This is the pressure, measured in cm H20, within the alveoli, the smallest gas exchange units ofthe lung. Alveolar pressure is given with respect to atmospheric pressure, which is always set tozero. Thus, when alveolar pressure exceeds atmospheric pressure, it is positive; when alveolarpressure is below atmospheric pressure it is negative.
Alveolar pressure determines whether air will flow into or out of the lungs. When alveolarpressure is negative, as is the case during inspiration, air flows from the higher pressure at the mouth down the lungs into the lower pressure in the alveoli. When alveolar pressure is positive,which is the case during expiration, air flows out.At end-inspiration or end-expiration, when flow temporarily stops, the alveolar pressure is zero (i.e., the same as the atmospheric pressure).