The purpose of the urinary system is to remove unwanted elements (urine) from the body.
The amount of urine expelled each day depends on many factors including the volume of fluid and food consumed, and the volume of fluid lost through sweating and breathing. A healthy bladder normally holds approx 500ml urine. After evacuation, the bladder usually retains approximately 100ml of urine.
Kidneys, take the waste from your bloodstream through nephrons which filter urea, forming your urine. Every 10-15 seconds, the urine moves down through ureters to the bladder. Ureters are between 8-10 inches long, but if anything gets stuck, or reverses here, then you may develop a kidney infection.
The bladder, like a small balloon, sits on your pelvis where it stores the urine ready for expelling. A healthy bladder can hold around 16oz or urine for 2-5 hours. The sphincter muscles are what keeps the bladder from leaking. Nerves in the bladder contact your brain to let you know it is time to empty. The sensation of needing to urinate will increase as the bladder fills. When urinating, the brain tells the bladder muscles to tighten and your sphincter muscles to relax, allowing urine to pass from the bladder to the urethra.