Costal and Mediastinal Surfaces

Each lung has distinct external surfaces that reflect its relationships with the thoracic wall, diaphragm, heart, mediastinum, and major vessels. Two of the most important are the costal surface and the mediastinal surface.

The costal surface is broad, smooth, and convex and lies against the ribs and intercostal spaces. The mediastinal surface faces medially toward the mediastinum and contains the hilum of the lung, through which the structures forming the root of the lung enter and leave. The mediastinal surface also bears impressions produced by the heart and neighboring mediastinal structures.

External Surfaces of the Lungs

The lungs are commonly described as having three major surfaces:

• Costal surface
• Mediastinal surface
• Diaphragmatic surface

These surfaces are continuous with one another around the borders of the lung rather than being completely separate anatomical compartments.

Costal Surface

The costal surface is the large convex surface facing the internal aspect of the thoracic wall.

It is shaped to conform to the curvature of the ribs and intercostal spaces and forms much of the lateral and posterolateral external surface of each lung.

Shape of the Costal Surface

The costal surface is smooth and strongly convex. Its curvature corresponds broadly to the concavity of the internal thoracic wall.

Posteriorly, the surface approaches the thoracic vertebral column, while anteriorly and laterally it lies against the rib cage.

Relations of the Costal Surface

The costal surface is related to:

• Ribs
• Costal cartilages
• Intercostal spaces
• Intercostal muscles
• Intercostal nerves and vessels
• Endothoracic fascia
• Costal pleura

The visceral pleura covering the lung is separated from the costal pleura by the potential pleural cavity.

Pleural Relationship

The external surface of the lung is covered by visceral pleura, which closely adheres to the pulmonary tissue and extends into the fissures.

Across the pleural cavity, the costal surface faces the costal pleura, the portion of parietal pleura lining the internal thoracic wall.

Rib Impressions

The soft pulmonary tissue conforms to the contour of the thoracic cage. In anatomical specimens, shallow grooves corresponding to the ribs may sometimes be visible on the costal surface.

These are relationships rather than fixed structural divisions of the lung.

Posterior Part of the Costal Surface

The posterior portion of the costal surface is broad and rounded and lies beside the thoracic vertebral column.

It is related to the posterior ends of the ribs and structures of the posterior thoracic wall.

Costal Surface During Respiration

During inspiration, expansion of the thoracic cavity allows the lungs to increase in volume. The costal surfaces move with the expanding thoracic walls as the ribs elevate and the dimensions of the thorax increase.

During expiration, elastic recoil reduces pulmonary volume and the costal surfaces return toward their resting configuration.

Mediastinal Surface

The mediastinal surface is the medial surface of each lung facing the mediastinum.

Unlike the smoothly convex costal surface, the mediastinal surface is more irregular because it is molded by the heart, great vessels, esophagus, and other mediastinal structures.

Hilum of the Lung

The most important feature of the mediastinal surface is the hilum.

The hilum is a depressed region through which structures forming the root of the lung pass between the mediastinum and pulmonary tissue.

Structures Passing Through the Hilum

Structures associated with the hilum and root include:

• Main and lobar bronchi
• Pulmonary arteries
• Pulmonary veins
• Bronchial arteries and veins
• Lymphatic vessels
• Bronchopulmonary lymph nodes
• Autonomic nerves
• Connective tissue

Cardiac Impression

The mediastinal surface of each lung bears a depression produced by the heart and pericardium known as the cardiac impression.

The impression is considerably deeper on the left lung because the heart projects predominantly toward the left side of the thoracic cavity.

Left Cardiac Impression

The left mediastinal surface has a prominent cardiac impression related primarily to the left ventricle and other portions of the heart enclosed within the pericardium.

The depth of this impression contributes to the characteristic shape of the left lung and is associated with the presence of the cardiac notch along its anterior border.

Right Cardiac Impression

The right lung also has a cardiac impression, but it is generally shallower than that of the left lung.

It is related principally to the right side of the heart, particularly the right atrium.

Right Mediastinal Surface

The right mediastinal surface is molded by several major structures of the right side of the mediastinum.

In addition to the cardiac impression and hilum, grooves or impressions may be produced by the superior vena cava, inferior vena cava, azygos vein, and esophagus.

