Pulmonary effusion or fluid in the lungs, the formal medical term for this is called pleural effusion. The normal human chest would have had a few milliliters of fluid that the body would normally produce and then absorb in a dynamic balance. If this balance is disturbed, a pleural effusion can result. Any factor that would make the production of fluid in the chest cavity too fast or slow absorption by the body would eventually lead to pleural effusion. A simple example: traumatic pleural effusion, when the chest suffers external damage, the blood vessels in the chest cavity or between the ribs are damaged and ruptured, and a large amount of blood flows out into the chest cavity in a short period of time, which cannot be absorbed by the body quickly, leading to the occurrence of pleural effusion. In addition to this, there are many factors that lead to the formation of fluid in the lungs, i.e. pleural effusion. For example, chest inflammation, cancer, immune diseases, cardiac insufficiency, hypoproteinemia, and so on, can cause excessive fluid production in the human chest cavity too quickly, or the human body can not absorb it, which eventually makes excessive fluid accumulation in the chest cavity, resulting in pulmonary effusion, or pleural effusion. In summary, pleural effusion is a condition caused by increased secretion or (and) decreased absorption of fluid in the chest cavity for a variety of reasons. Its causes include bacterial infections, tuberculosis, cancer and cardiac insufficiency.