The central organs of the urinary system are the kidneys, the “purification plant” of the body: they filter toxic substances and other substances that the organism cannot utilize out of the body. The substances are excreted with the urine produced in the kidneys. Water and substances such as urea, uric acid, salts or amino acids are thus removed from the blood. Every day, a person’s entire blood – five to six liters – flows through the kidneys about 300 times. In total, they filter about 1700 liters of blood every day. This produces about 170 liters of primary urine per day – a preliminary stage of urine (secondary urine).
Inside the kidney is the renal marrow, which is traversed by small tubes and larger collecting tubes. While the primary urine flows through this tubular system, the kidney cells absorb about 99% of the fluid and many other substances that can still be used, and at the same time release other substances. This leaves about 1.7 liters of secondary urine per day. It passes from the kidneys through the ureters into the bladder and is collected there.
Urinary bladder and urethra
Similar to a balloon, the bladder expands when it fills. Nerves in the bladder wall register the stretching and signal to the brain that the bladder is full.
The urinary bladder can store a maximum of about 500 (for women) to 700 ml of urine (for men). However, the urge to urinate arises from a bladder filling of 150 to 250 ml. To empty the bladder, the bladder muscle contracts and the sphincter muscles at the bottom of the bladder relax so that the urine can flow through the urethra.
In men, the urethra passes through the penis and is about 20 centimeters long. In women, the urethra opens above the vaginal outlet. Since their urethra is only three to five centimeters long, germs from the anus can enter their bladder more easily. This is one of the reasons why bladder infections are more common in women. In older men, an enlarged prostate can press on the bladder and urethra, making urination more difficult.
How does bladder control develop?
Urination and urine retention requires a complex interplay of muscles, nerve signals, and hormones controlled by the brain and spinal cord. Infants and young children are not yet able to voluntarily control bladder emptying – they are only gradually learning this. In addition, the muscles in the pelvic floor that stabilize the bladder must first develop. The brain must learn to control the internal organs. Although the most important bodily functions are congenital, the fine-tuning of the organs takes time. This also applies to bladder control. This development process can be very different in children and cannot be accelerated.
In babies, the brain responds spontaneously to the signal “bladder is full” by signaling the sphincter muscle of the bladder to relax. It then releases the opening to the urethra and the bladder empties. As the child gets older, he learns not to give in to this first reflex, but to consciously keep the bladder closed until there is an opportunity to urinate. With a little practice, this also works during sleep. Instead of emptying his bladder, the child wakes up. At the same time, the sleep rhythm develops.
The brain also has to learn how to control certain hormones. This also includes the hormone vasopressin. In early childhood, the brain begins to release large amounts of vasopressin at night. The hormone passes through the blood to the kidneys, where it reduces urine production. As a result, the bladder does not fill so quickly and the child can sleep at rest.
Urinary incontinence in adults
Although bladder control disorders are more common in children, they can occur at any age. If bladder emptying cannot be deliberately suppressed, this is known as urinary incontinence. If the bladder sphincter no longer functions properly, the urine can no longer be stored in the bladder. Causes can be for example a very weak pelvic floor musculature or a paralysis (disturbed nerve function) in the area of the pelvis.