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The functional residual capacity of the infant’s lungs is only one half that of an adult in relation to body weight. This difference causes excessive cyclical increases and decreases in the newborn baby’s blood gas concentrations if the respiratory rate becomes slowed because it is the residual air in the lungs that smooths out the blood gas variations.

The functional residual capacity  equals the expiratory reserve volume  plus the residual volume.  This is the amount of air that remains in the lungs at the end of normal expiration (about 2300 milliliters).