Costodiaphragmatic Recess

The costodiaphragmatic recess, also called the costophrenic recess, is a potential space within the pleural cavity formed where the costal pleura reflects onto the diaphragmatic pleura. It lies along the inferior margin of the pleural cavity and provides additional space into which the lower border of the lung can expand during inspiration.

The recess is particularly important clinically because fluid within the pleural cavity tends to collect in its dependent portions. Its surface anatomy is also important when interpreting chest imaging and performing procedures such as thoracentesis.

Formation

The costodiaphragmatic recess is formed at the junction between the costal and diaphragmatic portions of the parietal pleura.

The costal pleura lines the internal surface of the thoracic wall, while the diaphragmatic pleura covers the superior surface of the diaphragm. Where these two pleural surfaces approach one another inferiorly, they form a narrow potential space.

Location

The recess extends around the inferior part of the pleural cavity between the thoracic wall and diaphragm.

It is deepest laterally and posterolaterally, where the inferior boundary of the pleural sac extends substantially below the inferior border of the lung during quiet respiration.

Pleural Reflections

The boundaries of the recess are determined by the inferior reflection of the parietal pleura.

At this reflection, the pleura changes direction from the inner surface of the thoracic wall onto the superior surface of the diaphragm.

Costal Pleura

The costal pleura lines the internal surfaces of the ribs, intercostal spaces, and associated thoracic wall structures.

Inferiorly, it reflects onto the diaphragm and becomes continuous with the diaphragmatic pleura.

Diaphragmatic Pleura

The diaphragmatic pleura covers most of the superior surface of the diaphragm.

Near the thoracic wall, it approaches the costal pleura and forms the medial boundary of the costodiaphragmatic recess.

Potential Space

The costodiaphragmatic recess is described as a potential space because under normal conditions its opposing pleural surfaces are separated only by the thin film of fluid present within the pleural cavity.

During inspiration, the inferior border of the lung descends into this space and separates the opposing pleural surfaces.

Relationship to the Lung

During quiet respiration, the inferior border of the lung does not completely occupy the inferior extent of the pleural cavity.

The resulting space between the lower border of the lung and the inferior pleural reflection forms the functional costodiaphragmatic recess.

Inferior Border of the Lung

The inferior border of each lung follows the contour of the diaphragm and separates the costal surface from the diaphragmatic surface.

Its position changes during respiration as the diaphragm moves and the lungs expand or recoil.

Inferior Border of the Pleura

The inferior reflection of the parietal pleura extends approximately two ribs below the inferior border of the lung at several commonly used surface landmarks.

This difference creates the space available for inferior expansion of the lung.

Surface Anatomy

Surface Landmark	Inferior Border of Lung	Inferior Pleural Reflection
Midclavicular line	Approximately rib 6	Approximately rib 8
Midaxillary line	Approximately rib 8	Approximately rib 10
Scapular line	Approximately rib 10	Approximately rib 12

These levels are conventional surface anatomical approximations and can vary with respiration, posture, and individual anatomy.

Midclavicular Line

Near the midclavicular line, the inferior border of the lung is commonly approximated at the sixth rib.

The pleural reflection extends farther inferiorly, approximately to the eighth rib.

Midaxillary Line

At the midaxillary line, the inferior border of the lung is commonly approximated at the eighth rib.

The inferior pleural reflection reaches approximately the tenth rib, producing a clinically important lateral portion of the recess.

Posterior Extent

Posteriorly, the inferior border of the lung reaches approximately the tenth rib near the scapular line.

The pleural reflection may descend to approximately the twelfth rib, maintaining the characteristic difference between lung and pleural boundaries.

Changes During Inspiration

During inspiration, the diaphragm contracts and descends while the lungs expand.

The inferior borders of the lungs move downward into the costodiaphragmatic recesses, reducing the unoccupied portion of these spaces.

Deep Inspiration

During deep inspiration, greater diaphragmatic descent permits greater expansion of the lungs.

The inferior lung margins extend farther into the recesses than during quiet inspiration.

Changes During Expiration

During expiration, the lungs recoil and their inferior borders move superiorly.

The costal and diaphragmatic pleura therefore approach one another over a larger area, increasing the extent of the recess not occupied by lung tissue.

Role in Lung Expansion

The costodiaphragmatic recess provides reserve space for changes in lung volume.

Without this difference between the inferior boundaries of the lung and pleural sac, the lung would have less room to expand inferiorly during inspiration.

Right Costodiaphragmatic Recess

The right recess follows the inferior margin of the right pleural cavity and lies superior to the diaphragm.

The liver lies inferior to the right hemidiaphragm and influences the position and contour of the right diaphragmatic dome.

