bladder

The urinary bladder is a hollow, distensible organ that stores urine collected from the kidneys before elimination. In adults, normal capacity is about 400–600 mL; sensations of urgency typically appear around 150–200 mL. The bladder sits in the pelvis, with the apex directed toward the navel; in males it lies anterior to the rectum, and in females it lies anterior to the uterus and vagina.

The bladder wall consists of mucosa with transitional epithelium (urothelium), a lamina propria, and a muscularis propria (detrusor) arranged in interlacing layers. The detrusor contracts to expel urine. The lumen contains the trigone, a triangular region defined by the two ureteral orifices and the internal urethral orifice. The bladder neck houses the internal urethral sphincter; the external urethral sphincter is located in the urogenital diaphragm and is under voluntary control.

Blood supply comes from the vesical arteries. Innervation is autonomic: parasympathetic fibers from the pelvic splanchnic nerves stimulate detrusor contraction; sympathetic fibers via the hypogastric nerves promote detrusor relaxation and internal sphincter tone; somatic innervation by the pudendal nerve controls the external urethral sphincter.

Functionally, the bladder stores urine during filling and coordinates micturition with detrusor contraction and sphincter relaxation. Common clinical conditions affecting the bladder include cystitis (bladder infection), bladder outlet obstruction, overactive or neurogenic bladder, and bladder cancer, often of the transitional cell type.

Developmentally, the bladder originates from the upper part of the urogenital sinus. The trigone forms from the incorporation of the distal ends of the mesonephric ducts; the urachus connects the bladder apex to the umbilicus.