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. 2018 Aug 21;9:1168. doi: 10.3389/fphys.2018.01168

FIGURE 2.

FIGURE 2

This ecosystem is shaped by lung physiology, which changes radically from the upper respiratory tract to the alveoli. Indeed, in the trachea, the airway has a diameter of about 15 mm, and the partial pressures of oxygen and carbon dioxide are similar to those in the external environment. Temperature and contact area are low. Moving toward the alveoli, temperature, contact area, and the partial pressure of carbon dioxide increase, whereas the partial pressure of oxygen and airway diameter decrease. The barometric pressure in the lung is dependent on pulmonary ventilation. At steady state (no air flow) the pressure is similar in the alveoli and in the external environment. During inspiration and expiration, the pressure falls, and increases, respectively. The airway environment also changes and may favor the selection of certain bacteria, leading to the installation of pathogens.