The total lung capacity is TLC = IRV + Vt + ERV + RV
IRV - Inspiratory reserve volume - The additional air that can be inhaled after a normal tidal breath in. The maximum volume of air that can be inspired in addition to the tidal volume.
Vt- Tidal volume - The amount of air breathed in or out during normal respiration.
ERV -Expiratory reserve volume - The amount of additional air that can be pushed out after the end expiratory level of normal breathing
RV- Residual volume - The amount of air left in the lungs after a maximal exhalation. The amount of air that is always in the lungs and can never be expired.
Even after a maximum expiration, residual volume is always present in the lungs. This prevents the collapse of the alveoli and the lungs at large.
Functional Residual Capacity (FRC): It is the amount of air left in the lungs after a tidal breath out i. e. the amount of air that stays in the lungs during normal breathing. This is a reservoir for gas exchange .The FRC is the lung’s physiologic reserve.
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