Medical Definition of urinary bladder
: a distensible membranous sac that serves for the temporary retention of the urine, is situated in the pelvis in front of the rectum, receives the urine from the two ureters and discharges it at intervals into the urethra through an orifice closed by a sphincter, is lined with transitional hypoblastic epithelium, and develops from the proximal part of the allantois of the embryo
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