The abdomen is a roughly cylindrical chamber extending from the inferior margin of the thorax to the superior margin of the pelvis and the lower limb. The inferior thoracic aperture, closed by the diaphragm, forms its superior opening. Inferiorly, the deep abdominal wall is continuous with the pelvic wall at the pelvic inlet.

The diaphragm forms the superior boundary of the abdomen, separating it from the thoracic cavity.

The peritoneal cavity is a single large cavity enclosed by the abdominal wall that freely communicates with the pelvic cavity. It is lined by peritoneum.

Abdominal viscera include major elements of the gastrointestinal system (esophagus, stomach, intestines, liver, pancreas, gallbladder), the spleen, components of the urinary system (kidneys, ureters), suprarenal glands, and major neurovascular structures.

The abdomen houses and protects major viscera. Much of the liver, gallbladder, stomach, and spleen, and parts of the colon are protected by the thoracic wall as they lie under the domes of the diaphragm. Viscera not under the diaphragm are protected by the muscular walls of the abdomen.

The abdominal wall assists in breathing by relaxing during inspiration to accommodate thoracic expansion and the inferior displacement of abdominal viscera. During expiration, it contracts to help elevate the diaphragm, reducing thoracic volume and aiding in forced expulsion of air.

Contraction of abdominal wall muscles can dramatically increase intra-abdominal pressure when the diaphragm is fixed, which assists in voiding the bladder and rectum, and in childbirth.

The skeletal elements include the five lumbar vertebrae and their intervertebral discs, the superior expanded parts of the pelvic bones, and bony components of the inferior thoracic wall such as the costal margin, rib XII, the end of rib XI, and the xiphoid process.

The lateral parts of the abdominal wall are predominantly formed by three layers of muscles: transversus abdominis, internal oblique, and external oblique.

Anteriorly, the rectus abdominis is a segmented muscle on each side that spans the distance between the inferior thoracic wall and the pelvis.

Structural continuity between the posterior, lateral, and anterior parts of the abdominal wall is provided by thick fascia posteriorly and by flat tendinous sheets (aponeuroses) derived from the muscles of the lateral wall.

The abdominal cavity is lined by peritoneum, which consists of a single layer of cells (mesothelium) and a supportive layer of connective tissue.

Parietal peritoneum lines the abdominal wall, while visceral peritoneum covers the suspended organs.

Intraperitoneal structures are suspended from the abdominal wall by mesenteries, such as elements of the gastrointestinal system.

Retroperitoneal structures are not suspended by a mesentery and lie between the parietal peritoneum and the abdominal wall. Examples include the kidneys and ureters.

The omental foramen is a restricted opening that connects the omental bursa (lesser sac) to the rest of the peritoneal cavity (greater sac). It is formed by the secondary fusion of the duodenum to the body wall, massive growth of the liver, and fusion of the liver's superior surface to the diaphragm.

The omental bursa is the part of the abdominal cavity enclosed by the expanded dorsal mesentery, posterior to the stomach.

The greater omentum is an apron-like structure formed by the inferior enlargement and fusion of the dorsal mesentery. It is suspended from the greater curvature of the stomach and lies over other abdominal viscera.

The midgut develops into the distal duodenum, jejunum, ileum, ascending colon, and proximal two-thirds of the transverse colon. During development, it herniates into the umbilical cord and then returns to the abdominal cavity, undergoing rotation.

The anterior rami of thoracic spinal nerves T7 to T12, and the subcostal nerve T12, supply the abdominal wall. T5 and T6 supply upper parts of the external oblique muscle. Lumbar nerve L1 innervates the inguinal and suprapubic regions.

The inguinal canal is a passage through the anterior abdominal wall created by the processus vaginalis during the descent of the gonads. It is a weak area prone to hernias.

The transpyloric plane is a horizontal plane that transects the body through the lower aspect of vertebra LI. It is approximately midway between the jugular notch and the pubic symphysis and relates to the pyloric orifice, pancreas, and hila of the kidneys.

The three major unpaired arteries branching from the abdominal aorta are the celiac artery (supplying the foregut), the superior mesenteric artery (supplying the midgut), and the inferior mesenteric artery (supplying the hindgut).

The hepatic portal system is a vascular system where venous blood from the digestive tract, pancreas, gallbladder, and spleen passes through the liver before returning to the heart. This allows the liver to process nutrients and detoxify blood.