Bladder function

Urinating has a proper name in medicine: micturition. And it is not a single movement, it is an interplay between what the body does on its own and what you decide. At the bladder outlet there are two muscular rings, the urethral sphincters: the internal one, which is not under your voluntary control, and the external one, which is.

As the bladder fills, stretch receptors in its wall, sensors that report how far it has stretched, send impulses to the lower part of the spinal cord. There a spinal reflex fires, an automatic response that bypasses your own decision: the signal that comes back contracts the detrusor muscle of the wall and loosens the internal sphincter. What stays on your side is the other part: the external sphincter is skeletal muscle, the same type as in your arm, and that is why you can keep it clamped shut. It is like a door with two bolts, one that opens on its own and one you hold with your hand until you reach the bathroom.