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by Junior Resident (2.0k points)
Closing capacity is the volume of the lungs at which the airways begin to close during expiration. Its value is lowest in the late teens and above this age it progressively increases. If the FRC is less than the closing capacity some airways will always be closed during normal tidal breathing. Blood will pass (or shunt) through the closed (non-ventilated) areas of the lungs and the arterial PO2 will fall. In subjects with normal lungs, closing capacity becomes equal to FRC in the 60s (and in 40s when supine). Factors which increase closing capacity cause arterial hypoxemia and they are:

 1. Smoking and COPD
 2. Obesity
 3. Abdominal distension
 4. Rapid IV infusion
 5. Left ventricular failure
 6. Recent surgery
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by Doctor of Medicine (5.6k points)
It is the volume at which airway closes. Normally it is 1 litre less than functional residual capacity (FRC). If FRC falls below closing capacity (CC), there will be significant hypoventilation and V/Q mismatch. It is measured using Helium dilution technique.

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