This document discusses pleural effusion, which is an accumulation of fluid in the pleural cavity between the lungs and chest wall. It causes symptoms like shortness of breath and chest pain. Diagnosis is made through physical exam finding decreased breath sounds and imaging tests like chest x-rays that show blurred lung borders and opacity in the affected hemithorax. The images show examples of a pneumothorax where air is trapped in the pleural space, as well as pleural effusions.