Left Costodiaphragmatic Recess

The left recess follows the inferior margin of the left pleural cavity.

Its relationships differ from those on the right because of the underlying stomach, spleen, left lobe of the liver, and other upper abdominal structures.

Relationship to the Diaphragm

The diaphragm forms the muscular floor of the thoracic cavity and separates it from the abdominal cavity.

The diaphragmatic pleura covering its superior surface forms one side of the costodiaphragmatic recess.

Relationship to Abdominal Organs

Abdominal organs lie immediately inferior to the diaphragm and therefore indirectly beneath the recess.

On the right, the liver is particularly important. On the left, the stomach and spleen are among the important neighboring structures.

Pleural Fluid

A small amount of serous fluid normally occupies the pleural cavity.

This fluid lubricates the pleural surfaces and allows the visceral and parietal pleura to slide against one another during respiratory movement.

Pleural Effusion

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

Because gravity draws free pleural fluid toward dependent regions, the costodiaphragmatic recess is an important site of fluid accumulation, particularly in an upright individual.

Costophrenic Angle

On a frontal chest radiograph, the lateral junction of the diaphragm and thoracic wall forms the costophrenic angle.

In normal conditions this angle is relatively sharp. Pleural fluid can accumulate in this region and alter its radiographic appearance.

Blunting of the Costophrenic Angle

Accumulation of pleural fluid may produce blunting of the costophrenic angle on chest imaging.

This finding can provide evidence of pleural fluid, although imaging interpretation must consider the projection, patient position, and other possible causes of an abnormal costophrenic contour.

Posterior Costophrenic Angle

The posterior part of the costodiaphragmatic recess is relatively deep.

On lateral chest radiographs, small amounts of pleural fluid may become apparent posteriorly because fluid gravitates into dependent portions of the pleural space.

Effect of Body Position

The location of free pleural fluid changes with body position.

In an upright patient, fluid tends to accumulate inferiorly. In a supine patient, fluid may redistribute posteriorly rather than producing the same pattern seen in an upright chest radiograph.

Thoracentesis

Thoracentesis is a procedure in which a needle or catheter is introduced into the pleural cavity to obtain or remove pleural fluid.

The costodiaphragmatic recess is clinically relevant because pleural effusions commonly collect within its dependent portions.

Intercostal Neurovascular Bundle

The main intercostal neurovascular bundle runs within the costal groove along the inferior margin of a rib.

Procedures entering an intercostal space must account for this relationship to reduce the risk of injury to the intercostal vein, artery, and nerve.

Needle Relationship to the Rib

When accessing an intercostal space, the course of the principal intercostal neurovascular bundle along the inferior border of the rib is an important anatomical consideration.

Smaller collateral branches may also be present near the superior margin of the rib below, so procedural placement requires appropriate anatomical and imaging guidance.

Risk of Diaphragmatic Injury

The diaphragm lies immediately deep to the inferior pleural space.

Excessively inferior instrumentation can therefore injure the diaphragm or enter the abdominal cavity.

Risk to Abdominal Viscera

Because the liver, spleen, stomach, and other abdominal structures lie below the diaphragm, procedures performed too inferiorly may damage abdominal viscera.

This risk emphasizes the importance of identifying the diaphragm and pleural fluid before intervention.

Ultrasound Guidance

Ultrasound can identify pleural fluid, the diaphragm, lung movement, and neighboring structures.

It is therefore useful for selecting an appropriate site for pleural procedures and reducing the risk of injury to surrounding structures.

Pneumothorax

A pneumothorax occurs when air enters the pleural cavity and disrupts the normal mechanical relationship between the lung and thoracic wall.

Unlike free pleural fluid in an upright patient, pleural air tends to rise toward nondependent portions of the pleural cavity.

Hemothorax

A hemothorax is an accumulation of blood within the pleural cavity.

Like other pleural fluids, blood may collect within dependent portions of the pleural cavity, including the costodiaphragmatic recess.

Empyema

An empyema is a collection of infected purulent material within the pleural space.

Fluid may occupy the costodiaphragmatic recess, although inflammatory adhesions and loculations can alter its normal distribution.

Pleural Adhesions

Inflammation can cause the visceral and parietal pleura to adhere to one another.

Adhesions involving the inferior pleural cavity can restrict normal movement of the lung into the costodiaphragmatic recess during inspiration.

Innervation of the Costal Pleura

The costal pleura receives somatic sensory innervation from the intercostal nerves.

Irritation of this pleura can therefore produce relatively well-localized pain corresponding to the affected thoracic wall region.

Innervation of the Diaphragmatic Pleura

The diaphragmatic pleura has different sensory innervation centrally and peripherally.