Superior Vena Cava Impression

The superior vena cava lies anterior and superior to the root of the right lung and may produce a vertical groove on the mediastinal surface.

This relationship is particularly evident near the anterior portion of the right hilum.

Inferior Vena Cava Relationship

The inferior vena cava enters the thorax through the diaphragm and passes almost immediately into the right atrium.

Its relationship with the lower mediastinal surface of the right lung may contribute to the contour of this region.

Azygos Vein Impression

The azygos vein arches over the root of the right lung before entering the superior vena cava.

This arch can produce a curved groove superior to the right pulmonary hilum.

Esophageal Impression on the Right Lung

The esophagus lies posterior to the trachea and heart and has a close relationship with the posterior part of the mediastinal surface of the right lung.

A shallow esophageal groove may therefore be present posterior to the right lung root.

Left Mediastinal Surface

The mediastinal surface of the left lung is characterized by a deep cardiac impression and grooves related to major components of the aorta.

Important relationships include the arch of the aorta, descending thoracic aorta, left subclavian artery, and esophagus.

Aortic Arch Impression

The arch of the aorta passes superior to the root of the left lung.

It produces a curved groove on the mediastinal surface above the hilum.

Descending Aortic Impression

The descending thoracic aorta passes inferiorly posterior to the left lung root.

It commonly produces a prominent vertical groove on the posterior part of the left mediastinal surface.

Left Subclavian Artery Impression

The left subclavian artery ascends from the aortic arch toward the root of the neck.

Its close relationship with the superior medial aspect of the left lung can produce a groove near the apex.

Esophageal Relationship on the Left

The esophagus also relates to the posterior mediastinal surface of the left lung, although its relationship is influenced by the position of the descending thoracic aorta.

The exact prominence of an esophageal impression varies between individuals.

Right and Left Mediastinal Surface Comparison

Feature	Right Lung	Left Lung
Cardiac impression	Shallower	Deeper and more extensive
Azygos arch groove	Present	Absent
Superior vena cava groove	Prominent relationship	Absent
Aortic arch groove	Absent	Prominent
Descending aortic groove	Absent	Prominent
Cardiac notch	Absent	Present along anterior border

Cardiac Notch

The cardiac notch is a marked indentation along the anterior border of the left lung.

It accommodates the leftward projection of the heart and is closely related to the deep cardiac impression on the mediastinal surface.

Lingula

Inferior to the cardiac notch is the lingula, a tongue-shaped projection of the left superior lobe.

The lingula overlies part of the cardiac region and is often considered the left-sided anatomical counterpart of the right middle lobe.

Mediastinal Pleura

The mediastinal surface of each lung is covered by visceral pleura. Across the pleural cavity, it faces the mediastinal pleura, the portion of parietal pleura covering the lateral surfaces of mediastinal structures.

At the root of the lung, mediastinal pleura reflects onto the lung and becomes continuous with visceral pleura.

Pleural Reflection at the Root

The pleural reflection surrounds the structures forming the lung root and creates a sleeve of pleura around them near the hilum.

Inferior to the root, the anterior and posterior layers of this reflection extend downward as the pulmonary ligament.

Pulmonary Ligament

The pulmonary ligament is a double layer of pleura extending inferiorly from the lung root.

It provides mobility for the structures of the root and helps accommodate changes in pulmonary position and venous relationships during respiration.

Borders Between the Surfaces

The surfaces of the lung meet along identifiable borders. The anterior border separates the costal and mediastinal surfaces anteriorly, while the posterior border forms a broad rounded transition between these surfaces posteriorly.

The inferior border separates the diaphragmatic surface from the costal and mediastinal surfaces.

Anterior Border

The anterior border of each lung is relatively sharp and extends downward from the apex.

On the left lung, it is interrupted by the cardiac notch. The right anterior border is comparatively straighter.

Posterior Border

The posterior border is broad and rounded rather than sharply defined.

It lies alongside the thoracic vertebral column and forms the transition between the posterior portions of the costal and mediastinal surfaces.

Inferior Border

The inferior border is relatively sharp and separates the base of the lung from the costal and mediastinal surfaces.

Its position changes during respiration as the lungs expand and recoil with movements of the diaphragm and thoracic wall.

Relationship to the Diaphragmatic Surface

The costal and mediastinal surfaces become continuous inferiorly with the diaphragmatic surface, or base, of the lung.

