Your bladder stores hundreds of milliliters quietly
Urine arrives from the kidneys through two ureters and waits in a muscular sac on the pelvic floor. Most adults feel the urge somewhere between three hundred and five hundred milliliters. Rugae and a stretchy detrusor wall let the organ expand without a sharp pressure spike.
In humans the bladder sits at the pelvis base as a hollow, distensible organ with fundus, body, apex, and neck. Two ureteric openings and the urethral outlet form a smooth-muscle trigone; mucosal flaps at the ureteric orifices help block backflow into the ureters. Thick mucosal folds called rugae flatten as volume rises. The detrusor muscle—spiral, longitudinal, and circular smooth-muscle bundles—stays relaxed during filling and contracts during voiding. Wall thickness is usually three to five millimeters when empty and under three millimeters when well filled.
Embryologically the bladder comes from the upper part of the urogenital sinus after the cloaca divides between weeks four and seven. Sensation of stretch or irritation travels mainly via parasympathetic pathways through sacral nerves S2–S4 and onward in the spinal dorsal columns. Stretch receptors typically trigger the urge around three hundred to four hundred milliliters. Higher control involves the pontine micturition center in the brainstem. Muscarinic M3 receptors drive detrusor contraction; β3 adrenergic signaling helps keep the muscle relaxed while the bladder fills.
Cystitis—inflammation or infection of the bladder—often accompanies urinary tract infection and is more common in adult women than men because of a shorter urethra. Bacteria, most often Escherichia coli, are the usual cause. One working definition of overactive bladder is urinating more than eight times daily. Infants keep an abdominal bladder even when empty; adult female capacity is typically lower than male because the uterus sits above the bladder.
Source: Bladder
