Function of the Kidneys and Bladder

• On average, your kidneys produce about 1–2 litres (2–3.5 pints) of urine each day.
• Most people do not feel an urge to pass urine until the bladder holds around 150–200 mL (about a quarter of a pint) of urine.
• A typical adult bladder can hold just under 500 mL (1 pint or 16 ounces) of urine.

The kidneys, located near the back under the 13th rib (one on each side), filter waste and excess fluid from your blood to make urine. Each day, about 1.5 litres (3 pints) of urine flow from the kidneys to the bladder, which is a muscular organ that stretches like a balloon as it fills.

When the bladder reaches about 250 mL (½ pint or 8 ounces) of urine, it sends signals to the brain indicating it’s time to urinate. When you decide it’s the right time, the bladder muscle contracts to push the urine out. At the same time, the sphincter muscles (surround the urethra that keeps urine from leaking) and pelvic floor muscles relax to let the urine pass through the urethra – the tube that carries urine from the bladder out of the body.

• In women, the urethra is short and straight.
• In men, the urethra is longer and passes through the prostate gland and down the penis.

There are two important sphincters (valves) involved:

• The internal sphincter, located at the bladder neck or base, holds urine in.
• The external sphincter, located below the internal, is under under your voluntary control. It is supported by the pelvic floor muscles, which function like a trampoline stretched across your pelvis.

Pelvic Floor Differences

• Women have three openings through the pelvic floor: the urethra, vagina, and anus
• In men, the two tubes passing through the pelvic floor muscle are the rectum and its opening (the anus), and the urethra. These muscles surround the prostate.

There are two important sphincters (valves) involved:

• The internal sphincter, located at the bladder neck, holds urine in.
• The external sphincter, located below the internal sphincter, is under your voluntary control. It is supported by the pelvic floor muscles, which function like a trampoline stretched across your pelvis.

Age-Related Changes in the Kidneys and Bladder

Although urinary incontinence is not a normal part of aging, aging can affect how kidneys and bladder work:

• Nighttime urine production increases when lying flat, causing frequent nighttime urination (called nocturia).
• The bladder holds less urine than it did when you were younger, leading to more frequent urination.
• You may have difficulty fully emptying your bladder.
• Bladder sensation may decrease with age, so the urge to urinate may come later than it used to, reducing the ‘warning time’ before leakage.
• Involuntary bladder contractions can occur with movement (such as bending over), which may trigger symptoms of overactive bladder symptoms, including urinary urgency and frequency.

First published on embarrassingproblems.com
Reviewed and edited by Dr Diane K. Newman
Last updated: July 2025 by Dr Diane K. Newman