How kidneys clean our blood

Every day, two small organs wash all your blood. How?

You have two kidneys, each about the size of your fist, tucked in at the back of your body.

Think of your blood like a river carrying useful things and also rubbish, like extra water and waste.

Blood reaches each kidney through a tube called the renal artery, and it comes out cleaner through the renal vein.

Inside each kidney are about a million tiny filters called nephrons. Each one works like a tiny coffee filter.

• 1 million nephrons per kidney

First, blood is squeezed through a tangle of tiny vessels. Water, salts, sugar and waste get pushed out.

But the filter is clever. Blood cells and big proteins are too large to squeeze through, so they stay in the blood.

• Blood cells stay in

Next comes the clever part. The filtered liquid flows along a tiny tube, and the body takes back everything useful.

Useful things like sugar, most of the water and needed salts go back into the blood. Waste is left behind.

The leftover waste, called urea, and extra water together make urine.

• Waste + extra water = urine

Urine trickles down two thin tubes to the bladder, which stores it until you visit the toilet.

Here is one example. If you drink lots of water, your kidneys send out more water and your urine looks clear.

In a day your kidneys filter about one hundred and eighty litres of blood liquid, yet you only pass about one or two.

Quick check. What do kidneys do with the useful sugar and water that get filtered out? To sum up, kidneys filter your blood, keep what is useful, and send out waste and extra water as urine.