Why do people die if they stop breathing?

In order to stay alive, human beings need a constant supply of oxygen (a gas found in the air) to all parts of the body. They also need to rid their bodies of a waste gas called carbon dioxide which would otherwise poison them.

  These two gases are carried round the body in the blood. Veins carry blood to the heart and arteries carry blood away from the heart. Both veins and arteries divide into millions of tiny capillary blood vessels. Gases can move between the blood in the capillaries and tiny cells which make up the human body.

  When a human being breathes in, air goes into the lungs, which are like two spongy bags filled with millions of air sacs. Oxygen from the air passes through the sacs into the capillary blood vessels. The blood then carries the oxygen through a vein to the heart.

  The heart pumps this oxygen-carrying blood around the whole body through arteries which divide into capillaries to reach the body cells. Oxygen passes from the blood to the cells, and carbon dioxide (the waste gas) passes from the cells into the blood. Veins take this waste-carrying blood back to the heart which pumps it back to the lungs. There the carbon dioxide passes into the air sacs.

  When the human being breathes out, the carbon dioxide is pushed back into the air. Breathing in and out is therefore essential because it ensures that life-giving oxygen is constantly replaced and poisonous carbon dioxide expelled.