The thoracic cavity and the pleural cavity are two concepts that are easily confused, even by some doctors. In fact, there are some differences between these two concepts, the main differences are: First, the concept is different: the thoracic cavity refers to the medial wall of the thorax, the upper thoracic opening and the diaphragm enclosed in a relatively large cavity called the thoracic cavity; the pleural cavity refers to the potential cavity formed by the displacement of the dirty layer of the pleura and the wall layer, which is very small, and the normal pleural cavity contains only a very small amount of fluid. Second, the two contain different contents: the pleural cavity contains very many organs and tissues, including the heart, lungs, large blood vessels, etc. are within the scope of the pleural cavity; relative to the pleural cavity contains less tissue, only a very small amount of fluid. Third, the pathological significance of pleural cavity and thoracic cavity is different: because the pleural cavity is airtight, some open-heart surgery and heart surgery can be performed without going through the pleural cavity, thus affecting the lung and other tissues less and with fewer postoperative complications; because there is no gas in the pleural cavity and only a very small amount of fluid, if gas and fluid accumulate in the pleural cavity, it can have adverse effects on the heart and lung and needs to be discharged in time.