Mechanics of Breathing: Pressures, Compliance and Surfactant
Core facts on the four respiratory pressures, compliance and elastic recoil during one quiet breath.
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Questions Covered in This Set
10 cards to master
What reference point is used for all respiratory pressures?
Atmospheric pressure, defined as zero; units are cmH₂O (1 cmH₂O ≈ 0.74 mmHg).
Define transpulmonary pressure (P_L) and give its value at FRC.
P_L = P_alveolar − P_pleural. At FRC: 0 − (−5) = +5 cmH₂O — the distending pressure holding the lung open.
Why is pleural pressure negative?
The lung recoils inward while the chest wall springs outward; the two are coupled by incompressible pleural fluid, so the tug-of-war creates a subatmospheric pressure (~−5 cmH₂O at FRC).
What defines functional residual capacity (FRC)?
The volume where inward lung elastic recoil exactly balances outward chest wall recoil, with respiratory muscles relaxed.
What happens to alveolar pressure during quiet inspiration and expiration?
It falls to about −1 cmH₂O during inspiration (air flows in) and rises to about +1 cmH₂O during expiration (air flows out); it is 0 at end-inspiration and end-expiration when flow is zero.
What happens in a pneumothorax?
Pleural coupling is lost: the lung collapses toward its own resting volume and the chest wall springs out → hyperexpanded, hyperresonant hemithorax with absent breath sounds.
Define compliance and give normal values.
C = ΔV/ΔP. Lung alone ≈ 200 mL/cmH₂O; lung + chest wall ≈ 100 mL/cmH₂O (1/C_total = 1/C_lung + 1/C_wall).
Which conditions cause LOW compliance, and how do patients breathe?
Fibrosis, ARDS, pulmonary edema, surfactant deficiency. Stiff lung → rapid, shallow breathing to minimise elastic work; FRC falls.
Why is emphysema a high-compliance problem?
Elastase destroys elastin, so the lung fills easily but recoils poorly → expiratory airflow limitation, air trapping, raised FRC, barrel chest, pursed-lip breathing.
What muscles drive inspiration and quiet expiration?
Inspiration: diaphragm descends plus external intercostals lift the ribs. Quiet expiration is passive (elastic recoil); forced expiration adds abdominal and internal intercostal muscles, which can make P_pl positive.
