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What are the kidneys?

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The two kidneys lie to the sides of the upper abdomen (the loins), behind the intestines, and either side of the spine. Each kidney is about the size of a large orange, but bean-shaped.

A large renal artery takes blood to each kidney. The artery divides into many tiny blood vessels (capillaries) throughout the kidney. Tiny structures in the kidneys called nephrons 'filter' the blood contained in the capillaries. Water and waste materials which filter through the walls of the capillaries into the nephrons form urine. Urine passes along thin channels (tubules) which are part of each nephron, into larger channels (ducts) which drain the urine into the renal pelvis (the inner part of the kidney).

Urine passes down a tube called a ureter which goes from each kidney to the bladder. Urine is stored in the bladder until it is passed out through the urethra when we go to the toilet.

The 'cleaned' (filtered) blood from each kidney collects into a large renal vein which takes the blood back towards the heart.
Some specialized cells in the kidneys also make some hormones, including:

Renin - which helps to regulate blood pressure.
Erythropoietin - which helps to stimulate the bone marrow to make red blood cells.
Calcitriol - which helps to regulate the calcium level in the blood.

Although it is normal to have two kidneys, we can live perfectly well with just one healthy kidney.