Define functional residual capacity and closing volume, what is there importance in anaesthesia practice?enumerate factors affecting FRC and CV

Functional Residual Capacity is the lung volume at the end of a normal exhalation. This is the volume which  balances the outward pull of the structures of the thoracic cage and the inward elastic recoil of the lung parenchyma 

Importance in anaesthesia practice

Factors affecting the FRC and closing capacity

Closing capacity is the volume at which the small airways which lack the cartilaginous structures and depend on traction caused by the elastic recoil of the adjacent tissue for patency start to close especially in the dependent part of the lungs.

Induction with general anaesthesia reduces the FRC and closing capacity, this decrease is due to cephalic movement of the diaphragm.Other factors affecting the lung volume and compliance is the position of the patient, steep head down .The closing capping reduction can increase the intrapulmonary shunting which is more in elderly, obese and patients with lung diseases.

1) Body Habitus : FRC is directly proportional to height

2) Sex: FRC is less in females as compared to males

3) Postures:FRC is reduced in supine position 

4) Lung Disease: Restrictive lung diseases reduces the FRC and compliance of the lung

5) Diaphragmatic tone: affects the FRC