Description: The urinary bladder is the hollow, muscular organ central in the pelvis anteriorly. This muscle is called the detrusor m. The bladder receives tubular ureters one from each side. They enter at the postero-lateral portion externally, pass obliquely through the detrusor m. and open into the base of the trigone internally. At the apex of the trigone of the bladder inferomedially, the  internal urethral os, the outlet of the bladder merges with the urethra.The bladder is loosely attached to the lower portion of the uterus, and the trigone rests anterior to the vagina. The vesico-uterine pouch lies posterior to the extraperitoneal bladder, and the Space of Retzius is anterior, allowing upward expansion of the organ, and surgical access to the urethra. The circulation of the bladder is via the superior and middle vesical a., and the innervation from the inferior hypogastric plexus; S2-S4 (parasympathetic).