Expansion
The balance point where inward lung recoil equals outward chest wall recoil
Expansion
Functional residual capacity is the volume left after a normal expiration, about 2500 ml or 30 ml per kilogram. It is the point at which the inward elastic recoil of the lung exactly balances the outward recoil of the chest wall.
Reduced by
- Lying supine, pregnancy, obesity (abdominal contents push the diaphragm up)
- General anaesthesia and muscle relaxation, which lose chest wall tone
- Pulmonary fibrosis and oedema, which increase lung recoil
- Age extremes at either end
Increased by
- Emphysema, where recoil is lost
- Asthma with gas trapping
- Standing and PEEP
Two consequences follow. It is the oxygen reservoir, so a low value shortens the time to desaturation during apnoea, which is why the obese and pregnant desaturate rapidly. And when it falls below closing capacity, small airways close during tidal breathing, causing shunt and hypoxaemia.