The central portion is supplied primarily by the phrenic nerves, while the peripheral portion receives sensory fibers from lower intercostal nerves.

Pleural Pain

The parietal pleura is sensitive to pain, pressure, and other somatic stimuli.

Inflammation involving the costal or peripheral diaphragmatic pleura may produce localized thoracic pain carried by intercostal nerves.

Referred Shoulder Pain

Irritation of the central diaphragmatic pleura can produce pain referred toward the shoulder because sensory fibers travel through the phrenic nerve to cervical spinal cord levels C3-C5.

This pattern differs from pain arising from the peripheral diaphragmatic or costal pleura.

Costodiaphragmatic Recess and Auscultation

The inferior lung fields extend toward the recess during inspiration.

Knowledge of the changing inferior lung boundary is useful when examining breath sounds over the lower thoracic wall.

Costodiaphragmatic Recess and Percussion

Percussion over the lower thorax can provide information about underlying lung, pleural fluid, diaphragm, and abdominal organs.

Changes in the expected percussion note may accompany pleural fluid accumulation or alterations in lung expansion.

Surface Projection

The surface projection of the recess follows the gap between the inferior border of the lung and the inferior reflection of the pleura.

This gap is commonly approximated as two rib levels at the midclavicular, midaxillary, and posterior reference lines.

Relationship During Quiet Breathing

During quiet breathing, the inferior border of the lung moves within a limited range and does not fully occupy the recess.

The opposing costal and diaphragmatic pleural surfaces therefore remain closely apposed over part of the recess.

Relationship During Deep Breathing

Deep inspiration increases diaphragmatic descent and lung expansion.

The inferior lung border moves farther into the recess, allowing the lower lobes to expand into pleural space that was not occupied during quiet expiration.

Functional Importance

The recess permits substantial respiratory movement without requiring the lung to occupy the entire inferior pleural cavity at rest.

It therefore accommodates changes in lung volume associated with the respiratory cycle.

Costodiaphragmatic and Costomediastinal Recesses

Feature	Costodiaphragmatic Recess	Costomediastinal Recess
Pleural reflections	Costal and diaphragmatic pleura	Costal and mediastinal pleura
Location	Inferior pleural cavity	Anterior pleural cavity
Relationship to lung expansion	Receives inferior lung border during inspiration	Accommodates anterior lung expansion
Clinical association	Common dependent site for pleural fluid	Important anterior pleural reflection

Anatomical Boundaries

• Laterally: costal pleura lining the thoracic wall
• Medially and inferiorly: diaphragmatic pleura covering the diaphragm
• Superiorly: inferior border of the lung when it extends into the recess
• Within the space: the normal thin film of pleural fluid between parietal and visceral pleural surfaces where lung is present, and apposed parietal pleural surfaces in the unoccupied recess

Key Anatomical Relationships

• The costodiaphragmatic recess is formed between costal and diaphragmatic pleura.
• It is a potential space within the inferior pleural cavity.
• The inferior lung border does not normally extend to the inferior limit of the pleural sac during quiet respiration.
• The recess is particularly deep laterally and posteriorly.
• The lung descends into the recess during inspiration.
• The recess becomes less occupied by lung during expiration.
• The lung border is approximately at ribs 6, 8, and 10 along the midclavicular, midaxillary, and scapular lines.
• The pleural reflection is approximately at ribs 8, 10, and 12 at corresponding landmarks.
• Pleural fluid commonly accumulates in dependent portions of the recess.
• Pleural effusion can blunt the costophrenic angle on chest imaging.
• The costal pleura is supplied by intercostal nerves.
• The central diaphragmatic pleura is supplied by the phrenic nerve.
• The peripheral diaphragmatic pleura is supplied by lower intercostal nerves.
• The diaphragm and abdominal viscera lie immediately inferior to the recess.
• The recess is clinically important during pleural drainage and thoracentesis.

Clinical Importance

The costodiaphragmatic recess is an important anatomical space because it accommodates movement of the inferior lung during respiration. During inspiration, the lower border of the lung descends into the recess as the diaphragm contracts. During expiration, the lung recoils superiorly and a larger portion of the recess becomes unoccupied by lung tissue.

The recess is also a major site of pleural fluid accumulation. In an upright individual, free fluid tends to move into the dependent inferior pleural cavity, where it can produce blunting of the costophrenic angle on imaging. This relationship makes the recess important in the evaluation of pleural effusions.

Understanding the costodiaphragmatic recess provides the anatomical foundation for studying pleural reflections, respiratory lung movement, surface anatomy, pleural effusion, costophrenic angles, thoracentesis, pleural pain, and relationships between the thorax and diaphragm.