The ureters have valves that only let urine through one way and keep urine from being forced back into the kidney when the bladder contracts. The bladder stores urine and expels it into the urethra for excretion from the body. The bladder can stretch to hold about a pint of urine. The urethra is a tube through which urine flows from the bladder to the outside of the body. Usually we don't have problems with our urinary system but sometimes things go wrong.
People can get urinary tract infections, kidney stones, or suffer from kidney failure.
The most common disease of the urinary system is a urinary tract infection. If bacteria goes up the urethra into the bladder you can get a bladder infection. If this infection is not treated and it goes through the ureters into the kidneys you can get a very serious disease called pyelonephritis. This can be treated with antibiotics. When calcium salts in the urine separate they form crystals in the kidneys, this is called kidney sand.