Normally, the kidneys continually filter blood, removing waste and producing urine, which travels down tubes called ureters into the bladder, a muscular sac behind the pelvic bone. The bladder expands to store urine and contracts to expel it. The tube that empties the bladder is the urethra. A valve-like sphincter muscle encircles the top of the urethra at the bladder neck, preventing leakage. Spinal nerves coordinate the actions of the bladder and the sphincter.
Infants empty their bladders by reflex.
When the bladder fills and stretches beyond a certain point, it signals the spinal cord, which triggers the sphincter to relax and the bladder to contract. Young children gradually develop control over this process. Bladder stretching registers consciously as discomfort, and the brain learns to suppress the reflex. The more the bladder stretches, the more urgent the desire to urinate. Incontinence can occur for many reasons.