PLEURAL EFFUSION EPIDEMIOLOGY AND PATHOBIOLOGY Pleural effusions occur most frequently in patients with pneumonia(肺炎)or heart failure(心衰). Normally,a small amount of fluid in the pleural space forms a thin layer between the visceral and parietal pleural surfaces and acts as a lubricant(润滑剂)to minimize friction between the chest wall and lung as they move against each other during inspiration (and expiration (呼气). This flux of fluid depends on the oncotic pressure(渗透压) and hydrostatic pressure(静水压)_within the parietal and visceral pleura as well as the pressure within the pleural space itself. PLE第URA一L E部FF分USION EPIDEMIOLOGY AND PATHOBIOLOGY Pleural effusions occur most frequently in patients with pneumonia（肺炎） or heart failure（心衰）. Normally, a small amount of fluid in the pleural space forms a thin layer between the visceral and parietal pleural surfaces and acts as a lubricant（润滑剂） to minimize friction between the chest wall and lung as they move against each other during inspiration（吸气） and expiration （呼气）. This flux of fluid depends on the oncotic pressure （渗透压） and hydrostatic pressure （静水压） within the parietal and visceral pleura as well as the pressure within the pleural space itself