Pleura

Introduction

The lungs are enclosed within paired pleural sacs. Each pleural sac is lined by the pleura, a thin serous membrane that surrounds the lung and lines the corresponding part of the thoracic cavity.

Each pleura consists of two continuous layers:

1. Visceral pleura: This layer closely covers the external surface of the lung and extends into its fissures.
2. Parietal pleura: This layer lines the inner surface of the thoracic cavity and forms the boundary between the lungs and the mediastinum.

The visceral and parietal layers become continuous with each other around the hilum of the lung and along the pulmonary ligament. The narrow potential space between these two layers is known as the pleural cavity.

Embryological Aspects of Pleural Cavities

During embryonic development, the future pleural cavities arise from the paired pericardioperitoneal

As each lung bud develops, it grows into the corresponding pleural cavity and becomes invested by the surrounding serous membrane. With continued growth, the lung expands and progressively occupies the pleural cavity.

The layer of the pleural membrane directly covering the developing lung forms the visceral pleura, whereas the layer lining the body wall forms the parietal pleura.

A narrow space persists between the visceral and parietal layers and constitutes the pleural cavity. The surfaces of both pleural layers facing this cavity are lined by mesothelium, which consists of simple squamous epithelium.

The pleural cavity contains a small quantity of serous pleural fluid, which lubricates the opposing pleural surfaces and allows them to glide smoothly over one another during respiration.

Mediastinum

The mediastinum is the median compartment of the thoracic cavity situated between the right and left pleural cavities. It contains the heart, pericardium, major blood vessels, principal bronchi, esophagus, and several other thoracic structures.

Pulmonary Ligament

The pulmonary ligament is a double-layered fold of pleura that extends inferiorly from the root of the lung. It contains loose areolar tissue and provides sufficient mobility for displacement of the lung root and associated structures during respiratory movements.

Developmental and Functional Correlations

The visceral pleura develops from the splanchnopleuric layer of the lateral plate mesoderm. Consequently, its blood supply and autonomic nerve supply are closely related to those of the lungs.

The parietal pleura develops from the somatopleuric layer of the lateral plate mesoderm. Its blood supply and somatic sensory innervation are therefore derived mainly from structures associated with the thoracic wall.

The left pleural cavity is relatively smaller than the right because the heart projects predominantly toward the left side of the thoracic cavity.

Layers of Pleura

The pleura is composed of two continuous layers:

1. Visceral pleura, which forms the inner layer and closely covers the surface of the lung.
2. Parietal pleura, which forms the outer layer and lines the thoracic cavity.

Parietal Pleura

The parietal pleura is thicker than the visceral pleura.

According to the region of the thoracic cavity that it lines, the parietal pleura is divided into four parts:

1. Costovertebral pleura
2. Diaphragmatic pleura
3. Mediastinal pleura
4. Cervical pleura

Table 7.1: Differences between visceral and parietal pleurae

Feature	Visceral (pulmonary) pleura	Parietal pleura
Position	Forms the inner layer of the pleura	Forms the outer layer of the pleura
Distribution	Closely covers the surfaces of the lungs and extends into their fissures	Lines the inner surface of the thoracic wall, diaphragm, and mediastinum
Thickness	Thinner than the parietal pleura	Thicker than the visceral pleura
Embryological origin	Derived from the splanchnopleuric layer of lateral plate mesoderm	Derived from the somatopleuric layer of lateral plate mesoderm
Blood supply	Mainly supplied by the bronchial arteries	Supplied by vessels of the thoracic wall, including intercostal, internal thoracic, and musculophrenic arteries
Nerve supply	Receives autonomic innervation through the pulmonary plexus, with sympathetic fibers mainly from thoracic sympathetic segments and parasympathetic fibers from the vagus nerve	Receives somatic sensory innervation from the intercostal and phrenic nerves
Pain sensitivity	Relatively insensitive to pain	Highly sensitive to pain

Costovertebral Pleura (Previous Name: Costal Pleura)

The costovertebral pleura lines the inner surface of the thoracic wall, including the sternum, costal cartilages, ribs, intercostal spaces, and vertebral bodies.

It is separated from the thoracic wall by a thin layer of loose connective tissue known as the endothoracic fascia. This fascial layer allows the costovertebral pleura to be separated relatively easily from the thoracic wall.

