Figure 5.01. Effect on distribution of ventilation due to inspiration
from FRC (panel A) vs. starting from RV (panel B).
Because of the way in which the lungs are suspended in the chest cavity and are subjected to gravity, a gradient in pleural pressure exists from the apex (top, blue) to the base, of about 7.5 cm H2O. If a subject starts his inspiration from Functional Residual Capacity (FRC) (apex -10, base -2.5; both in cm H2O), apical alveoli will start off at a larger volume than the basal ones due to the more negative apical pressure. Thus apical alveoli will be less able to accommodate further increase in volume (i.e. they are already stretched and thus less compliant than basally). Thus, if inspiration starts from FRC, most of the incoming air goes preferentially to basally located alveoli which began less distended and thus more compliant.
If inspiration starts from RV (following a very energetic expiration to get below FRC), all the incoming air goes initially to apical alveoli where pleural pressure is still negative (-4 cm H2O). Basal areas are experiencing a positive pleural pressure of +3.5 cm H2O, which causes the collapse of small airways. Thus air inflow is initially prevented in that area until basal pressures become negative again and allow airway opening.
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