The function of the respiratory mechanism is inspiration and expiration. The inspiration will cause the diaphragm to move downward, rib moves upward and make the chest cavity volume increases. The air pressure in the area around the lungs is lower than outside air. Then, outside air will move into the nose to the trachea and lungs. In contrast, if the diaphragm moves upward, rib move downward and make the chest cavity volume decreased. The air pressure in the area around the lungs is higher than outside air ,the air within the lung, thus moving from the lung to the trachea and out through the nose, it is called expiration.