Human Excretory System

Every kitchen produces rubbish. Your body is like a kitchen that never stops cooking — and the "rubbish" it makes must be removed, or it would build up and harm you.

Excretion is the removal of waste products made by the body's own chemical reactions. These wastes are made inside cells and would be poisonous if they piled up.

It is easy to confuse excretion with another word, egestion:

• Excretion removes waste that the body has made itself — for example carbon dioxide and urea.
• Egestion removes undigested food — the leftovers of a meal that the body never absorbed in the first place. This waste leaves as faeces.

The key difference: egested waste was never really part of your body's reactions; excreted waste was made by your body. So when you breathe out carbon dioxide, that is excretion. When undigested food leaves the gut, that is egestion.

Your body makes two main waste products that it must get rid of.

Carbon dioxide is made during respiration — the reaction in every cell that releases energy from food. Carbon dioxide travels in the blood to the lungs and is breathed out.

Urea is made in the liver. When the body has more protein than it needs, the liver breaks the extra down, and urea is formed as a waste product. Urea is poisonous if it builds up, so the blood carries it to the kidneys to be removed.

So two organs do most of the excreting: the lungs remove carbon dioxide, and the kidneys remove urea. There is also excess water and salt, which the kidneys remove too.

You have two kidneys, one on each side of your lower back. They are about the size of your fist and shaped like a bean.

Their job is to filter the blood. Blood full of waste flows into each kidney, the kidney removes the waste, and clean blood flows back out.

As the blood is filtered, the kidney removes three things:

• Urea — the poisonous waste from the liver.
• Excess water — any water the body does not need.
• Excess salt — any salt above what the body needs.

Useful substances, such as the right amount of water and salt and all the blood's nutrients, are kept in the blood. The waste removed becomes a liquid called urine.

Once the kidneys have made urine, it follows a clear pathway out of the body.

The pathway has three clear stages:

• Ureters — two tubes that carry urine down from the kidneys.
• Bladder — a stretchy, muscular bag that stores urine until you are ready to release it.
• Out of the body — when the bladder is full, the urine is released through a tube to the outside.

The bladder is useful: it lets the kidneys keep working all the time, while you only need to release urine now and then.

Your skin is also part of excretion. When you sweat, the liquid that comes out — sweat — contains water, a little salt and a tiny amount of urea. So sweating removes a small amount of waste.

But sweat does another important job: it helps control your body temperature. When you are hot, sweat spreads over your skin and evaporates. Evaporation takes heat energy away from your body, which cools you down.

Your body also has to keep its water balance steady — the right amount of water inside. This is a team effort:

• On a hot day you sweat a lot and lose water, so the kidneys keep more water in the blood. Your urine becomes more concentrated.
• On a cold day you sweat little, so the kidneys remove more water as urine.

This is why your urine looks paler when you have drunk plenty of water and darker when you have not. The excretory system is always adjusting to keep the inside of your body just right.

Understanding excretion links to several later topics:

• Respiration — the carbon dioxide your lungs excrete is the waste product of releasing energy in cells.
• The circulatory system — blood carries waste to the lungs and kidneys, so transport and excretion work hand in hand.
• Keeping the body balanced — the idea that your body controls water, salt and temperature is a theme you will meet again and again.
• Health and the kidneys — knowing how kidneys work helps you understand why kidney health, and drinking enough water, matter.

Whenever a later topic asks how the body gets rid of something it has made, picture the clean-up team: lungs, kidneys and skin.