Respiratory system consists of certain air channels and the lungs. The respiratory passage consists nostril, nasal chamber, posterior nares, pharynx,epiglottis, glottis, larynx or voice box, trachea or windpipe and two bronchi. In expiration, the ribs and diaphragm return to their original position, the chest cavity volume is decreased. The distended elastic lungs then contract and the air is forced out. Changes in the intrapleural pressure are responsible for air entering and leaving the lungs. In inspiration, expansion of the thorax, aided by descent of the diaphragm, decreases intrathoracic pressure from 4 to 10 mm Hg, and air pushes into the lungs.