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How does the urinary system work?

The central organs of our urinary system are the kidneys, our body’s “sewage treatment plant”: they filter toxins and other substances that we no longer need out of our body. These substances are sent out along with the urine produced in the kidneys. You can read more about the kidneys here.

This is how water and substances like urea, uric acid, salts and amino acids are removed from the blood. About 1,700 liters of blood pass through the renal corpuscles every day. This leads to the daily production of about 170 liters of primary urine – which later becomes urine.

Inside of the kidney there is the renal medulla, which has small tubules and larger collecting tubes running through it. While the urine flows through this system of tubes, the kidney cells absorb about 99% of its liquid and many substances that can still be used, and at the same time release other substances. Eventually, the finished urine – about 1.7 liters a day – reaches the renal pelvis. From there it passes through the ureter and into the urinary bladder, where it is stored.

The urinary bladder can store, with conscious effort, between 500 and 700 ml of urine. The desire to urinate is already felt after the bladder is between 200 and 350 ml full. When the bladder is emptied, the detrusor muscle tightens and the sphincters at the base of the bladder relax. In this way the bladder is emptied through the urethra several times a day.

In women the urethra ends above the opening to the vagina and is only 3 to 5 cm long. In men it leads through the penis and is about 20 cm long. Because women have a shorter urethra, germs can more easily enter the bladder from the anus – a possible cause of an inflammation of the bladder (cystitis). Older men often have a benign enlarged prostate pressing against the bladder and urethra, making urination difficult.

Holding and passing water are the result of a complex interplay between muscles and nerve signals that must be correctly recognized and processed by the brain. Infants and young children up to the age of about 2 to 3 years cannot yet voluntarily control when they empty their bladder – they gradually learn to do so.

You can read more about kidneys and the urinary system here.