Abdominal Wall

The abdominal wall forms the outer boundary of the abdominal cavity, enclosing and protecting the abdominal organs. It is a layered structure composed of skin, fascia, muscles, and peritoneum, and serves both protective and functional roles. The abdominal wall is involved in posture, trunk movement, breathing assistance, and abdominal pressure regulation, making it vital for actions like coughing, urination, and childbirth. Its composition varies across regions, especially between the anterior, lateral, and posterior aspects, but overall it functions as a unified structural envelope for the abdominal contents.

Structure

The abdominal wall is organized into distinct anatomical layers from superficial to deep. These include the skin, superficial fascia, muscular layers, deep fascia, transversalis fascia, extraperitoneal fat, and parietal peritoneum.

1. Skin

The outermost layer, the skin, varies in thickness and pigmentation. The umbilicus is a central landmark on the anterior abdominal wall and is used as a reference in clinical examinations and surgical procedures.

2. Superficial Fascia

Beneath the skin lies the superficial fascia, which in the lower abdomen differentiates into two layers:

• Camper’s fascia – a fatty superficial layer.
• Scarpa’s fascia – a membranous deeper layer that continues into the perineum and forms the fundiform ligament of the penis or clitoris.

3. Muscular Layers

The muscle layer consists of three flat muscles on each side and two vertical muscles in the midline:

Muscle	Position	Fiber Direction
External Oblique	Lateral	Inferomedial ("hands-in-pockets")
Internal Oblique	Intermediate	Superomedial
Transversus Abdominis	Innermost	Transverse
Rectus Abdominis	Anterior midline	Vertical
Pyramidalis (if present)	Inferior midline	Vertical

These muscles are surrounded by and connected through aponeuroses that contribute to the formation of the rectus sheath.

4. Rectus Sheath

The rectus sheath is a fibrous compartment enclosing the rectus abdominis and pyramidalis muscles. It is formed by the aponeuroses of the three flat abdominal muscles. The composition of the sheath differs above and below the arcuate line:

• Above arcuate line: Internal oblique splits to enclose rectus abdominis.
• Below arcuate line: All three aponeuroses pass anterior to rectus abdominis.

5. Deep Fascia and Transversalis Fascia

Deep to the muscles lies the transversalis fascia, a thin connective tissue layer separating the muscle from the peritoneum. It plays a role in maintaining the structural integrity of the abdominal cavity and contributes to the posterior wall of the inguinal canal.

6. Extraperitoneal Fat and Parietal Peritoneum

These are the innermost layers. Extraperitoneal fat cushions abdominal contents, while the parietal peritoneum is a serous membrane lining the inner surface of the abdominal wall, providing a frictionless interface with abdominal organs.

Regions of the Abdominal Wall

The abdominal wall can be divided into specific zones based on anatomical and functional landmarks:

• Anterior abdominal wall – contains most of the abdominal muscles and the umbilical region.
• Lateral abdominal wall – formed primarily by the oblique and transversus muscles.
• Posterior abdominal wall – includes the lumbar vertebrae, psoas major, iliacus, and quadratus lumborum muscles.

Surface anatomy often divides the abdomen into quadrants (RUQ, LUQ, RLQ, LLQ) or nine regions (e.g., epigastric, umbilical, hypogastric), which helps in clinical evaluation and surgical planning.

Function

The abdominal wall serves several crucial physiological roles:

• Protection: Shields abdominal organs from external trauma.
• Movement: Allows flexion, lateral bending, and rotation of the trunk.
• Respiration: Assists in forced expiration, coughing, and sneezing.
• Pressure regulation: Supports intra-abdominal pressure during activities like defecation, micturition, parturition, and lifting.
• Posture: Contributes to spinal stability and upright posture.

Neurovascular Supply

Nerve Supply

The abdominal wall is innervated by:

• Lower six thoracoabdominal nerves (T7–T11)
• Subcostal nerve (T12)
• Iliohypogastric and ilioinguinal nerves (L1)

These nerves run between the internal oblique and transversus abdominis and provide motor innervation to the muscles and sensory innervation to the skin.

Blood Supply

Arterial supply comes from:

• Superior epigastric artery (branch of internal thoracic)
• Inferior epigastric artery (branch of external iliac)
• Deep circumflex iliac artery
• Intercostal and lumbar arteries

Venous drainage mirrors the arterial supply and ultimately drains into the internal thoracic, external iliac, and azygos venous systems.

Clinical Significance

• Hernias: Weakness in the abdominal wall can lead to hernias (inguinal, femoral, umbilical).
• Diastasis recti: Separation of rectus abdominis muscles, common in pregnancy and obesity.
• Surgical access: The abdominal wall is traversed during many surgeries (laparotomy, cesarean section).
• Trauma: Blunt force to the abdomen can result in muscle tears or hematoma within the wall layers.