pleura

The pleura consists of two layers: the visceral pleura, which directly covers the lung surface and has no somatic sensory innervation (pain fibers), and the parietal pleura, which lines the thoracic wall, diaphragm, and mediastinum, and is innervated by intercostal nerves (peripheral pleura) and the phrenic nerve (central/diaphragmatic pleura), making it pain-sensitive. Between these layers lies the pleural cavity, containing a small amount of serous fluid (~10-20 mL) that reduces friction during breathing and creates negative pressure to keep the lungs inflated. Clinically, disruption of pleural integrity or fluid balance leads to important pathologies: pneumothorax (air in pleural space causing lung collapse), pleural effusion (excess fluid accumulation, transudative vs exudative based on Light's criteria), hemothorax (blood), and pleuritis/pleurisy (inflammation causing sharp pleuritic chest pain worsened by breathing, often from infection, pulmonary embolism, or autoimmune disease). The parietal pleura's segmental innervation explains referred pain patterns—diaphragmatic pleura irritation can refer pain to the shoulder via the phrenic nerve (C3-C5). Understanding pleural anatomy is essential for procedures like thoracentesis and chest tube placement, and for interpreting physical exam findings such as decreased breath sounds, dullness to percussion, or a pleural friction rub on auscultation.