The human body needs water to function but it also needs a way to get rid of waste water that it no longer needs. This is where the urinary system comes into play.
The central nervous system has most of the control over bladder emptying but normally we have some control over when and where that happens.
The part of the nervous system that controls most of the process is called the autonomic system. The autonomic nervous system works without you having to think about it. It controls things like your heart rate and blood pressure, as well as your bowel and bladder function.
When the bladder needs to empty the main bladder muscle and the valves (sphincters) work together:
- The external sphincter (the one you control) relaxes.
- The internal sphincter (the one that’s controlled automatically) relaxes and at the same time the bladder muscle contracts. This allows the urine to drain out of the urethra.
Once the bladder is empty the process is reversed; the sphincters tighten to prevent urine leaking out, and the bladder muscle relaxes so it may stretch and fill up again.