Thoracic Cavity and Pleura

Topic Overview

Thoracic Cavity and Pleurae — Thoracic Cavity and Dissection

Thoracic Cavity

• The thoracic cavity is the upper part of the ventral body cavity, enclosed by the thoracic wall, diaphragm, and superior thoracic aperture.

• It is divided into three compartments:

  1. Two pleural cavities (right and left) containing the lungs.

  2. One mediastinum, situated between the pleural sacs, containing the heart, great vessels, trachea, oesophagus, and thymus.

• The walls of the cavity are formed by:

  • Anteriorly: Sternum and costal cartilages.

  • Posteriorly: Thoracic vertebrae.

  • Laterally: Ribs and intercostal spaces.

  • Inferiorly: Diaphragm (separating thorax from abdomen).

  • Superiorly: Thoracic inlet (communication with neck).

• The mediastinum acts as the central partition between the two lungs. It is divided into:

  • Superior mediastinum (above the level of sternal angle).

  • Inferior mediastinum, further subdivided into anterior, middle, and posterior parts.

• The pleural cavities are lined by pleurae, which are serous membranes enclosing each lung.

Dissection of Thoracic Cavity

1. Position: Place the cadaver supine with the thorax elevated slightly.

2. Skin incision:

   • A midline incision from the suprasternal notch to the xiphoid process, extended laterally along the costal margins and upwards along midaxillary lines.

3. Reflection:

   • Reflect the skin and superficial fascia to expose pectoralis major and minor.

   • Cut through the intercostal muscles along selected intercostal spaces to open the thoracic cage.

4. Exposure:

   • Remove anterior thoracic wall by cutting costal cartilages close to their junction with ribs.

   • Detach diaphragm along the costal margins to view the thoracic viscera.

5. Observation:

   • Note the two pleural sacs, each containing a lung, and the mediastinum between them.

   • Identify heart within pericardium, thymus (in children), and great vessels emerging from the heart.

6. Further dissection:

   • The pleural reflections and recesses (costodiaphragmatic and costomediastinal) can be traced by following the parietal pleura over the inner thoracic wall and diaphragm.

   • The mediastinal pleura can be gently separated to expose the pericardium and related structures.

The thoracic cavity thus serves as a protective chamber for vital organs of respiration and circulation and provides mechanical support for lung expansion during breathing

Pleura

• The pleura is a serous membrane similar to the peritoneum, composed of mesothelium (flattened epithelium).

• Each lung is enclosed within a pleural sac consisting of two continuous layers:

  • Parietal pleura → lines thoracic wall, mediastinum, diaphragm.

  • Visceral (pulmonary) pleura → covers the lung surface and fissures, inseparable from the lung tissue.

• Both layers enclose a potential space — the pleural cavity — containing a thin film of serous fluid that allows frictionless movement during respiration.

Parts of the Parietal Pleura

1. Costal pleura – lines inner surface of ribs and intercostal spaces.

2. Diaphragmatic pleura – covers the diaphragm’s upper surface.

3. Mediastinal pleura – forms the lateral wall of the mediastinum.

4. Cervical pleura (cupula) – extends into the root of the neck above the first rib.

Pulmonary Ligament

• A double fold of pleura extending down from the lung root.

• Allows movement of pulmonary vessels and descent of lung roots during inspiration

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Pleural Recesses

• Costomediastinal recess → between costal and mediastinal pleura; more prominent on the left near the cardiac notch.

• Costodiaphragmatic recess → between costal and diaphragmatic pleura; deepest at the midaxillary line, between the 8th and 10th ribs.

  • These act as reserve spaces into which the lungs expand during deep inspiration

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Nerve Supply of the Pleura

• Parietal Pleura:

  • Derived from somatopleuric mesoderm, hence supplied by somatic nerves.

  • Costal and peripheral diaphragmatic pleurae: Intercostal nerves → pain referred to thoracic wall.

  • Mediastinal and central diaphragmatic pleurae: Phrenic nerve (C4) → pain referred to shoulder tip via supraclavicular nerves

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• Visceral Pleura:

  • Derived from splanchnopleuric mesoderm, supplied by autonomic nerves accompanying bronchial vessels.

  • Sympathetic: from T2–T5 ganglia → bronchodilation.

  • Parasympathetic: from vagus nerve → bronchoconstriction and glandular secretion.

  • Insensitive to pain

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• Blood Supply:

  • Parietal pleura: Intercostal, internal thoracic, musculophrenic arteries.

  • Venous drainage: Azygos and internal thoracic veins.

  • Lymphatics: Intercostal, internal mammary, posterior mediastinal, and diaphragmatic nodes

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Clinical Anatomy

• Pleurisy (Pleuritis): Inflammation of pleura; may be dry (painful) or wet with pleural effusion. Pain arises from parietal pleura friction.

• Pleural Effusion: Collection of fluid in pleural cavity; obliterates costodiaphragmatic recess.

• Pneumothorax: Air in pleural cavity causing lung collapse.

• Haemothorax: Blood in pleural cavity.

• Hydropneumothorax: Air and fluid together.

• Empyema: Pus in pleural cavity

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Referred Pain:

• Irritation of costal pleura → pain along intercostal nerves (chest/abdominal wall).

• Irritation of mediastinal or central diaphragmatic pleura → pain at shoulder tip (C4 dermatome).

  • Right shoulder pain: Gallbladder inflammation.

  • Left shoulder pain: Splenic rupture.

Important Procedures:

• Paracentesis thoracis: Needle inserted in lower part of intercostal space to avoid main vessels and nerve

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• Pleural effusion is a frequent feature in pulmonary tuberculosis

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In summary, the pleura serves as a protective and lubricating membrane for the lungs. Its dual nerve supply explains the distinct patterns of referred pain, and its recesses and folds (like the pulmonary ligament) are essential for lung mechanics during respiration.