Pleural Cavity — MRCS Part B Anatomy

The pleura and the lungs in the opened chest. The station is half surface anatomy and half nerve supply, because the two together explain where chest pain is felt and why a drain hurts where it does.

The pleural cavity

Each lung is wrapped in a closed sac of serous membrane: the visceral pleura on the lung, continuous at the hilum and the pulmonary ligament with the parietal pleura lining the chest wall, diaphragm and mediastinum. The cavity between them is a potential space holding a film of fluid, at a pressure below atmospheric, which is what keeps the lung expanded against the chest wall. The two sacs are separate, so a pneumothorax on one side leaves the other lung working, unless the mediastinum is pushed across by tension. Where the parietal pleura folds back on itself beyond the lung's edge, at the costodiaphragmatic and costomediastinal recesses, fluid collects first.

Nerve supply

The parietal pleura is somatic: the costal pleura and the outer diaphragmatic pleura by the intercostal nerves, which is why their pain is sharp, local and referred to the chest wall; the mediastinal pleura and the central diaphragmatic pleura by the phrenic nerve (C3, 4, 5), which is why their pain is felt at the shoulder tip, where C4 supplies the skin. The visceral pleura has only autonomic fibres and no pain sensation, so a lung can be needled without discomfort while the parietal layer cannot.

Diaphragmatic irritation and hiccough

Blood or pus under the diaphragm, a ruptured spleen or a subphrenic abscess irritates the phrenic-supplied peritoneum and pleura, and because the phrenic nerve and the supraclavicular nerves share the C4 root, the pain is referred to the tip of the shoulder (Kehr's sign). A hiccough is a reflex spasm of the diaphragm: an afferent from the phrenic or vagus nerves or the sympathetic chain, triggered by gastric distension, diaphragmatic irritation, or brainstem disease, reaches a centre in the medulla and upper cord, and the efferent phrenic nerve fires the diaphragm while the glottis snaps shut, making the sound.

Surface markings of the pleura

From behind the sternoclavicular joint, where the apex rises about 2.5 cm above the medial third of the clavicle, the two pleural reflections meet in the midline at the sternal angle (second costal cartilage). The right runs straight down to the sixth cartilage; the left deviates laterally at the fourth cartilage to leave the heart bare. From the sixth cartilage each crosses the eighth rib in the mid-clavicular line, the tenth rib in the mid-axillary line, and reaches the twelfth rib at the side of the spine: 2, 4, 6, 8, 10, 12. Below the twelfth rib the pleura lies under the kidney and can be opened in a loin incision.

Surface markings of the lungs

The lung follows the pleura but stops two ribs short: the sixth rib in the mid-clavicular line, the eighth in the mid-axillary line, and the tenth at the back, leaving the costodiaphragmatic recess between. The oblique fissure runs from the spine of T3 (with the arm raised, along the medial border of the scapula) round to the sixth costal cartilage; the horizontal fissure on the right runs from the fourth costal cartilage back to meet it in the mid-axillary line. So the upper lobes are heard at the front, the lower lobes at the back, and the middle lobe only on the right, between the fourth and sixth ribs in front.

Blood supply

The parietal pleura is supplied by the intercostal, internal thoracic and musculophrenic arteries of the wall it lines, and drains to the systemic veins; the visceral pleura by the bronchial arteries, draining to the pulmonary veins. Lymph from the parietal pleura goes to the intercostal, parasternal and diaphragmatic nodes, and from the visceral pleura to the hilar nodes.