The main organs of the respiratory system are the lungs (soft, spongy organs that fill most of the space in the chest) and the airways that lead to the lungs, such as the trachea (windpipe). Inflated by the muscles surrounding them, the lungs suck in air and allow oxygen to pass into the blood, while waste carbon dioxide gas passes in the other direction.
The human body is powered by the same chemical process that powers a car. Inside a car engine, fuel reacts with oxygen to release energy, which turns the wheels. In the human body, food molecules react with oxygen inside cells. The process of obtaining oxygen from the air and then using it in cells is called respiration. The organs that bring oxygen into the body make up the respiratory system.
Food molecules contain trapped chemical energy, just as gasoline put into a car contains trapped energy. Living cells release this energy through a process called cellular respiration. Molecules such as sugars are made to react with oxygen molecules.
The reaction breaks the chemical bonds in sugar and releases trapped energy. Carbon dioxide and water are produced as by-products. Carbon dioxide is poisonous in large amounts, so the bloodstream carries it away to be breathed out by the lungs.
Sugar + Oxygen → Carbon dioxide + water + energy
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Cellular respiration takes place in microscopic power plants called mitochondria, which are found in every cell. Some cells have only one or two mitochondria, but a cell that uses a lot of energy - such as a muscle cell - may have hundreds. Each mitochondrion is enclosed by two membranes: a flat outer membrane and a deeply folded inner membrane. The chemical reactions of respiration happen on the inner membrane.