Diaphragmatic Pleura

The diaphragmatic pleura is thin and firmly adherent to the superior surface of the diaphragm.

Laterally, it is continuous with the costovertebral pleura, while medially it merges with the mediastinal pleura.

Cervical Pleura (Cupola)

The cervical pleura, or cupola, is the dome-shaped superior extension of the pleura that covers the apex of the lung.

It projects into the root of the neck, reaching approximately 5 cm above the first costal cartilage and about 2.5 cm above the medial end of the clavicle.

Superiorly, the cervical pleura is reinforced by the suprapleural membrane.

Relations

The relations of the cervical pleura are:

• Anterior: Subclavian artery, medial border of the first rib, and scalenus anterior muscle
• Posterior: Neck of the first rib
• Lateral: Medial border of the first rib and scalenus medius muscle
• Medial: Major vessels of the neck
• Superior: Suprapleural membrane and supraclavicular part of the brachial plexus
• Inferior: Apex of the lung

Mediastinal Pleura

The mediastinal pleura forms the lateral boundary of the mediastinum and covers the mediastinal aspect of the thoracic cavity.

At the root of the lung, it becomes continuous with the visceral pleura. Inferior to the root, the reflected pleural layers form the pulmonary ligament.

Pulmonary Ligament

The pulmonary ligament is a double-layered fold of mediastinal pleura that extends inferiorly from the root of the lung (Fig. 7.5).

At the root of the lung, the parietal pleura reflects over the structures entering and leaving the lung and becomes continuous with the visceral pleura. Below the root, this pleural reflection continues downward as the pulmonary ligament.

Contents

The pulmonary ligament contains:

1. Loose areolar tissue
2. A few lymphatic vessels
3. Occasionally, an accessory bronchial artery

Relations

• Medial: Esophagus
• Lateral: Mediastinal surface of the lung
• Superior: Hilum of the lung
• Inferior: Ends in a free, sickle-shaped margin

Functions

1. Accommodation of pulmonary veins: The pulmonary ligament provides a potential space that permits expansion and movement of the pulmonary veins.
2. Mobility of the lung root: During inspiration, descent of the diaphragm causes downward movement of the lung. The pulmonary ligament allows the root of the lung and associated structures to move without excessive tension.

Recesses of Pleura

Pleural recesses are potential spaces within the pleural cavity formed where adjacent parts of the parietal pleura come into contact.

Normally, only a narrow potential space exists between the visceral and parietal pleurae. At certain sites where the parietal pleura changes direction, deeper pleura-lined gutters are formed. These are known as pleural recesses.

The recesses provide reserve space into which the lungs can expand during deep inspiration.

Types of Pleural Recesses

There are two major pleural recesses on each side:

1. Costodiaphragmatic recess
2. Costomediastinal recess

The minor pleural recesses are:

1. Right and left retroesophageal recesses
2. Infracardiac recess

Costodiaphragmatic Recess (Right and Left)

The costodiaphragmatic recess, also called the phrenicocostal sinus, is a cleft-like potential space between the costal and diaphragmatic parts of the parietal pleura.

In the midaxillary line, it is approximately 5 cm in vertical extent.

Its approximate extent is:

• Midclavicular line: Between the 6th and 8th ribs
• Midaxillary line: Between the 8th and 10th ribs
• Paravertebral region: Between the 10th and 12th ribs

The costodiaphragmatic recess forms the most dependent part of the pleural cavity. Consequently, fluid within the pleural cavity, as in pleural effusion, tends to collect in this recess, particularly in an upright individual.

Costomediastinal Recess (Right and Left)

The costomediastinal recess is a small potential space situated anteriorly, behind the sternum and adjacent costal cartilages. It is formed between the costal and mediastinal pleurae.

The right costomediastinal recess is relatively small because it is largely occupied by the anterior border of the right lung, even during quiet respiration.

The left costomediastinal recess is larger because the cardiac notch of the left lung leaves a greater part of this recess unoccupied.

Nerve Supply of Pleura

Parietal Pleura

The parietal pleura develops from the somatopleuric layer of the lateral plate mesoderm and therefore receives somatic sensory innervation. It is sensitive to pain.

• The intercostal nerves supply the costovertebral pleura and the peripheral part of the diaphragmatic pleura.
• The phrenic nerve (C3–C5) supplies the mediastinal pleura and the central part of the diaphragmatic pleura.

