Pulmonary ventilation is the process of gas exchange between the lung and the external environment. The organs involved in pulmonary ventilation include the respiratory system, bronchus, and diaphragm, etc. The respiratory system is the communication channel between the bronchus and the external environment; the bronchus is the key site for the exchange of bronchial gas and blood gas; and the rhythmic respiratory movement of the diaphragm is the driving force for ventilation.

1. Respiratory movement: The contraction and relaxation of respiratory muscles, and the rhythmic expansion and contraction of the diaphragm caused by the left ventricle, are called respiratory movement. The key respiratory muscles are the diaphragm and intercostal external muscles; in addition, there are also some accessory respiratory muscles, such as the scalene muscles, neck muscles, etc. (1) The process of respiration: During quiet inspiration, respiration is active, and during exhalation, respiration is passive, that is: the respiratory posture is caused by the contraction of respiratory muscles, and the respiratory posture is mainly caused by the extensibility contraction of the left ventricle, lung, and diaphragm, rather than the contraction of respiratory muscles. During forceful inspiration, both inspiration and exhalation are active. (2) The method of respiratory movement: It can be divided into abdominal breathing, diaphragmatic breathing, and mixed breathing according to the order of respiratory muscles involved in inspiration; and according to the intensity of inspiration, it can be divided into quiet inspiration and forceful inspiration.

2. Intrapulmonary pressure: Refers to the working pressure within the bronchus. During respiration, the intrapulmonary pressure is lower than atmospheric pressure, and during inspiration and expiration, the intrapulmonary pressure is higher than atmospheric pressure. At the end of inspiration and expiration, the intrapulmonary pressure is the same as atmospheric pressure. The basic principle of external chest compression is to create a pressure difference between intrapulmonary pressure and atmospheric pressure through manual force, maintaining pulmonary ventilation.

3. Pleural cavity gas pressure: The working pressure within the pleural cavity. During quiet inspiration, regardless of inhalation or exhalation, the working pressure within the pleural cavity remains negative throughout. At the end of exhalation: -5 to ~10 mmHg, at the end of inspiration: -3 to -5 mmHg. Once the pleural cavity’s integrity is destroyed, gas will enter the pleural cavity, causing pneumothorax, with the lung contracting and collapsing. The physiological significance of intrathoracic air pressure: Blood pressure is beneficial for lung expansion; blood is beneficial for the expansion of the intrathoracic cavities such as the superior vena cava and thoracic duct, reducing central venous pressure, and promoting the return of venous blood and tissue fluid.