Outside us is our skin. Inside us is a skeleton and lots parts. All of these are called organs. An organ is a part of the body which does a special task to help the body survive. For example the stomach is an organ which helps digest a substance called protein in food.

Organs are even arranged into groups which help each other to perform a larger task. For example, the stomach forms a group with the mouth, intestines, liver and pancreas called the digestive system. The purpose of this group of organs is to digest all the substances in food so that they can be taken into the blood.

When the digested food enters the blood it travels through another organ system. This is called the circulatory system. It is composed of the heart and three kinds of blood vessels called arteries, veins and capillaries. The purpose of the circulatory system is to circulate materials in the blood to all places in the body.

As the blood passes through the lungs it picks up oxygen from the air and releases carbon dioxide. The lungs form part of the breathing or respiratory system. This is made up by the lungs, windpipe, ribs and diaphragm. They work together to draw air into the chest then blow it out again.

Although we can divide the body up like this, some parts of us are used for more than one job. For example the ribs form part of the skeleton (holding us up) as well as the respiratory system. Did you know it was the liver that provides heat to the blood which the blood then takes round the body to keep us warm?

Perhaps the strangest organ is the one you can see on the surface of your body - your skin. This is the largest organ of the body. This may seem surprising, but although the skin is very thin it also covers a large area and in an adult may be the size of a table cloth.

The skin cools you down with sweat when you are too hot and makes "goose pimples" to save heat when you are cold. It is also sensitive to heat, cold and pressure, and it forms a protective coat which stops germs entering the body. Sometimes when you have been swimming you may dry yourself and flakes of skin fall away. If you looked at them with a microscope you would see that the flakes are made of tiny cells just like the rest of your body.