>Significance Physiological Distend the lung make inspiration easier Promote venous lymph return in the chest Pathological (Pneumothorax Collapse the lung make inspiration difficult Affect venous lymph return Fatal in pulmonary circulatory failure cases Importance Keep the interpleural cavity hermetically sealed➢Significance Physiological ~ Distend the lung & Make inspiration easier Promote venous & lymph return in the chest Pathological ~ (Pneumothorax) Collapse the lung & Make inspiration difficult Affect venous & lymph return Fatal in pulmonary & circulatory failure cases Importance Keep the interpleural cavity hermetically sealed