The diaphragmatic surface is concave and rests upon the dome of the diaphragm.

Surface Anatomy and Thoracic Imaging

The external surfaces of the lungs help explain their appearance on radiographs, computed tomography, and magnetic resonance imaging.

The costal surfaces define much of the peripheral pulmonary contour, while the mediastinal surfaces form interfaces with the heart, great vessels, mediastinum, and hilar structures.

Mediastinal Impressions on Imaging

Cross-sectional imaging demonstrates the close relationships between the lungs and neighboring mediastinal structures more clearly than the shallow grooves seen in anatomical specimens.

Knowledge of these normal relationships helps distinguish expected pulmonary contours from masses, vascular enlargement, lymphadenopathy, and other abnormalities.

Hilar Assessment

Because the hilum lies on the mediastinal surface, evaluation of this surface is closely related to assessment of pulmonary vessels, bronchi, and hilar lymph nodes.

Abnormal hilar enlargement can reflect vascular enlargement, lymphadenopathy, tumors, or other pathological processes.

Pleural Disease

The distinction between pulmonary surfaces and the adjacent parietal pleura is clinically important in pleural disease.

Fluid, air, blood, infection, or other material may accumulate within the pleural cavity and separate the visceral surface of the lung from the thoracic wall or mediastinum.

Pneumothorax

In a pneumothorax, air enters the pleural cavity and disrupts the normal mechanical coupling between visceral and parietal pleura.

The lung may recoil away from the thoracic wall, changing the relationship of its costal surface to the costal pleura.

Pleural Effusion

A pleural effusion is an abnormal accumulation of fluid within the pleural cavity.

Fluid commonly collects in dependent pleural recesses and may compress adjacent portions of the costal and diaphragmatic surfaces of the lung.

Surgical Importance

The external surfaces provide important orientation during thoracic surgery. Identification of the costal, mediastinal, and diaphragmatic surfaces helps establish the position of lobes, fissures, pulmonary borders, hilum, and root.

The mediastinal surface is particularly important because major bronchovascular structures enter and leave the lung through the hilum.

Costal and Mediastinal Surfaces Compared

Feature	Costal Surface	Mediastinal Surface
Orientation	Faces thoracic wall	Faces mediastinum
Shape	Broad and convex	More irregular and generally concave centrally
Major relationships	Ribs and intercostal spaces	Heart, great vessels, esophagus, and mediastinum
Hilum	Absent	Present
Cardiac impression	Absent	Present
Adjacent parietal pleura	Costal pleura	Mediastinal pleura

Key Anatomical Relationships

• The costal surface is broad, smooth, and convex.
• The costal surface faces the ribs and intercostal spaces.
• The mediastinal surface faces the mediastinum.
• The hilum is located on the mediastinal surface.
• The mediastinal surface contains impressions from neighboring thoracic structures.
• The cardiac impression is deeper on the left lung.
• The right mediastinal surface relates to the superior vena cava and azygos vein.
• The azygos arch passes above the right lung root.
• The left mediastinal surface bears grooves for the aortic arch and descending thoracic aorta.
• The aortic arch passes above the left lung root.
• The left anterior border contains the cardiac notch.
• The lingula lies inferior to the cardiac notch.
• Visceral pleura covers both pulmonary surfaces.
• Costal pleura faces the costal surface across the pleural cavity.
• Mediastinal pleura faces the mediastinal surface.
• Mediastinal pleura reflects around the lung root to become visceral pleura.

Clinical Importance

The costal and mediastinal surfaces reflect the position of the lungs within the thoracic cavity and their relationships with surrounding structures. The smooth convex costal surface conforms to the thoracic wall, while the more complex mediastinal surface is shaped by the heart, great vessels, esophagus, and structures passing through the pulmonary hilum.

Differences between the right and left mediastinal surfaces provide important anatomical landmarks. The right lung is related to structures such as the superior vena cava and azygos arch, whereas the left lung bears prominent impressions from the heart, aortic arch, and descending thoracic aorta.

Understanding the costal and mediastinal surfaces provides the anatomical foundation for studying lung borders, pulmonary hilum, lung root, pleural reflections, cardiac impressions, mediastinal relationships, thoracic imaging, and surgical anatomy of the lungs.