Urine is drained from the renal pelvis and carried by the ureter to the urinary bladder (Fig. 1). Urine is stored in the bladder until fullness stimulates a re?ex contraction of the bladder muscle and expulsion of urine through the urethra. The female urethra is short (4 cm; 1.5 in) and carries only urine. The male urethra is longer (20 cm; 8 in) and carries both urine and semen.

The voiding (release) of urine, technically called micturition or urination, is regulated by two sphincters (circular muscles) that surround the urethra. The upper sphincter, just below the bladder, functions involuntarily; the lower sphincter is under conscious control.

FIGURE 1. Interior of the urinary bladder, shown in a male subject. The trigone is a triangle in the ?oor of the bladder marked by the openings of the ureters and the urethra. (Reprinted with permission from Cohen BJ, Wood DL. Memmler’s The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

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