Pleura and Lungs

Each lung is enclosed by a continuous serous membrane, folded to form visceral and parietal layers, with a thin, potential pleural cavity between them containing a small volume of lubricating pleural fluid.

Visceral pleura:

• Closely invests the lung surface, including into the fissures between lobes.
• Insensitive to pain (no somatic sensory innervation): receives only autonomic innervation from the pulmonary plexus; visceral pleural irritation does not itself cause localised chest wall pain.

Parietal pleura:

• Lines the inner surface of the thoracic wall and related structures, subdivided by location into four continuous parts:
• Costal pleura: lines the internal surface of the ribs and intercostal spaces; innervated by the intercostal nerves, producing well-localised somatic pain when irritated.
• Mediastinal pleura: lines the lateral surface of the mediastinum; reflects at the lung root to become continuous with the visceral pleura.
• Diaphragmatic pleura: covers the thoracic (superior) surface of the diaphragm; the central part is innervated by the phrenic nerve (referred shoulder-tip pain when irritated), while the peripheral part is innervated by the lower intercostal nerves (localised pain), mirroring the diaphragm's own sensory pattern.
• Cervical pleura (cupula/pleural dome): extends above the 1st rib into the root of the neck, covered and reinforced by Sibson's fascia (suprapleural membrane); clinically relevant to apical lung pathology (Pancoast tumour) and to the risk of pneumothorax during central line placement or brachial plexus block near the neck root.

• Potential spaces where parietal pleural surfaces are in contact (or nearly so) but not occupied by lung tissue even during maximal inspiration, important reservoirs where pleural fluid tends to collect abnormally, and landmarks for clinical procedures.

Costodiaphragmatic recess:

• Between the costal and diaphragmatic pleura, at the lower margin of the pleural cavity.
• The deepest and most clinically significant recess: the most dependent part of the pleural cavity in an upright person, and therefore the classic site where pleural effusions first collect and are best drained (thoracentesis).
• Lung tissue does not normally extend into this recess even at full inspiration, so needle insertion here (at an appropriate level) avoids puncturing the lung when performed correctly.

Costomediastinal recess:

• Between the costal and mediastinal pleura, anteriorly.
• Smaller and less clinically significant than the costodiaphragmatic recess; most prominent on the left side around the cardiac notch, where the pleura (and lung) deviates laterally to accommodate the heart's leftward bulge.

• Anterior reflection: The parietal pleura on each side runs close to the midline behind the sternum from roughly the sternal angle down to about the 4th costal cartilage, where the left pleura deviates laterally (due to the cardiac notch) while the right continues closer to the midline down to about the 6th costal cartilage.
• Inferior reflection (costal margin extent): The pleura extends down to approximately the 8th rib in the midclavicular line, 10th rib in the midaxillary line, and 12th rib (or the level of the T12 spinous process) posteriorly: descending progressively more inferiorly moving from anterior to posterior, tracking the costodiaphragmatic recess.
• Apex: Extends approximately 2–3 cm above the medial third of the clavicle into the root of the neck.

• Parietal pleura: Somatic sensory innervation, costal and peripheral diaphragmatic parts via intercostal nerves (localised pain); central diaphragmatic and mediastinal parts via the phrenic nerve (referred shoulder-tip pain).
• Visceral pleura: Autonomic (pulmonary plexus) innervation only, insensitive to pain, touch, and temperature; pain arising from lung pathology itself is therefore typically only appreciated once it involves or irritates the adjacent parietal pleura or chest wall.

• Parietal pleura: Segmental supply from the intercostal, internal thoracic, musculophrenic, and pericardiacophrenic arteries, matching the systemic supply of the adjacent chest wall structures it overlies.
• Visceral pleura: Supplied by the bronchial circulation (systemic) and drains partly via the pulmonary venous system, reflecting its close association with the lung itself.

General features:

• Apex: Rounded superior tip, extends into the root of the neck (2–3 cm above the clavicle), related to the subclavian vessels and the cervical pleural dome.
• Base: Concave inferior surface, rests on the diaphragm; the right base is higher due to the underlying liver.
• Surfaces: Costal (convex, against the ribs), mediastinal (medial, contains the hilum), diaphragmatic (base, concave).
• Borders: Anterior (thin, marks the anterior pleural reflection; contains the cardiac notch on the left), posterior (rounded, along the vertebral column), inferior (separates the base from the costal surface).
• Lobes and fissures: These differ between the right and left lungs.
• Hilum: The region on the mediastinal surface where structures enter/exit the lung.
• Root: The collection of structures (bronchus, pulmonary artery, pulmonary veins, bronchial vessels, lymphatics, nerves) connecting the lung to the mediastinum at the hilum, surrounded by a pleural sleeve.

Right lung:

• Three lobes: Superior (upper), middle, and inferior (lower).
• Horizontal fissure: Separates the superior and middle lobes; runs roughly along the 4th rib/costal cartilage anteriorly.
• Oblique fissure: Separates the inferior lobe from the superior/middle lobes; runs from around the T4 spinous process posteriorly to the 6th costal cartilage anteriorly (a useful general rule shared with the left, tracing an oblique line down and around the chest wall).
• Right lung is generally larger and heavier than the left, but slightly shorter (due to the higher right hemidiaphragm from the underlying liver).

Left lung:

• Two lobes: Superior and inferior.
• Oblique fissure only (no horizontal fissure, since there is no middle lobe).
• Cardiac notch: A deep indentation of the anterior border of the superior lobe, accommodating the leftward bulge of the heart.
• Lingula: A tongue-shaped projection of the superior lobe, inferior to the cardiac notch, considered the left lung's embryological/functional counterpart to the right middle lobe.
• Left lung is smaller than the right overall, reflecting the space occupied by the heart on this side.

