bladder. Urine drains into the bladder from the kidneys (not seen), which filter waste products out of the bloodstream continuously. The size of the bladder varies from person to person, but on average it can hold up to half a litre of fluid before the urge to empty it becomes painful. MRI scans show a slice" through the human body in any chosen direction, revealing soft tissues as well as harder material like bone."