visceral pleura

**Anatomy** - Pleura is a serous membrane with two continuous layers: the visceral pleura (adherent to lung parenchyma, including the interlobar fissures) and the parietal pleura (lines the thoracic wall, diaphragm, and mediastinum). The two layers are continuous at the hilum, where the pulmonary ligament forms a reflection inferior to the hilum. - Between the layers is the pleural cavity, a potential space holding about 5–15 mL of serous fluid. The fluid lubricates the surfaces. Surface tension and the lung's elastic recoil (opposed by outward chest wall recoil) create negative intrapleural pressure that keeps the lung expanded. - Histology: a single layer of mesothelial cells on a thin connective tissue layer. Mesothelium is cytokeratin+, calretinin+, and WT-1+.

**Innervation** - Visceral pleura: autonomic innervation (pulmonary plexus; vagus and sympathetic fibers). It has no general sensory (pain) fibers, so it is insensitive to pain, touch, and temperature. - Parietal pleura: somatic innervation, so it is pain-sensitive. - Costal and peripheral diaphragmatic pleura: intercostal nerves. Pain is felt locally over the chest wall. - Mediastinal and central diaphragmatic pleura: phrenic nerve (C3–C5). Pain is referred to the shoulder or neck.

**Blood supply and drainage** - Visceral pleura: bronchial arteries (systemic circulation), with venous drainage largely via pulmonary veins. The parietal pleura is supplied by intercostal, internal thoracic, and musculophrenic vessels. - Lymphatics: visceral pleural lymph drains to the bronchopulmonary (hilar) nodes. Parietal pleural lymph drains to parasternal, intercostal, and diaphragmatic nodes.

**Pleural fluid physiology** - Starling forces govern fluid movement. Fluid is mostly formed from the parietal pleural capillaries (systemic) and is absorbed by parietal pleural lymphatics (stomata). Visceral pleural contribution is small.

**Clinical significance** - Pneumothorax: air enters the pleural space, either from rupture of the visceral pleura (blebs/bullae in primary spontaneous pneumothorax in tall, thin young men; COPD or emphysema in secondary) or through the chest wall (trauma). Findings: hyperresonance, decreased breath sounds and tactile fremitus, and absent lung markings on CXR. Tension pneumothorax causes tracheal deviation away from the affected side, hypotension, and JVD, and needs immediate needle decompression. - Pleural effusion: transudate (Light's criteria negative, such as CHF, cirrhosis, nephrotic syndrome) versus exudate (pneumonia, malignancy, TB, PE). Findings: dullness to percussion, decreased breath sounds and fremitus, and blunting of the costophrenic angle. - Pleuritis/pleurisy: inflammation of the parietal pleura (viral infection, SLE, PE) causes sharp pleuritic chest pain that is worse with inspiration and a pleural friction rub. Inflammation of the visceral pleura alone is not painful. - Mesothelioma: malignancy of the pleura associated with asbestos exposure. Features include pleural plaques (benign, calcified, parietal), a latency of 20–40 years, a hemorrhagic effusion, and an increased risk with smoking only for lung cancer (asbestos plus smoking raises lung cancer risk, not mesothelioma risk). It is a highly aggressive tumor. - Visceral pleural invasion by lung cancer upstages non-small cell lung cancer (T2 if invading the visceral pleura). - Chest tubes and thoracentesis: insert above the superior border of the rib to avoid the neurovascular bundle (which runs inferior to each rib). Entry through the parietal pleura (pain-sensitive) requires adequate local anesthesia.

**High-yield pearls** - Visceral = autonomic (painless). Parietal = somatic (painful). - Costodiaphragmatic recess: the lowest extent of the pleural cavity, where fluid collects first on upright imaging (about 200 mL is needed to blunt the angle on a PA film). Thoracentesis is performed at the midscapular line, 8th–10th intercostal space. - Pleural reflections: the pleura extends lower than the lung (the lung reaches rib 6 at the midclavicular line, rib 8 at the midaxillary line, and rib 10 posteriorly, whereas the pleura reaches ribs 8, 10, and 12).