QS Study

Mechanism of changes in intra-alveolar and intraplural pressure during a different phase of respiration.

Three main pressures come into play during a different phase of respiration.

Intra alveolar pressure

Intra-alveolar pressure is the pressure of the air within the alveoli, which changes during the different phases of breathing. The expansion of the lungs causes the pressure in the lungs (and alveoli) to become slightly negative relative to atmospheric pressure. Because the alveoli are connected to the atmosphere via the tubing of the airways, the interpulmonary pressure of the alveoli always equalizes with the atmospheric pressure.

The air pressure inside the lung alveoli. (Normally 0 cm H2O)

  • In quite an inspiration is small negative (- 1 cm H2O)
  • This is generated by the expansion of the alveolar walls and this draw 0.5L air in from the atmosphere within 3 seconds.
  • During expiration, the elastic recoil of the stretch lung products a positive (+ 1 cm H2O) intra alveolar pressure.

Infra pleural pressure

Intrapleural pressure is the pressure of the air within the pleural cavity, between the visceral and parietal pleurae. However, due to certain characteristics of the lungs, the intrapleural pressure is always lower than, or negative to, the intra-alveolar pressure. Although it fluctuates during inspiration and expiration, intrapleural pressure remains approximately –4 mm Hg throughout the breathing cycle.

The pressure between the parietal and visceral pleura –

  • During inspiration, contractor of the diaphragm and inspiratory muscle, external intercostals muscle increase the outward forces causes the intrapleural pressure to fall about – 7.5 cm H2
  • In forced inspiration, it is much more negative (-30 cm H2O).
  • In forced expiration, it is strongly positive (+ 20 cm H20); which deflate the lungs more rapidly.