A printer friendly page is available here. Pleural fluid secreted by the pleural membranes help the pleura slide over each other Lung tissue is elastic Movement of the thoracic cavity walls causes movement of lung walls. Diaphragm Dome-shaped skeletal muscle that forms floor of thoracic cavity. Passive process Contraction of abdominal muscles during forced breathing causes rapid expiration. Active process Ribs Form walls of thoracic cavity.
Air flows in.
Air flows out. Passive process Internal intercostals contract during forced breathing.