4. Parietal Pleura Visceral Pleura Space Pleura Hydrostatic Pressure +30 - 5 + 24 35 29 6 0 Net 29 29 + 34 + 5 +34 Oncotic Pressure Diagrammatic representation of the pressures involved in the formation and absorption of pleural fluid.

6. Pathophysiology of pleural effusion Pleural Effusion

7. General Causes of Pleural Effusions Decreased pleural fluid absorption Increased pleural fluid formation Increased interstitial fluid in the lung Increased intravascular pressure in pleura. Increased permeability of the capillaries in the pleura. Increased pleural fluid protein level. Decreased pleural pressure. Increased fluid in peritoneal cavity. Disruption of the thoracic duct. Disruption of blood vessels in the thorax. Obstruction of the lymphatics draining the parietal pleura. Elevation of systemic vascular pressures

8. Increased Pleural Fluid Formation Increased Interstitial Fluid It is the most common cause of increased pleural fluid formation. whenever the amount of edema in the lung exceeds 5 g/gram of dry lung weight, pleural fluid accumulates. This is the predominant mechanism of pleural effusions in patients with -- congestive heart failure, -- parapneumonic effusions, -- acute respiratory distress syndrome, -- after lung transplantation.

9. Increased Hydrostatic Pressure Gradient Increased intravascular pressure a decrease in the pleural pressure can occur with right ventricular failure, left ventricular failure, pericardial effusions, or superior vena cava syndrome. The most common cause is bronchial obstruction leading to atelectasis of the lower lobe or complete lung. A decrease in the pleural pressure also occurs when the visceral pleura becomes coated with a collagenous peel and the lung becomes trapped.

11. Increased levels of vascular endothelial growth factor (VEGF) increase the permeability of the capillaries

12. VEGF receptors have been demonstrated on mesothelial cells , and the levels of VEGF are higher in exudative effusions than in transudative pleural effusions .

14. Presence of Free Peritoneal Fluid If there is free fluid in the peritoneal cavity, it will lead to pleural fluid accumulation through defects in the diaphragm, e.g. in - hepatic hydrothorax, - peritoneal dialysis, - meig’s syndrome ( Benign ovarian tumour with ascites and pleural effusion usually resolve after resection of the tumour )