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This part of the respiratory tract consists of the nose, oral cavity (mouth), pharynx, larynx (voice box), and trachea (wind pipe). Atmospheric air—which is mostly nitrogen— enters through the nose or oral cavitywhere it is filtered and humidified, passes through the structures of the throat, and then enters the lower respiratory tract. Upper respiratory tract infections (e.g. sinus infections, common cold, strep throat) are some of the most common infections in the world and are largely non-fatal.

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 CO₂ builds up in his system, he will be forced to exhale!</ref> There are muscles between In humans, the ribs known as ''intercostal muscles'' which apply pressure diaphragm is the primary muscle of respiration. At rest, the diaphragm is a dome that projects into the lungs for exhalationchest. When it contracts, it flattens, and relax for inhalationthe volume of the chest cavity increases. The diaphragm also plays an integral part 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.

The process of gas exchange that occurs within Gas is exchanged in the lungs is by diffusion&mdash;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 diffuses into the blood from the alveoli, and the CO₂ molecules from the blood (a waste product of cellular respiration) enter into the alveoli and are then exhaled. In order for the this diffusion to workoccur, there must be a higher saturation concentration of oxygen molecules in the alveolialveolar air than in the blood, and a lower higher CO₂ saturation concentration in the bloodthan in the alveoli. This is achieved through constant respiration, bringing in air that is rich in oxygen and scant in CO₂.