Visceral Pleura

The visceral pleura develops from the splanchnopleuric layer of the lateral plate mesoderm and is supplied by autonomic nerves. It is relatively insensitive to pain.

Its autonomic innervation reaches it through the pulmonary plexus:

• Sympathetic fibers: Mainly from the upper thoracic sympathetic segments
• Parasympathetic fibers: From the vagus nerve

Blood Supply and Lymphatic Drainage of Pleura

Parietal Pleura

The vascular and lymphatic supply of the parietal pleura generally follows that of the adjacent thoracic wall.

Its arterial supply is derived mainly from:

• Intercostal arteries
• Internal thoracic artery
• Musculophrenic artery

Venous blood drains mainly into the intercostal, internal thoracic, and azygos venous systems.

Lymphatic drainage occurs through lymph vessels that reach the intercostal, parasternal, posterior mediastinal, and diaphragmatic lymph nodes.

Visceral Pleura

The visceral pleura receives its arterial supply mainly from the bronchial arteries.

Venous drainage occurs predominantly through the bronchial venous system, with communication with pulmonary veins.

Its lymphatic vessels drain mainly into the bronchopulmonary (hilar) lymph nodes.

Pleural effusion

Definition

Pleural effusion is an abnormal accumulation of excess fluid within the pleural cavity. Normally, the pleural cavity contains approximately 5–10 mL of serous fluid.

Types of Pleural Effusion

Depending on the nature of the accumulated fluid, pleural collections may be classified as follows:

• Hydrothorax: Accumulation of serous, usually transudative, fluid
• Hemothorax: Accumulation of blood
• Empyema (pyothorax): Accumulation of pus
• Chylothorax: Accumulation of chyle
• Hydropneumothorax: Presence of both fluid and air within the pleural cavity

Causes

Pleural effusion may occur due to several conditions, including tuberculosis, congestive heart failure, liver cirrhosis, and infections.

Treatment

Management includes:

• Thoracentesis (thoracocentesis): Aspiration of excess pleural fluid using a needle or catheter.
• Treatment of the underlying cause responsible for the pleural effusion.

Thoracocentesis

Thoracocentesis, also called thoracentesis or pleural tap, is a procedure in which excess fluid is aspirated from the pleural cavity.

Procedure

The procedure is performed under strict aseptic precautions, preferably with ultrasound guidance (Fig. 7.14).

The patient is usually seated upright and asked to lean slightly forward with the arms supported.

A needle or cannula is introduced through an appropriate intercostal space, commonly in the lateral chest wall, just above the upper border of the rib (conventionally, in the lower part of the 8th or 9th intercostal space in the midaxillary line). This approach reduces the risk of injury to the main intercostal neurovascular bundle, which runs along the inferior border of the rib.

The site of needle insertion is selected according to the level of the pleural fluid. Care should be taken to avoid injury to the lung, diaphragm, and abdominal viscera.

Thoracocentesis requires particular caution in patients with uncorrected coagulation abnormalities because of the increased risk of bleeding.

Pneumothorax is an important complication of thoracocentesis. Other possible complications include bleeding, infection, and injury to adjacent structures.

Pneumothorax

Definition

Pneumothorax is the presence of air within the pleural cavity (Fig. 7.13).

Normally, intrapleural pressure is subatmospheric. Entry of air into the pleural cavity reduces or abolishes this negative pressure, allowing the elastic recoil of the lung to produce partial or complete lung collapse (atelectasis).

A large or tension pneumothorax may also displace the mediastinal structures toward the opposite side.

Causes

Common causes include:

• Injury to the thoracic wall or pleura
• Spontaneous rupture of a pulmonary bleb or bulla

Tension Pneumothorax

Tension pneumothorax is a life-threatening emergency in which air enters the pleural cavity but cannot escape. A one-way valve mechanism causes progressive accumulation of air and a continuous rise in intrapleural pressure.

This results in:

• Progressive collapse of the affected lung
• Severe respiratory distress
• Hyperexpansion of the affected side of the chest
• Displacement of the mediastinum and trachea toward the opposite side
• Impaired venous return to the heart, which may lead to circulatory compromise

Treatment

Tension pneumothorax requires immediate decompression of the pleural cavity, followed by definitive drainage with a chest tube (tube thoracostomy).

Emergency decompression may be performed by needle or other appropriate decompression technique at a suitable intercostal site, depending on established clinical protocols.

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