Respiration

Describe the inspiratory and expiratory process involving the chest wall, diaphragm, pleura and lung parenchyma

Explain the significance of the vertical gradient of pleural pressure and the effect of positioning

Change in lung volume occurs due to change in intrapleural pressures. Therefore, respiration relies on the thoracic cavity being airtight, with the trachea being the only method gas can enter or exit the chest.

Intrapleural pressure (P_Pl)

Intrapleural pressure is the pressure in the space between the visceral and parietal pleura, or (physiologically) between the lungs and the chest wall.

• Usually negative, typically -5cmH₂O at rest
  Balance between the:
  • Outwards recoil of the chest wall
  • Inwards recoil of the lungs (P_el)
• Varies with vertical distance in the lung
  • Gravity pulls the lung parenchyma inferiorly
  • Intrapleural pressure is therefore:
    • More negative in the apex
      Typically -10cmH₂O at FRC
    • Less negative in the base
      Typically -3cmH₂O at FRC
• This changes the degree of inflation at FRC
  • Apical alveoli are maximally inflated
  • Basal alveoli are relatively deflated
• During inspiration, the pleural pressure changes evenly throughout the lung, however the basal alveoli are better ventilated because their compliance is increased (due to lower resting volume)

Inspiration

• Diaphragmatic and external intercostal/accessory muscle contraction causes an increase in the volume of the thorax
• Intrapleural pressure becomes more negative, typically to -8cmH₂O
• When P_pl > P_el, the lungs expands
• Alveolar pressure (P_A) becomes sub-atmospheric, and inspiration occurs
• At end inspiration:
  • P_pl = P_el
  • P_A = P_atmospheric

Expiration

• Muscular relaxation causes the chest wall to passively return to their resting position
• Thoracic volume falls
• P_pl falls to -5cmH₂O
• The elastic recoil of the lung causes it to collapse until P_A = P_atmospheric

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.