During inspiration and expiration external intercostal muscles and diaphragm are involved. Movement of gases is due to Boyle's law. (Pressure is inversely proportional to volume.) During inspiration the pressure of atmospheric oxygen is more as compared to the pressure inside the lungs i.e. there is a negative pressure in the lungs. This is due to the large volume of the lung cavity. This is due to the muscles that contract to use energy which means muscles are performing work Thus inspiration is an active process. During expiration, the muscles of diaphragm relax. The pressure inside the lungs increases than the atmospheric pressure and thus air leaves the lung cavity and expiration takes place. This process does not use energy and hence it is a passive process.