Lung Volumes, Spirometry & Airway Resistance
Core facts on lung volumes, capacities, spirometric patterns and airway resistance for first-year physiology.
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Questions Covered in This Set
12 cards to master
What is the difference between a lung volume and a capacity?
A volume is a primary, non-overlapping quantity (TV, IRV, ERV, RV); a capacity is two or more volumes added together (IC, FRC, VC, TLC).
List the four lung volumes with typical values.
Tidal volume ≈500 mL; inspiratory reserve volume ≈3000 mL; expiratory reserve volume ≈1100 mL; residual volume ≈1200 mL.
What is FRC and what determines it?
FRC = ERV + RV, the volume at end of quiet expiration where inward lung recoil exactly balances outward chest-wall recoil.
Which volumes/capacities cannot be measured by spirometry, and how are they measured?
Any value containing RV — RV, FRC and TLC. They require helium dilution, nitrogen washout, or body plethysmography.
Why can dilution/washout underestimate lung volume in emphysema?
They measure only communicating gas, so trapped gas in bullae is missed; plethysmography (Boyle's law) measures all compressible gas, so a discrepancy indicates gas trapping.
How do FEV1, FVC and the ratio behave in obstructive disease?
FEV1 falls markedly, FVC normal or slightly reduced, FEV1/FVC <0.7; TLC, FRC and RV increase from air trapping.
How do FEV1, FVC and the ratio behave in restrictive disease?
Both FEV1 and FVC are reduced (FVC more), so FEV1/FVC is normal or increased; TLC, FRC and RV are all reduced.
FEV1 = 1.8 L, FVC = 4.0 L. Interpret.
Ratio = 0.45 → obstructive pattern. The ratio, not FEV1 alone, is the discriminator.
How does DLCO help subclassify restriction?
DLCO is low in intrinsic lung disease (fibrosis, and in emphysema) but normal or high in chest-wall/neuromuscular restriction and in asthma.
Where is the site of greatest airway resistance, and why?
Medium-sized bronchi (generations ~4–8). Small airways have huge total cross-sectional area in parallel, so their combined resistance is low — making early small-airway disease clinically silent.
State Poiseuille's relationship for airway resistance.
R ∝ 8ηl/πr⁴ — halving the radius increases resistance 16-fold.
What is the equal pressure point and dynamic compression?
During forced expiration pleural pressure is positive; at the equal pressure point airway pressure equals surrounding pressure, and downstream the airway is compressed (dynamic compression), limiting flow.
