Define residual air capacity?

That portion of air contained in the lungs which can not be expelled even by the most violent expiratory effort is called residual air.

Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration.It can also be caller Residual air capacity.
FRC is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV) and measures approximately 2400 mL in an 80 kg, average-sized male.

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