Exudative pleural fluid refers to pleural fluid as exudate, characterized by hematogenous, purulent and celiac appearance, turbidity, specific gravity higher than 1.018, and self-clotting. There are many causes of exudative pleural fluid, which are summarized into the following categories: 1. Increased hydrostatic pressure in the pleural capillaries: Increased hydrostatic pressure in the body circulation is the most important factor in generating pleural effusion, such as congestive heart failure or constrictive pericarditis, which can increase the hydrostatic pressure in the body circulation and/or pulmonary circulation, increase the filtration of pleural fluid and form pleural effusion, and increase the hydrostatic pressure in the body circulation alone, such as superior vena cava or odd vein obstruction, the wall layer pleural fluid exudation exceeds the ability of the dirty layer pleura back absorption, resulting in pleural effusion, this kind of pleural effusion is mostly leakage fluid. 2.Increased permeability of pleural capillaries: Inflammation of the pleura or infection of tissues and organs adjacent to the pleura, pulmonary infarction or systemic diseases involving the pleura can increase the permeability of the pleural capillaries, and a large number of cells, proteins and liquids in the capillaries leak into the pleural cavity, which increases the protein content in the pleural water and increases the colloid osmotic pressure of the pleural water, further promoting the increase of pleural effusion, and this pleural effusion is exudate. 3.Decrease in plasma colloid osmotic pressure: protein loss diseases such as nephrotic syndrome, liver cirrhosis, chronic infection and other diseases with reduced or impaired protein synthesis reduce plasma albumin, decrease plasma colloid osmotic pressure, increase capillary fluid filtration from the wall pleura, and decrease or stop absorption in the dirty layer, then form leaky pleural effusion. 4, wall layer pleural lymphatic reflux obstruction: wall layer pleural lymphatic reflux system (mainly lymphatic vessels) plays a role in the absorption of pleural fluid back, when congenital abnormalities or cancer embolism, parasitic blockage, or trauma causing lymphatic reflux obstruction, it is easy to produce high protein pleural exudate. 5, injury pleural effusion: trauma (such as esophageal rupture, thoracic duct rupture) or disease (such as thoracic aortic aneurysm rupture) and other reasons, the thoracic cavity appears bloody, purulent (secondary to infection), celiac pleural effusion, which is exudate.