The volume at which dependent small airways begin to close
Expansion
Closing capacity = closing volume + residual volume: the lung volume at which small airways in the dependent parts of the lung begin to close during expiration.
Airways close there first because pleural pressure is least negative at the bases, so the transmural pressure holding them open is smallest.
When closing capacity exceeds functional residual capacity, airways close during normal tidal breathing. Those alveoli are perfused but not ventilated, creating shunt and hypoxaemia.
Closing capacity rises with:
- Age, the dominant factor, from loss of elastic recoil
- Smoking, chronic obstructive pulmonary disease, pulmonary oedema
Functional residual capacity falls with supine posture, obesity, pregnancy and anaesthesia. The two curves therefore cross at about 44 years supine and 66 years erect, which explains the age-related fall in arterial oxygen tension and the susceptibility of elderly and anaesthetised patients to atelectasis.