HOW THE HUMAN BLADDER WORKS

When babies are six months old, emptying the bladder is an automatic reflex, in response to the distension of the bladder that usually occurs when the volume of urine reaches around 30 ml.
By the time children are one or two years old, they learn to recognise when their bladder is full because their parasympathetic nervous system has matured and begun to send sensations from the bladder to the brain.

After the age of two, most children are able to tell other people of their need to urinate.

By the age of three, children can hold their urine for a few minutes when their bladder is full and postpone urinating, doing it voluntarily once they reach the toilet. By this age most children are able to do for themselves almost everything involved in going to the toilet—pulling their pants up and down, pulling the chain, etc.

When they are four or five, most children can control their bladder voluntarily when it is full, postpone urinating until it is appropriate, start and stop the flow of urine and decide when and where they urinate.

The bladder is a bag lined on the inside with an epithelium (like the skin or mucous membrane of the mouth), with most of the rest of its thickness made of muscle.

The body’s system to stop urine from leaking out—the muscular valve or sphincter—is quite complex, with two major areas: the internal sphincter and the external sphincter (which in turn is divided into two sections).

In order for the bladder and urination process to work properly, all the nervous systems of the body need to work together in a coordinated way.
In simple terms, the inner wall of the bladder is controlled from the conscious system (somatic nerves), the internal sphincter by the sympathetic system (involuntary), the innermost section of the external sphincter by the parasympathetic system and the outermost section by the somatic system again. In order for urination to be normal, all these nerves must work together so that the bladder contracts while the internal and external sphincters relax.
The bladder dilates as it fills up, remaining under little pressure and sending the sensation of fullness to the brain when the amount of liquid contained reaches 100-150 ml. However, the musculature of the wall remains relaxed until the bladder’s maximum capacity (which varies greatly from person to person) is reached.
When the brain “gives the okay” for urination to take place, the bladder contracts and the sphincters relax. The final part of the process is controlled by activity of the outermost section of the external sphincter, which is voluntary.