Urine produced in kidney passes down two urethers to the bladder.
The bladder has a sphincter (shutter), that restrains the urine in the bladder. It's opening and shutting down depends on the urine volume in the bladder.
When the urine in the bladder reaches a certain degree, signal is sent to the spinal cord by nerves, that the sphincter must open and the bladder must empty.
In the case of a new-born child an order to open the sphincter comes at once and the bladder is emptying. (The child urinates)
An adult has a possibility to control urine output until he/she reaches WC. For that he/she blocks impulses, which come from the spinal cord to open the sphincter. At that moment human feels that he/she wants to urinate, but controls his/her bladder - urine outpute. But even this inhibition has a limit. If it lasts very long time, impulses from the spinal cord will "break" the "block of the brain" and open the sphincter, that will course wetting. This means that, if a person feels, that the bladder is full he/she must walk to WC and urinate immediately.
Children often feel the signal, that indicates that the bladder is full, but they are so interested in game they are playing in that moment, that they try to delay urination and restrain urine in the bladder. It means, that the brain delays the signal of the spinal cord. During the certain period child manages to do this but when the bladder is filling and requirement of wetting is growing up, the child, as a rule, is trying to "clip" the sphincter he/she also uses "tricks of inhibition "(crossing legs, sitting on the heel and etc.). This measures help the child to restrain urine only for a while, but soon this "barrier" is extracted too.
To void the untimely secreted urine, it's necessary for a child to walk to WC, when he/she gets the signal that the bladder is filled. The child must urinate in relaxed and comfortable position.
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