During a breathing cycle, why do lungs always contain a residual volume of air in it?
It gives sufficient time for O2 absorption and CO2 release.
Residual volume is the amount of air that remains in a person's lungs after exhaling fully. During the breathing cycle, when air is taken in and let out, the lungs always contain a residual volume of air to ensure that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released.