Kidneys and bladder
The kidneys process fluids, maintain blood pressure, acidity and mineral levels, help produce red blood cells and clean the blood. The bladder takes the kidneys' waste and holds it until you are ready to go to the toilet.
There are two of these organs. Healthy kidneys are bean-shaped, about 11 cm long, 5 cm wide and 3 cm thick. They're located about half way down the torso, near the back on either side of the spine.
After processing fluids, the kidneys pass the waste, as urine, to the bladder. The ureters carry a constant trickle of urine from the kidneys to the bladder. The amount of urine produced depends on how much you drink and eat or exercise and sweat.
The bladder holds the urine. It is made up of four layers. The innermost is a membrane, the next has nerves and blood vessels, and then there is the wall of smooth muscle called the detrusor muscle which is surrounded by the outer layer.
As the bladder receives urine from the kidneys via the ureters its elastic tissues expand. When it's expanded enough, you feel the sensation of needing to go to the toilet. When it has expanded to its full storage capacity, you feel an urgent need to get to the toilet.
Urine is expelled from the bladder and out of the body through the urethra. It extends from the bladder to the penis or urethral opening.
The urethra passes through a group of muscles called the pelvic floor. The opening of the urethra is controlled by the urethral sphincter muscle.
Women have a short urethra, only 4 cm. A man's urethra is up to 20 cm long.