Respiration is a biochemical process in the cytoplasm or mitochondria of cells in which sugar molecules are broken down to release the energy contained in their bonds. This energy is captured to create ATP, a molecule containing small amounts of energy and used by all cells.

Respiration is most efficient when the cell uses oxygen to create many ATP. This is known as aerobic respiration. If a cell needs to create more ATP for its activities, then aerobic respiration will need an increase in the oxygen supply. At the same time, aerobic respiration will be creating greater numbers of carbon dioxide molecules which is one of the end products of aerobic respiration.

This is where the respiratory system of higher animals comes in. The typical respiratory system supplies cells with oxygen and removes the carbon dioxide. This process of swapping one gas for another is called gas exchange.

The site of gas exchange in the respiratory system is the alveoli, which are found in the lungs. Alveoli are small, thin-walled air sacs which are clustered at the ends of the bronchioles within the lungs (See diagrams.). Although each is tiny, the multiple alveoli in the lungs help increase the surface area across which gas exhange may occur.