How does the bladder normally work?1,2

The bladder is a sac where urine is stored. It is about the size of a grapefruit and can hold 300–500 mL of fluid.

  1. Urine is filtered by the kidneys and drains into the bladder through two tubes called the ureters
  2. The bladder expands as it stores urine until signals from the brain signal the need to urinate
  3. Contraction, or “squeezing”, of the thickest layer of the bladder wall, called the detrusor muscle, pushes the urine out
  4. The urine exits the bladder through a tube called the urethra
Icon depicting kidneys, ureter, bladder, and urethra

What causes OAB?3,4

The symptoms of OAB occur in most cases because the muscles of the bladder contract involuntarily, which creates an urgent need to urinate. In some people, this urge is purely sensory, meaning that you might feel an urge to urinate even though the muscles of your bladder are not contracting.


  1. The Canadian Continence Foundation. The Source: Your Guide to Better Bladder Control. 2012.
  2. National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC). Urinary incontinence in women. Available at: Accessed February 25, 2022.
  3. Mayo Clinic. Overactive bladder. Available at: Accessed February 25, 2022.
  4. Corcos J, et al. CUA guideline on adult overactive bladder. Can Urol Assoc J. 2017;11(5):E142-E173.