The Human Body is the most studied object in all of science. Yet every year we learn even more about its most detailed structure and its innermost workings. Even in ancient times people have known basic facts-for examples, there are 206 bones in its skeleton. Since the invention of the microscope nearly 400 years ago, people have studied the body's billions of tiny building blocks, known as cells. In more recent years we have learned about the instructions or "blueprints" for making its body-its genes. There are also over 100 body parts.

The body's main parts, like the brain, heart, lungs and stomach, are called organs. Different groups of organs work together as systems. Each system has a vital job to keep the whole body alive and healthy. For example, the heart, the body-wide network of tubes, blood vessels, and the red liquid called blood, together form the circulatory system. This carries essential nutrients and oxygen to all our body parts and collects waste nutrients for disposal. There are about a dozen major systems in are body.

Pumped by the heart, blood collects oxygen from the lungs and dissolved food from the liver and delivers it to all parts of the body. It gets to the other parts of the body through Veins and arteries.

If you ever stop breathing, you would very rapidly die. Oxygen in the air is vital to survival of every cell of the body. Just as fire burns only if there is plenty of oxygen, so cells need oxygen to break down food they get from blood. Without oxygen we would die. Brain cells cannot live for more than few minutes without oxygen- which is which why the brain can be damaged if the heart stops pumping blood.