The Urinary Bladder

Key points

• • It is a pelvic organ when empty, lying behind the pubic symphysis, but as it fills it rises into the abdomen and can be entered above the pubis without crossing the peritoneum.
• • Its wall is the detrusor smooth muscle, and the smooth triangular trigone lies between the two ureteric openings and the internal urethral orifice.
• • The ureters pierce the bladder wall obliquely, forming a flap-valve that stops urine refluxing back up to the kidneys as the bladder fills.
• • Parasympathetic fibres from the pelvic splanchnic nerves (S2 to S4) contract the detrusor to empty the bladder, while sympathetic fibres keep it relaxed during filling.
• • The internal urethral sphincter is smooth muscle under autonomic control, whereas the external urethral sphincter is skeletal muscle supplied by the pudendal nerve, giving voluntary control.
• • In males the neck of the bladder rests on and is closely related to the prostate, so prostate enlargement can obstruct the outflow of urine.