Human beings are adopted for terrestrial mode of life. They conduct pulmonary respiration. This system consists of external nostrils, nasal cavities, pharynx, larynx, trachea, bronchi, bronchioles and lungs. The functional units of the lungs are the thin walled chambers called alveoli.
MECHANISM OF RESPIRATION: Respiration in man occurs in two stages: inspiration and expiration.
a) INSPIRATION: Air enters the lungs through nostrils, nasal cavities, pharynx, and lastly into lungs. The thoracic cavity is increased by pulling the diaphragm down and pushing the ribs forward. Gaseous exchange occurs in the alveoli of lungs.
b) EXPIRATION: During expiration, upward movement of diaphragm and inward movement of ribs reduces thoracic cavity. So the column in lungs is decreased and the air under pressure rushes out of the lungs. In man diaphragm plays a major role in respiration. While in women the ribs play a major role.