The urinary system

Urination can be defined as the voluntary act of passing urine out of the bladder. You may sometimes also hear the medical term “micturition” used for this.
This natural urge is the result of complex processes.
  • The kidneys filter the blood to remove some of the water and waste from the body.
  • This filtration process produces urine, a mixture of water and nitrogen waste (urea and uric acid).
  • The urine flows through tubes (ureters) to the bladder, a bag made of muscle membrane. It collects there until there is enough liquid to make you feel that you need to urinate.
The bladder fills and empties with the help of various muscles:
  • the detrusor (bladder wall muscle), which contracts to stimulate urination;
  • the sphincters (circular muscles), which close off the bladder and stop urine flowing through the urethra. They relax to allow urination to take place.
All these muscles are controlled by the nervous system.

Another set of muscles has a key role in how well the bladder and sphincters work: the pelvic floor. This supports the internal organs and helps the sphincters keep the bladder “locked”.
Normally, people start to sense the need to urinate when the bladder contains about 150 ml (a cupful) of urine. When you are in the toilet, ready to urinate, your brain gives an order to:
  • the bladder, to contract and push the urine out;
  • the sphincters and the pelvic floor muscles, to relax enough to allow the bladder to be emptied through the urethra tube.
Once the bladder is empty the sphincters tighten again and the pelvic floor resumes its role as a support.

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