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<small>''This section contains information on the mechanism of respiration and gas exchange. For further information on systemic oxygenation, see [[pulmonary circulation]].''<ref>Under construction</ref></small>
Breathing is an active process controlled by the hypothalamus.<ref>So if your child is throwing a tantrum and holds his breath, don't fret! As the CO<sub>2</sub> builds up in his system, he will be forced to exhale!</ref> There are muscles between the ribs known as ''intercostal muscles'' which apply pressure into the lungs for exhalation, and relax for inhalation. The diaphragm also plays an integral part in respiration.
The process of gas exchange that occurs within the lungs is by diffusion—the process of molecules moving from an area of high concentration to an area of low concentration. Upon inhalation, oxygen molecules are directed into the alveoli by the bronchioles. The oxygen is then diffused into the blood from the alveoli, and the CO<sub>2</sub> molecules from the blood (a waste product of cellular respiration) enter into the alveoli and are then exhaled. In order for the diffusion to work, there must be a higher saturation of oxygen molecules in the alveoli, and a lower CO<sub>2</sub> saturation in the blood. This is achieved through constant respiration, bringing in air that is rich in oxygen and scant in CO<sub>2</sub>.
