The contraction of the diaphragm and the inter coastal muscles produces expansion of the chest cavity, so air enters into the lungs during inspiration. A forced inspiration can produce additional expansion, so more air can enter the lungs . Similarly, a forced expiration can expel an extra volume of air.
Vital capacity is defined as the volume of the air that can be expelled by a forced expiration after a forced inspiration.
Tidal air is the volume of the air passing in and out of the lungs with ordinary quiet breathing.
Inspiratory reserve is the additional volume of air that can be taken in by forced inspiration.
Expiratory reserve is the volume of air that can be expelled by forced expiration after normal inspiration.
Residual volume is the volume of the air which remains in the lungs on forced expiration after normal inspiration.
It is the sum of vital capacity and residual volume.