Function of the Kidneys and Bladder
- On average, about 1–2 litres (2–3.5 pints) of urine are produced by the kidneys every day.
- Most people do not feel an urge to pass urine until there is about 150–200 mL (about a quarter of a pint) in the bladder.
- The normal adult bladder can hold a little less than 500 mL (1 pint or 16 ounces) of urine.
The two kidneys (one on each side) are located behind your 13th rib. They make urine from fluids and dissolve waste matter in the blood. Each day, the kidneys empty about 1.5 litres (3 pints) of urine into the bladder. The wall of the bladder is made of muscle that stretches like a balloon as it fills with urine.
When the bladder contains about 250 mL (½ pint or 8 ounces) of urine, it sends signals to the brain that you need to urinate soon. When it is convenient to do so, the bladder muscle stops stretching and begins to contract, squeezing the urine out. At the same time, the bladder, the sphincter (valve that surrounds the urethra that keeps urine from leaking) and pelvic floor muscles relax, allowing the urine to come out. The urine then passes down a tube, called the urethra, to the outside. In women, the urethra is short. In men, the urethra is long; it passes through the prostate gland, and then along the penis to the hole at the end.
A ring of muscle, called the internal sphincter, at the neck or base of the bladder acts like a tap or valve, keeping the urine in. Below this sphincter is a second one, called the external sphincter. You control this sphincter by controlling the pelvic floor muscles that surround it. The pelvic floor muscles are like the trampoline that stretches from the pelvic bone in the front to the backbone or tailbone. Imagine a springy trampoline with tubes passing through it – three tubes in women, but only two in men.
- In women, the three tubes passing through the pelvic floor muscle are the rectum and its opening (the anus), the vagina and the urethra. The bladder and womb (uterus) lie on top of the pelvic floor.
- 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.
Age-Related Changes in the Kidneys and Bladder
Although urinary incontinence is not a normal part of aging, there are several age-related changes that can occur in your kidneys and bladder:
- The kidneys make the largest amount of urine during the night when you are lying flat. This may cause you to wake often at night to urinate (called nocturia).
- Your bladder holds less urine than when you were younger, so you may urinate more frequently.
- You may not empty your bladder completely and may find that you need to urinate more often.
- Bladder sensation changes with age, which delays the desire to urinate and decreases the ‘warning’ time (the time between the first urge to urinate and actually urinating).
- Involuntary bladder contractions can occur when someone changes position (e.g. bends over), causing overactive bladder symptoms. Combined with the delayed desire to urinate, this often results in urinary urgency and frequency.
First published on: embarrassingproblems.com
Reviewed and edited by: Dr Diane Newman
Last updated: October 2020