Pleural Pressure Gradient
- Lower regions of the lung ventilate more than upper zones
- This is because intrapleural pressure is less negative at the bottom than the top of the lung, due to the weight of the lung
- Because of this, basal lung is relatively compressed in its resting state, but expands more on inspiration than the apex.
- The base of the lung has a small expanding pressure, a small resting volume, and a big change in volume on inspiration
- The apex of the lung has a large expanding pressure, a big resting volume, and small change in volume on inspiration
- At low lung volumes, intrapleural pressures are less negative because the lung is not well expanded, and elastic recoil forces are smaller
- Intrapleural pressure at the base exceeds airway pressure, and the lung is not expanded but compressed. Ventilation is impossible until local intrapleural pressure < atmospheric pressure.
- The apex of the lung is on a favourable part of the pressure-volume curve, and ventilates well, therefore the normal distribution of ventilation is inverted, and upper regions ventilate better than lower regions
- The compressed region of the lung at the base does not have all its gas squeezed out
- Respiratory bronchioles close first, trapping gas in the distal alveoli
- This occurs at very low lung volumes in young normal subjects, but at higher volumes and even at FRC in elderly people and those with some types of lung disease
- This is due to reduction in lung elastic recoil, and therefore intrapleural pressures become less negative and pressure at the bases exceed airway pressures