How does the bladder work?

Authors: SCIRE Community Team | Reviewer: Bonnie Nybo | Published: 18 January 2018 | Updated: 16 October 2024

The urinary system helps the body filter and remove waste products and excess fluids. It consists of the kidneys, ureters, bladder, urethra, and bladder and sphincter muscles.

The kidneys filter the blood to produce urine, which is passed through narrow tubes called ureters to the bladder. The bladder is a sac that collects urine. Urine passes out of the body from the bladder through a tube called the urethra.

Filling and emptying of the bladder are partly controlled by the bladder muscles:

• The bladder wall muscle (detrusor muscle) is smooth muscle that covers the outside of the bladder. When it contracts, it squeezes the bladder and pushes urine out through the urethra. When it is relaxed, the bladder is loose and can be filled with urine.
• The bladder sphincter muscles (urethral sphincter or valve muscles) are two muscles which surround the exit of the bladder like a ring. When they tighten, they close off the urethra and hold urine in the bladder. When they relax, they allow urine to drain. The internal sphincter muscle is controlled unconsciously and the external sphincter muscle is controlled consciously.

When the bladder is not full, the bladder wall muscle is relaxed and urine produced by the kidneys passes through the ureters to fill the bladder. The bladder sphincter muscles are tightened so urine does not leak out.

When there is enough urine to stretch the bladder walls, a nerve signal is sent up the spinal cord to tell the brain that the bladder is full. Because the brain controls the external sphincter muscle, urine can be held until an appropriate time to empty.

When the bladder is to be emptied, signals are sent from the brain down the spinal cord to cause the coordinated squeezing of the bladder wall muscle and relaxation of the bladder sphincter muscles to allow urine to pass through the urethra and out of the body. Control of urination involves both bladder reflexes (in which emptying is triggered when the bladder is full) and voluntary control (in which urine can be held until a socially appropriate time to empty).