After the kidney makes urine, the ureters carry it to the bladder, which stores it until it is released through the urethra. Nerves and two sphincters decide when that happens.
Ureters
Each ureter is a muscular tube about 25 to 30 cm long that moves urine by waves of contraction. It is narrowest where it leaves the kidney, where it crosses the pelvic brim and where it enters the bladder, the places where stones get stuck.
Bladder
The bladder is a stretchy muscular bag behind the pubic bone. Adults usually feel the urge to void at about 200 to 300 mL and can hold about 400 to 600 mL.
Control of voiding
The internal sphincter works automatically and the external sphincter is under voluntary control. Voiding happens when the bladder muscle contracts and both sphincters relax, coordinated by centers in the sacral spinal cord and brainstem.
Urethra
The female urethra is about 4 cm long, which is why bladder infections are more common in women. The male urethra is about 20 cm long and passes through the prostate, which can narrow it with age.
At the bedside
A bladder scan after voiding measures residual urine, and a large residual suggests retention. Knowing the length and curves of the urethra guides safe urinary catheter insertion.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).