- Closing Capacity is the volume in the lungs at which its small airways begin to collapse.
- The closing capacity = residual volume + closing volume. There is normally enough air within the lungs to keep these airways open throughout both inhalation and exhalation.
- Measured with a single breath of O2 to washout N2, and 4 phases are observed:
- 1) Pure dead space exhaled - 100% O2
- 2) Mixture of dead space and alveolar gas exhaled
- 3) Pure alveolar gas exhaled - constant N2 content
- 4) Abrupt increase in N2 content seen indicating closure of the airways in the lung base, and preferential emptying of the apex. This is because with a vital breath of O2, this region expands less and so N2 there will be less diluted than O2
- Closing volume is ~10% of VC in normal subjects
- As the lungs age, there is a gradual increase in the closing capacity - 40% of VC - or equal to FRC at age 65
- This also occurs with certain disease processes, such as asthma, chronic obstructive pulmonary disease, and pulmonary edema.
- Any process that lowers the functional residual capacity (FRC) can increase an individual's risk of hypoxemia, as the small airways may collapse during exhalation, leading to air trapping and atelectasis.