TOPICS: Obstructive atelectasis, tumor, foreign body, mucus plug, non-obstructive atelectasis, compressive atelectasis, loculated pleural effusions, elevated hemidiaphragm, solid mass, functional residual capacity, passive atelectasis, pleural effusion, pneumothorax, hemothorax, emphysema, contraction atelectasis, granulomatous disease, sarcoidosis, adhesive atelectasis, NRDS, ARDS, surfactant, surface tension, absorption atelectasis, PEEP, percussion, tactile fremitus, opacification, mediastinal shift, PaO2, PaCO2