• General arrangement (superior to inferior), differs slightly by side:
• Right lung root: Eparterial bronchus (upper lobe bronchus, above the level of the pulmonary artery) is the most superior structure; pulmonary artery next; then the (hyparterial) bronchus continuing to the other lobes; pulmonary veins are the most inferior/anterior.
• Left lung root: Pulmonary artery is the most superior structure (no eparterial bronchus on this side, since the left main bronchus is entirely hyparterial); bronchus next; pulmonary veins most inferior/anterior.
• A simplified general mnemonic across both sides, anterior to posterior: Veins, Artery, Bronchus (pulmonary veins anterior, pulmonary artery in the middle, bronchus posterior), this anteroposterior relationship is generally more consistent between the two lungs than the superior-inferior relationship, which differs due to the eparterial bronchus on the right.
• Bronchial vessels (small systemic branches supplying the lung tissue itself, particularly the airway walls) also pass through the root, along with pulmonary lymphatics and autonomic nerve fibres from the pulmonary plexus.

• Functionally and surgically independent units of lung tissue, each supplied by its own segmental (tertiary) bronchus, segmental branch of the pulmonary artery, and segmental lymphatics, while the pulmonary veins typically drain across segmental boundaries (intersegmental) rather than staying confined to one segment, making the vein the useful surgical landmark for identifying the plane between adjacent segments.
• Each segment is a discrete, pyramid-shaped unit with its apex at the hilum, allowing individual segments to be surgically resected without disturbing the function of the remainder of the lobe (segmentectomy), a key surgical advantage over the older, less precise understanding of lobar anatomy alone.
• Right lung: typically 10 segments (3 in the superior lobe, 2 in the middle lobe, 5 in the inferior lobe).
• Left lung: typically 8–10 segments depending on classification convention (commonly grouped as 4 in the superior lobe/lingula combined, 4–5 in the inferior lobe, since some apical and posterior segments fuse on the left).

• Carry deoxygenated blood from the right ventricle to the lungs, via the pulmonary trunk, which bifurcates into right and left pulmonary arteries.
• Branch alongside the bronchial tree (bronchovascular bundles), following the segmental pattern.

• Carry oxygenated blood from the lungs to the left atrium; typically two veins per lung (superior and inferior), draining directly into the left atrium (unlike the systemic venous system, no major valve mechanism at this junction, relevant to the pathophysiology of some arrhythmias originating near the pulmonary vein ostia).
• Cross segmental boundaries within the lung (intersegmental course).

• Bronchial arteries: Small systemic branches (typically from the descending thoracic aorta, sometimes from intercostal arteries), supplying the bronchial tree, visceral pleura, and lung connective tissue (the "nutrient" supply for lung tissue itself, as opposed to the pulmonary circulation's gas-exchange function).
• Bronchial veins: Partly drain into the azygos/hemiazygos system, and partly into the pulmonary veins directly, contributing a small amount of relatively deoxygenated blood into the otherwise oxygenated pulmonary venous return (part of the normal small "physiological shunt").

• Follows the bronchial tree, draining sequentially through bronchopulmonary (hilar) nodes → tracheobronchial nodes (at the carina/bronchial bifurcation) → paratracheal nodes → bronchomediastinal trunks, which drain into the venous system (typically near the junction of the subclavian and internal jugular veins, or into the thoracic duct on the left).

• Pulmonary plexus: Formed at the lung root by contributions from the vagus nerve (parasympathetic, bronchoconstriction, increased glandular secretion) and the sympathetic trunk (via thoracic sympathetic ganglia, bronchodilation, vasoconstriction of pulmonary vessels).
• Visceral afferent fibres (pain-insensitive but carrying reflex information, e.g., stretch receptors relevant to the Hering-Breuer reflex) travel with the vagus nerve back to the CNS.

Pneumothorax:

• Air within the pleural cavity, disrupting the normal negative intrapleural pressure and causing partial or complete lung collapse.
• Can be spontaneous (often from ruptured subpleural blebs, classically in tall, thin young men), traumatic (penetrating or blunt chest injury), or iatrogenic (central line placement, thoracentesis, mechanical ventilation barotrauma).

Pleural effusion:

• Abnormal accumulation of fluid within the pleural cavity, collecting first (and most) in the costodiaphragmatic recess given gravity in the upright position.
• Causes are broadly transudative (e.g., heart failure, hypoalbuminaemia) or exudative (e.g., infection, malignancy, inflammation), a distinction with different underlying pathophysiological/anatomical mechanisms of fluid formation.

Lung collapse (atelectasis):

• Can result from airway obstruction (mucus plug, foreign body, tumour), external compression (pleural effusion, pneumothorax), or loss of surfactant.
• The pattern of collapse (whole lung, lobe, or segment) reflects the level of the underlying bronchial obstruction, given the segmental organisation of the bronchial tree.

Aspiration patterns:

• Aspirated material tends to enter the right main bronchus preferentially, given its more vertical orientation, wider calibre, and shorter length compared to the left (see Trachea and Bronchial Tree section for full detail).
• Within the right lung, aspiration in a supine patient classically affects the posterior segment of the right upper lobe (or the superior segment of the right lower lobe, depending on positioning), reflecting gravity-dependent airway anatomy.

Segmental lung resection:

• The independence of bronchopulmonary segments (each with its own segmental bronchus, artery, and lymphatics) allows a segmentectomy to remove a localised area of diseased lung tissue (e.g., a small tumour, localised infection) while preserving the function of the remaining segments within that lobe, a more lung-conserving alternative to lobectomy when anatomically and oncologically appropriate.