The role of the kidneys is to cleanse the blood and balance its fluids:
They manufacture urine to filter out substances that the body does not need.
They also take nutrients and other substances from the urine and return them to the blood.
Urine is manufactured in the kidney's renal tubules. There are approximately one million of these in each kidney. The raw material the body uses to manufacture urine is filtered blood serum, which is the plasma portion of the blood (minus proteins and blood cells).
The renal tubules process the filtered blood serum and remove the substances that will become urine
These tubules lead into collecting ducts.
These ducts empty the urine into small chambers referred to as renalcalyces.
The calyces funnel the urine into the renal pelvis, a basin-shaped cavity at the base of each kidney.
The ureters are tubular organs that lead from the renal pelvis of the kidney to the bladder. The ureters transport small quantities of urine when their muscular walls contract rhythmically. This pushes the urine in a wave-like fashion through the ureters from the kidneys to bladder.
The bladder is a storage organ for urine. The bladder wall has the capacity to stretch like a balloon, enabling the bladder to expand as the volume of urine increases.
When enough urine is in the bladder, receptors that respond to the stretch in the bladder wall will send electrical signals to the brain. This creates the need to urinate.
The urethra is a tubular organ that transports urine from the bladder to the outside of the body.
In men, the urethra runs through the penis.
In women, the urethra has its own opening within the vagina
Inside the body, the urethra is surrounded by a muscle that a person can control. This muscle must relax in order for urination to occur.