Respiratory

The volume of air moved into or out of the lungs during a single normal quiet breath, approximately 500 mL in an average adult.

The air remaining in the lungs after a maximal expiration. Spirometry cannot measure it because this air is never exhaled; it requires body plethysmography or helium dilution.

FRC = ERV + RV; it is the lung volume at the end of a normal passive expiration, where the inward elastic recoil of the lung balances the outward recoil of the chest wall.

VC = TV + IRV + ERV — the maximum volume of air that can be exhaled after a maximal inspiration. It equals TLC minus RV.

VD = VT × (PaCO2 − PeCO2)/PaCO2, where PeCO2 is expired CO2. It estimates the volume of inspired air that does not participate in gas exchange (anatomic + functional dead space).

Anatomic dead space is the conducting airways (no gas exchange, ~150 mL). Physiologic dead space = anatomic + alveolar dead space (alveoli ventilated but not perfused). In healthy lungs they are nearly equal.