The urine, in its final form, flows from the collecting tubules into the renal pelvis, a small pocket emptying into the urethra. The urethra carries it to the leak-proof and highly expandable bladder, where it is stored between urinations. A sphincter between the bladder and the urethra prevents urinary incontinence. When the bladder is sufficiently full, urination (miction) can occur. The bladder, which is composed of many smooth muscle fibers, contracts and the urethral sphincter relaxes. The urine is then forced out by the pressure.
Urination is regulated by the nervous system, with the brain providing voluntary and conscious control. Nerves originating in the lumbar, sacral and pelvic regions cause contraction of the bladder and relaxation of the sphincter.
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