Analysis: The process of gas exchange between the lungs and the external environment is called lung ventilation. Pulmonary ventilation is achieved through respiratory movement. Respiratory movement refers to the rhythmic expansion or contraction of the chest. It includes two processes: inspiratory movement and expiratory movement.

(1) inspiratory movement

In the state of calm breathing, the external intercostal muscles contract, the ribs are lifted, and the sternum moves upward and outward, which increases the anterior and posterior diameter and the left and right diameter of the thorax. At the same time, the diaphragm contracts and the top of the diaphragm drops, which increases the upper and lower diameters of the chest. At this time, the thorax expands, the lung expands, the lung volume increases, the air pressure in the lung drops, and the outside air enters the lung through the respiratory tract. Complete the inhalation action.

(2) Exhale exercise

When the external intercostal muscle relaxes, the ribs descend due to gravity, which reduces the anterior-posterior diameter and left-right diameter of the thorax. At the same time, the diaphragm relaxes and the top of the diaphragm rises, thus reducing the upper and lower diameters of the thorax. At this time, the chest contracts, the lungs also retract, the lung volume becomes smaller, and the air pressure in the lungs rises, forcing some gas in the alveoli to be discharged through the respiratory tract to complete the exhalation action.