Explanation

Your kidneys are two bean-shaped organs that clean and filter your blood. They are about the size of your fist, and sit just inside the bottom of your ribcage at the back - one on each side of your spine. About one quarter of the blood that your heart pumps goes to your kidneys. Your kidneys also tell your bone marrow when to make more red blood cells.

Unfiltered blood enters each of your kidneys through your renal arteries. Once inside, these blood vessels start to branch off, and become smaller and smaller. Each one eventually ends in a nephron, into which blood is forced at high pressure. The smaller parts and plasma are forced out through the walls, but the blood cells cannot pass through and stay in the blood vessels.

The plasma, supplies and wastes pass into a different tube, called the ureter. The blood vessels wind around the ureter and suck up anything your body still needs, like supplies and water. This clean, filtered blood then leaves your kidneys through your renal vein to go back into the rest of your body. The leftover waste is urine, and this continues straight on through the ureter to your bladder.

Your kidneys control the amount of supplies and water in the blood vessels inside them. If there's too much water in your blood, your kidneys get rid of the extra as urine, and you wee more. If there's not enough, your kidneys keep hold of more water. Your body controls your blood pressure by controlling the amount of water in your blood.