Figure 2.03. Changes in lung volume, air flow, intrapleural pressure, and alveolar pressure during normal (tidal) breathing. The dashed intrapleural pressure line would be followed if there were no airway resistance). The diagram at the left shows the lung and a spirometer measuring the changes.
This figure depicts changes in the main parameters of interest during a normal ventilatory cycle in which a Tidal Volume of 400 - 500 ml is being inspired and expired with each breath (quiet breathing). Inspiration is indicated by a downward deflection in the first panel.
Note that during the entire ventilatory cycle, the intrapleural pressure (Ppl) remains negative (2nd panel). The plot normally follows the solid blue line; alternatively it follows the dashed line when where there is no resistance to air flow (imaginary situation).
Note that alveolar pressure (bottom panel) has a non-zero value only when there is air flowing.
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