How does the urinary system work?
The main organs of our urinary system are the kidneys – our body’s “sewage treatment plants”: They filter toxins out of the body, as well as other substances that we no longer need. These waste products 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 (glomerular filtrate) – 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.
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.
Position of the kidneys and bladder
The urinary bladder can store up to 500 ml of urine in women and 700 ml in men. People already feel the need to urinate (pee) when their 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 through your urethra.
In men, the urethra leads through the penis and is about 20 cm long. In women, it ends above the opening of the vagina. The urethra is shorter in women (only 3 to 5 cm long), so it’s easier for germs from the anus to enter their bladder. This is one of the reasons why urinary tract infections (UTIs) 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 normally.
How does bladder control develop?
The ability to hold your urine and pass urine is complex and involves the coordination of muscles, nerve signals and hormones, which is regulated by the brain and the spinal cord. Babies and toddlers can’t yet voluntarily control the emptying of their bladder – they only learn to do so gradually. Also, the pelvic floor muscles that stabilize the bladder need to develop first. The brain has to learn how to control the internal organs, too. 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 and can’t be sped up.
In babies, 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. As children get older, they learn to ignore this reflex and keep their urine in voluntarily until they get a chance to go to the bathroom. Eventually, they can do this in their sleep too. Instead of emptying their bladder, they wake up. At the same time, their sleep pattern develops.
The brain also has to learn to regulate the production of certain hormones, including vasopressin. During early 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 doesn’t fill up as quickly and the child doesn’t have to wake up at night.
Urinary incontinence in adults
Although bladder control problems are more common in children, they can affect people of all ages. If people can’t voluntarily hold urine back and it leaks out of their bladder, it is known as urinary incontinence. This happens if the sphincter muscle stops working properly and it can no longer keep urine in the bladder. The possible causes include very weak pelvic floor muscles or paralysis (problems with nerve function) in the pelvic area.
Schmidt RF, Lang F. Physiologie des Menschen. Berlin: Springer; 2017.
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