The Kidneys and Keeping Clean Inside

Your built-in cleaning system

Every second, your cells are busy making energy, building and repairing — and that work creates waste, a bit like a kitchen that makes rubbish while cooking. If this waste built up in your blood, it would quickly make you very ill. So your body has a brilliant cleaning system that filters your blood and washes the waste away. At the centre of it are two bean-shaped organs called the kidneys.

Your kidneys are part of the urinary system, and they work closely with the blood pumped around by the circulatory system: heart and blood. In this lesson we will follow a drop of blood through the kidneys and see exactly how they keep you clean inside.

Where the kidneys are

You have two kidneys, one on each side of your spine, tucked just below your ribs at the back of your body. Each one is about the size of your clenched fist and shaped like a large bean. (In fact, the kidney bean is named after this shape.)

Even though they are small, the kidneys do an enormous job. All of your blood — around 5 litres — passes through them again and again, so that over a single day they filter the equivalent of about 180 litres of fluid. Almost all of that is cleaned and returned to your body; only a small amount leaves as waste.

How the kidneys clean your blood

Each kidney contains around a million tiny filters called nephrons. Here is what happens inside them:

1. Blood arrives. Blood full of waste flows into the kidney through a large blood vessel.
2. Filtering. Inside each nephron, the blood is pushed through a fine filter. Small things, like water, salts and waste, pass through. Useful big things, like blood cells and proteins, are too large and stay in the blood.
3. Taking back the good stuff. Your body does not want to lose all that water and salt, so the nephron carefully reabsorbs the amount you still need and returns it to your blood.
4. Making urine. What is left — the waste plus the extra water your body does not need — becomes a liquid called urine.

The clean blood flows back out to the body. The urine trickles down two tubes called ureters into a stretchy storage bag called the bladder, where it waits until you go to the toilet.

Balancing water and salts

The kidneys do more than remove waste. They are also your body's balancing experts, keeping the right amount of water and salts inside you.

• On a hot day, or when you exercise and sweat, you lose water. Your kidneys save water, so your urine becomes darker and more concentrated.
• When you drink a lot, your kidneys let the extra water go, so your urine becomes paler and there is more of it.

This balancing act keeps your blood at just the right strength for all the systems of the human body to work. Getting it wrong, in either direction, would harm your cells, so the kidneys adjust the balance minute by minute, all day and night.

Why keeping clean inside matters

Because the kidneys filter your whole blood supply, looking after them keeps your entire body healthier. Here is how to help them:

• Drink enough water. Water lets the kidneys flush waste away easily instead of straining. A good sign is pale yellow urine.
• Eat a balanced diet. Too much salt makes the kidneys work harder, so eat salty snacks only in small amounts.
• Be active. Exercise keeps your blood pressure healthy, which protects the delicate filters in your kidneys.

If kidneys ever stop working, waste builds up dangerously. Doctors can use a dialysis machine to clean the blood instead, or give a person a healthy donated kidney in a transplant. But for most people, simply drinking water and eating well keeps these two quiet, hard-working organs filtering happily for a whole lifetime.

Try it: be a kidney filter

This safe activity shows you how filtering separates waste from useful liquid.

You will need a cup of water, a spoonful of sand or soil, a small spoonful of salt, a coffee filter or kitchen paper, and an empty cup.

1. Stir the sand and salt into the cup of water. This dirty water stands for blood carrying waste.
2. Place the coffee filter over the empty cup and slowly pour the dirty water through it.
3. Watch what happens. The sand stays behind in the filter, while clearer water passes through into the cup below.
4. Taste-free test: notice the water that came through is cleaner but still slightly salty. (Do not drink it.)

Why it works: Your kidneys filter your blood in a similar way. The filter trapped the big bits (the sand), just as the nephron keeps blood cells in your blood, while letting water and tiny things through. Real kidneys are far cleverer, though — they can also choose to keep the right amount of salt and water and send only the true waste away as urine. That careful filtering is what keeps you clean and balanced inside.