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How does the urinary system work?

Last Update: January 7, 2015; Next update: 2018.

The main organs of our urinary system are the kidneys – our body’s “sewage treatment plants”: they filter toxins and other substances that we no longer need out of the body. These substances leave your body in the urine produced in your kidneys. This is how water and substances like urea, uric acid, salts and amino acids are removed from the blood. Every day, all of the blood in your body (between five and six liters) passes through the kidneys about 300 times. So your kidneys filter about 1,700 liters of blood per day in total. This leads to the daily production of about 170 liters of primary urine – which later becomes urine.

Inside the kidney there is a renal medulla, which has small tubules and larger collecting tubes running through it. As the primary urine flows through this system of tubes, the kidney cells re-absorb about 99 percent of the fluid in it, as well as many substances that can still be used, and at the same time release other substances. About 1.7 liters of urine are produced like this each day. The urine passes from the kidneys through the ureter into the urinary bladder, where it is stored.

Illustration: Position of the kidneys and bladder

Position of the kidneys and bladder

Urinary bladder and urethra

The bladder expands when it fills up, like a balloon. Nerves in the bladder wall detect the expansion and send a signal to the brain, letting it know that the bladder is full.

The urinary bladder can store up to between 500 and 700 ml of urine. The urge to urinate is already felt when the bladder has between 200 and 350 ml of urine in it. When you empty your bladder, the muscle in your bladder wall tightens to squeeze the urine out of your bladder, while at the same time the sphincter muscles at the base of your bladder relax, allowing the urine to flow out of your urethra.

In men the urethra leads through the penis and is about 20 cm long. In women it ends above the opening to the vagina. Because the urethra of women is only 3 to 5 cm long, it is easier for germs from the anus to enter the bladder. This is one of the reasons why urinary tract infections like cystitis are more common in women. In older men, a benign enlarged prostate might push against the bladder and urethra, making it difficult to urinate.

Development of bladder control

The ability to hold your urine and pass urine is complex and depends on the coordination of muscles and nerve signals, controlled by the brain and the spinal cord. Infants and young children up to the age of about two to three years cannot yet voluntarily control the emptying of their bladder – they gradually learn to do so. Also, the pelvic floor muscles that stabilize the bladder need to develop first.

The brain develops when nerve cells create new connections with each other. The brain also has to learn how to control the internal organs. Although the most important bodily functions work right after birth, the fine-tuning of the organs takes time. This also applies to bladder control, which takes longer to develop in some children. Parents cannot speed things up.

In early childhood, the brain reacts to the signal “bladder is full” by telling the sphincter muscles of the bladder to relax. The muscles then open the passage to the urethra and the bladder is emptied. When children get older, they learn to ignore this reflex and to keep their urine in voluntarily until they get a chance to go to the bathroom. With a little practice they can also do this in their sleep. Instead of emptying their bladder, they wake up. At the same time, their sleep pattern develops. Over time, children gradually learn to recognize body signals that mean "wake up."

The brain also has to learn to control the production of certain hormones, including vasopressin. During childhood, the brain starts releasing larger amounts of vasopressin at night. This hormone travels through the bloodstream to the kidneys, where it decreases urine production. As a result, the bladder does not fill up as quickly and the child does not have to wake up at night.

These processes develop at different times in different children. Although many five-year-old children can control their bladder during the day, they are often not yet able to wake up when they have a full bladder at night. Bladder control problems can affect people at any age, but are more common in children.

Sources

  • Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer. 2011.
  • Menche N. Biologie Anatomie Physiologie: mit www.pflegeheute.de – Zugang. München: Urban & Fischer/ Elsevier GmbH. 2012.
  • Psychrembel W. Klinisches Wörterbuch.  Berlin: De Gruyter. 2014.
  • IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

    Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. We do not offer individual consultations.

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