Structure
Each lung is enclosed in a double-layered serous sac: visceral pleura on the lung surface and parietal pleura on the walls of the cavity. The two layers are continuous around the root of the lung, and the sleeve extends below the root as the pulmonary ligament.
The parietal pleura is named by the surface it lines:
- Costal pleura: on the inner surfaces of the ribs, costal cartilages and intercostal muscles, separated from them by endothoracic fascia.
- Mediastinal pleura: on the side of the mediastinum, including the fibrous pericardium.
- Diaphragmatic pleura: on the upper surface of the diaphragm.
- Cervical pleura: the dome over the lung apex, strengthened by the suprapleural membrane from the transverse process of C7 to the inner border of the first rib.
The cavity between the layers is only a potential space. Negative pressure within it holds the lung against the chest wall. The left and right sacs are separate, so a pneumothorax on one side normally leaves the other lung expanded.
Recesses
Where the parietal pleura extends beyond the lung edge, it folds on itself to form recesses that the lung enters only in deep inspiration.
- Costodiaphragmatic recess: between the costal and diaphragmatic pleura, deepest in the midaxillary line. Fluid collects here first.
- Costomediastinal recess: behind the sternum, larger on the left over the heart.
Surface markings
The parietal pleura runs about two ribs lower than the lower border of the lung in quiet breathing.
| Line | Lower border of lung | Lower limit of pleura |
|---|---|---|
| Midclavicular line | 6th rib | 8th rib |
| Midaxillary line | 8th rib | 10th rib |
| Beside the vertebral column | 10th rib | 12th rib |
In front, each pleura rises from about 2.5 cm above the medial third of the clavicle, passes behind the sternoclavicular joint and meets its partner in the midline at the sternal angle, level with the second costal cartilage. The right pleura continues straight down to the sixth costal cartilage. The left diverges laterally at the fourth costal cartilage, leaving part of the pericardium uncovered, then reaches the sixth. Posteriorly the pleura drops just below the twelfth rib next to the vertebral column.
The oblique fissure runs from about the spine of T3 to the sixth costal cartilage, and the right horizontal fissure along the fourth costal cartilage.
Relations
The pleura separates each lung from every surrounding structure, and its extent determines which of them a needle or incision can reach. The cervical pleura is related to the subclavian artery, the lower trunk of the brachial plexus, the sympathetic trunk and the first rib. The mediastinal pleura lies against the pericardium, with the phrenic nerve and pericardiophrenic vessels running between them. Below the diaphragm lie the liver on the right and the stomach and spleen on the left.
Blood supply and innervation
The parietal pleura has a somatic nerve supply and is sensitive to pain; the visceral pleura has only autonomic fibres and is not.
- Costal pleura and peripheral diaphragmatic pleura: intercostal nerves. Pain is sharp and localised to the overlying chest wall.
- Mediastinal pleura and central diaphragmatic pleura: phrenic nerve (C3–C5). Pain is referred to the shoulder tip, in the C4 dermatome.
- Visceral pleura: autonomic fibres via the pulmonary plexus, with no pain sensation.
The parietal pleura is supplied by the intercostal, internal thoracic and musculophrenic arteries and drains to systemic veins. The visceral pleura is supplied by the bronchial arteries. Parietal lymph drains to intercostal, parasternal and diaphragmatic nodes; visceral lymph drains to the hilar nodes.
Clinical relevance
Pleural anatomy explains the signs of air or fluid in the cavity and the sites where the pleura is at risk.
- Pneumothorax: air in the cavity lets the lung collapse. In a tension pneumothorax the mediastinum shifts to the opposite side.
- Pleural effusion: fluid first fills the costodiaphragmatic recess, blunting the costophrenic angle on an erect radiograph.
- Pleurisy: inflamed parietal pleura gives sharp pain on breathing; diaphragmatic involvement may be felt at the shoulder tip.
- Pleura at risk: above the clavicle in subclavian line insertion and supraclavicular brachial plexus block, and below the twelfth rib in loin incisions for renal surgery.
On the specimen
This station shows the thorax opened from the front with the lungs in situ and the upper abdomen visible below the diaphragm.
- Costal or mediastinal pleura: costal pleura is the shiny lining on the inner chest wall; mediastinal pleura covers the pericardium and great vessels in the midline.
- Fibrous pericardium: the tough sac around the heart, under the mediastinal pleura.
- Right lung lobes: the horizontal fissure runs forwards at the level of the fourth costal cartilage between upper and middle lobes; the oblique fissure runs down and forwards below it.
- Left oblique fissure: the only fissure on the left, separating upper and lower lobes.
- Right apex: the rounded top of the right lung rising behind the clavicle.
- Manubrium: the upper part of the sternum; its junction with the body is the sternal angle.
- Below the diaphragm: the right and left anatomical lobes of the liver, separated by the falciform ligament.