Exhalation

Exhalation is the outward movement of gas from the lungs through the airways. During ordinary quiet breathing, relaxation after inhalation allows elastic recoil to reduce lung volume. Forceful expiration can additionally involve active muscles.^[ref] The term identifies a flow phase, rather than one universal muscle-activation pattern.

For an ideal passive compartment, let $V$ be the excess volume above equilibrium with the outlet pressure. With airway resistance $R$ and respiratory-system compliance $C$,An original example uses $R=10$ cm H${}_2$O·s/L, $C=0.06$ L/cm H${}_2$O and $V_0=0.60$ L. The time constant is 0.60 s, leaving about 0.221 L excess volume after one time constant. After three time constants, only about 0.030 L remains. These are model calculations, not recommended expiration times.

The flow-driving pressure difference is between alveolar pressure and the pressure at the open airway. Pleural pressure affects the distension and geometry of the lung and airway but is not interchangeable with either of those endpoint pressures. Active expiration can also narrow intrathoracic airways, so greater effort need not produce proportionally greater flow.^[ref]

The linear decay model is therefore useful for isolating resistance, compliance and available time, while measured flow can reflect changing airway geometry, active effort or multiple compartments. Completing one expiratory interval is a mechanical event; removing a given amount of carbon dioxide additionally depends on the composition and origin of the expired gas.