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The human '''respiratory system''' is responsible for the exchange of [[carbon ]] dioxide (CO<sub>2,</sub>) and [[oxygen ]] (O<sub>2</sub>). Oxygen is essential in the production of adenosine triphosphate (ATP)—the nucleotide responsible for cell metabolism the "fuel" of the cell— by the mitochondria. [[Human ]] beings cannot live without a properly functioning respiratory system unless some sort of mechanical intervention (e.g. ventilator, heart-lung bypass) is implemented.
==Anatomy==
====Lower Respiratory Tract====
This part of the respiratory tract consists the of the bronchi, bronchioles, and alveoli. All of these structures are contained within the [[lungs]]. The alveoli are responsible for the gas exchange mentioned above.
==Physiology==
<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> In humans, the [[diaphragm ]] is the primary muscle of respiration. At rest, the diaphragm is a dome that projects into the chest. When it contracts, it flattens, and the volume of the chest cavity increases. This change in volume causes the pressure in the chest cavity to drop, and air moves into the lungs due to this pressure difference. Contraction and relaxation of the intercostal muscles, which stretch between the ribs, play a smaller role in respiration. The intercostal muscles can increase or decrease the volume of the chest depending on which specific muscles contract.
Gas is exchanged in the lungs 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 then diffuses into the blood from the alveoli, and the CO<sub>2</sub> molecules from the blood (a waste product of cellular respiration) enter the alveoli and are then exhaled. In order for this diffusion to occur, there must be a higher concentration of oxygen molecules in the alveolar air than in the blood, and a higher CO<sub>2</sub> concentration in the blood than in the alveoli. This is achieved through constant respiration, bringing in air that is rich in oxygen and scant in CO<sub>2</sub>.
