Breathing is a process of inhalation of air into the lungs through mouth or nose and exhalation of air. Inhalation or inspiration is a active process whereas expiration is passive process. During inhalation the diaphragm and the intercostal muscles contract which enlarges the chest cavity and draws air into the lungs. Relaxation of these muscles forces air out of the lungs at exhalation. Breathing is a part of respiration and is sometimes called as external respiration.