How to enlarge the chest? The internal and external causes of lung enlargement are expounded.

When the human body is quiet, it breathes peacefully and evenly, which is called calm breathing. It is caused by the relaxation and contraction of diaphragm and external intercostal muscle. When inhaling quietly, the diaphragm contracts, the diaphragmatic crest descends, and the upper and lower thoracic diameter increases. At the same time, the contraction of the external intercostal muscle affects the rib lifting and slightly abduction, and the sternum also moves forward and upward, which increases the anterior and posterior diameter and left and right diameter of the thorax. When the thorax expands, the lung expands and the volume increases, causing inhalation; When exhaling calmly, the diaphragm and external intercostal muscles relax, and the diaphragm top, ribs and sternum return to their original positions, which reduces the volume of the thorax and lungs and produces exhalation. When the human body is working or exercising, it is called forced breathing. Different from calm breathing, when breathing hard, in addition to diaphragm and intercostal muscle contraction, other auxiliary inspiratory muscles (such as sternocleidomastoid muscle and pectoralis major muscle, etc. ) also contracts, further expanding the chest and increasing the inspiratory volume. When forcibly exhaling, in addition to the relaxation of inspiratory muscles, there are also contractions of expiratory muscles such as intercostal muscles and abdominal muscles, which make the volume of chest and lungs smaller and the expiratory volume increase. Therefore, when breathing hard, inhalation and exhalation are active processes. The reason why the lung can expand and contract with the expansion and contraction of the thorax is not only due to the expandability of the lung, but also related to the characteristics of pleural cavity and the pressure in pleural cavity. Pleural cavity is a closed potential cavity surrounded by pleural wall and visceral layer. There is no gas in the pleural cavity, only a small amount of serosity. The cohesion of serous molecules makes the two pleura stick together, which is not easy to separate, so the lung can expand and contract with the expansion and contraction of the thorax. Respiratory movement caused by contraction and relaxation of respiratory muscles is the power of lung ventilation, that is, thoracic cavity expansion caused by contraction of respiratory muscles is the external cause of lung expansion, and the pressure difference between pulmonary pressure and atmospheric pressure caused by respiratory movement is the direct power to realize lung ventilation, that is, pressure difference is the internal cause of lung expansion.