A 35-yr-old man has had increasing dyspnea for the past 24 hrs. A chest radiograph shows large, bilateral pleural effusions. Thoracentesis yields 500 mL of slightly cloudy yellow fluid from the right pleural cavity. Cytologic examination of the fluid shows many neutrophils, but no lymphocytes or RBCs. Which of the following mechanisms contributes most to the accumulation of the fluid in the pleural space?
A. Arteriolar vasoconstrictionB. Neutrophil release of lysosomesC. Endothelial contractionD. Inhibition of platelet adherenceE. Lymphatic obstruction

The primary function of the vascular endothelium is to act as an effective barrier system and controls the diffusion of micro and macromolecules and keep the vascular compartments and interstitial spaces intact.

When this endothelial barrier system fails to function, exudation or leakage of fluid, solutes and proteins from the vasculature takes place leading to accumulation in the interstitial spaces.

Failure of endothelial vasculature like endothelial contraction occurs due to binding with mediators or specific receptors like histamine or leukotrienes or also due to any injury.

When there is an endothelial contraction, the intercellular gaps present in between postcapillary venules widen giving space for the fluid leaked from the vasculature to accumulate.

This exudation of fluids is a factor of an acute inflammatory process. In the above case, the high neutrophil count in the pleural fluid confirms a lung inflammatory process. The neutrophils act as the first line of defence against the inflammation.