The kidneys serve a number of different functions, with their primary role being to filter the blood to remove waste products and excess water. By removing any unneeded water from the blood and thus creating urine, the kidneys also help to keep the body's water level balanced.
When the kidneys filter the blood, they remove toxins, excess salts and urea, which is then mixed with the excess water to create urine. Urine is then transported from the kidneys to the bladder through tubes known as ureters.
The kidneys also help to regulate acid levels in the body, as well as red blood cells. When the kidneys don't receive enough oxygen, they produce a hormone known as erythropoietin, which sends out a signal that causes the body to produce more red blood cells.
In addition, they also play a major role in helping the body to regulate its blood pressure as well. They do this by producing a protein known as angiotensin when blood pressure is too low, which works to constrict the blood vessels and raise the pressure back up. This protein signals the body to hold on to water and salts as well, which also helps to boost the blood pressure.