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Kidneys are a critical part of your health.  They “clean” your blood by removing wastes from the body through your urine, help your body to make red blood cells, and regulate blood pressure.  One of the main jobs of the kidneys is to remove wastes from the blood and return the cleaned blood back to the body. Every minute, about one litre of blood (one fifth of all the blood pumped by the heart) enters the kidneys through the renal arteries. After the blood is cleaned, it flows back into the body through the renal veins.
A normal kidney can greatly increase its workload: if you were born with one kidney or if one kidney is injured or donated, the remaining kidney can work harder to keep your body healthy.
Inside each kidney there are more than one million tiny units called nephrons. Each nephron is made up of a very small filter called a glomerulus, which is attached to a tubule. Water and waste products are separated from the blood by the filters and then flow into the tubules. Much of this water is reabsorbed by the tubules and the wastes are concentrated into urine.

The urine is collected from the tubules in the funnel-like renal pelvis and then flows through tubes called the ureters into the bladder. Urine passes out of the body through a tube called the urethra. Together, the kidneys normally make one to two litres of urine every day depending on how much you drink.