Fluid in the lungs and pleural effusion are essentially the same disease, both refer to fluid in the pleural cavity, they are just called differently. The pleural cavity is a chamber between the lungs and the chest wall, and between the wall pleura and the dirty pleura, there is a very thin layer of fluid that serves as a lubricant. The production and consumption of this layer of fluid is in a state of dynamic equilibrium. When pathological factors break the equilibrium, it can lead to an increase in this layer of fluid, which accumulates in the pleural cavity and is called pleural effusion. The pleural cavity is originally a negative pressure chamber, which adsorbs the lung tissue to the chest wall, and the lung tissue is in a state of expansion and dilatation, which is conducive to exhalation and gas exchange. When fluid builds up in the pleural cavity, the fluid will break the balance of negative pressure in the chest cavity and compress the lung tissue, which is also called pulmonary effusion. There is no difference between fluid in the lungs and pleural effusion. With pleural effusion, it is recommended to seek timely medical treatment and standardize the treatment under the guidance of professional doctors.