Anatomy of the Abdomen

The abdomen is the largest body cavity, extending from the diaphragm superiorly to the pelvic inlet inferiorly. It contains the majority of the digestive, urinary, and reproductive organs and is traversed by major vascular highways. Regional anatomy of the abdomen is foundational for surgery, gastroenterology, and emergency medicine. This guide is for educational purposes only.

## Abdominal Wall Layers

The anterior abdominal wall consists of several layers, from superficial to deep: skin, superficial fascia (with a fatty Camper's fascia and a membranous Scarpa's fascia), and then three flat muscular layers — the external oblique (fibers running inferomedially, "hands in pockets"), internal oblique (fibers running superomedially), and transversus abdominis (fibers running transversely) — with the rectus abdominis and pyramidalis muscles flanking the midline within the rectus sheath. The rectus sheath has an anterior and posterior wall above the arcuate line; below the arcuate line (approximately at the level of the anterior superior iliac spine), the posterior sheath is absent and the transversalis fascia directly backs the rectus. Deep to the muscles lies the transversalis fascia, the extraperitoneal fat, and the parietal peritoneum.

Weak points of the abdominal wall are sites of potential herniation. The inguinal canal contains the spermatic cord (males) or round ligament (females); indirect inguinal hernias emerge laterally through the deep inguinal ring (lateral to the inferior epigastric vessels), while direct inguinal hernias push through Hesselbach's triangle medially. The umbilical ring, the femoral canal (below and lateral to the inguinal ligament), and surgical incision sites are additional hernia sites.

## Peritoneal Cavity

The peritoneum is a mesothelial serous membrane lining the abdominal cavity (parietal peritoneum) and covering most abdominal organs (visceral peritoneum). Intraperitoneal organs (stomach, spleen, liver, jejunum, ileum, transverse colon, sigmoid colon) are almost entirely covered by visceral peritoneum and suspended by mesenteries. Retroperitoneal organs (kidneys, ureters, adrenal glands, aorta, IVC, duodenum [most], ascending and descending colon, rectum [partly], pancreas [most]) lie behind the peritoneum.

The peritoneal cavity communicates between the greater sac (the main peritoneal space) and the lesser sac (omental bursa), through the epiploic foramen of Winslow (bounded anteriorly by the hepatoduodenal ligament containing the portal triad). The greater omentum — an apron of fat hanging from the greater curvature of the stomach — is an important immunological and surgical structure that can wall off infections ("the policeman of the abdomen").

## GI Tract Organs

The esophagus passes through the esophageal hiatus of the diaphragm at T10 to join the stomach at the gastroesophageal junction. The stomach has a fundus, body, and pyloric antrum/canal, with a lesser curvature (supplied by the right and left gastric arteries) and a greater curvature (supplied by the right and left gastro-omental/gastroepiploic arteries). The pyloric sphincter controls gastric emptying.

The small intestine (duodenum, jejunum, ileum — total length approximately 6–7 meters) is the primary site of digestion and absorption. The duodenum (approximately 25 cm, C-shaped) is largely retroperitoneal and receives bile and pancreatic juice at the major duodenal papilla (ampulla of Vater) in its second part. The jejunum and ileum are suspended by the mesentery, which carries blood vessels, lymphatics, and nerves from the root (originating to the left of L2 and extending to the right iliac fossa).

The large intestine (cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum) is approximately 1.5 meters long and is characterized by teniae coli (three longitudinal muscle bands), haustra (sacculations), and omental appendices (fatty tags). The ileocecal valve controls flow from ileum to cecum. The appendix arises from the posteromedial cecum; its surface marking (McBurney's point) corresponds to one-third of the distance from the ASIS to the umbilicus.

## Hepatobiliary System

The liver, the largest intraabdominal organ, occupies predominantly the right hypochondrium and epigastrium. Surgically, it is divided into eight functional segments (Couinaud segments), each with its own portal pedicle and hepatic venous drainage — allowing anatomical resections. The hepatic portal vein (formed by the union of superior mesenteric and splenic veins at the transpyloric plane) carries nutrient-rich blood from the GI tract to the liver; it, the hepatic artery, and the bile duct run together in the hepatoduodenal ligament (the portal triad, compressible at the Pringle maneuver to control hemorrhage during liver surgery).

The gallbladder is situated in a fossa on the visceral surface of the liver. Calot's triangle — bounded by the cystic duct, common hepatic duct, and visceral surface of the liver — contains the cystic artery (usually from the right hepatic artery) and must be carefully dissected during cholecystectomy. The pancreas lies retroperitoneally, with its head nestled in the duodenal C-loop, its neck overlying the superior mesenteric vessels, and its tail reaching the splenic hilum.

## Renal System

The kidneys lie retroperitoneally on the posterior abdominal wall at the T12–L3 level, with the right kidney slightly lower than the left (displaced by the liver). The renal hilum — where the renal vein (anterior), renal artery (middle), and renal pelvis (posterior) enter and leave — faces anteromedially. The ureters descend from the renal pelvis along the anterior surface of the psoas major muscle, cross the pelvic brim anterior to the bifurcation of the common iliac artery (a critical crossing point in pelvic surgery), and enter the posterior bladder wall obliquely at the ureterovesical junction.

## Abdominal Aorta and Branches

The abdominal aorta enters the abdomen through the aortic hiatus at T12 and bifurcates into the common iliac arteries at the L4 level (approximately at the level of the umbilicus). Major branches include: the celiac trunk (L1) — supplying the foregut (esophagus, stomach, proximal duodenum, liver, gallbladder, spleen, pancreas) via its three divisions (left gastric, splenic, hepatic arteries); the superior mesenteric artery (L1, just below celiac) — supplying the midgut (distal duodenum through proximal two-thirds of transverse colon); and the inferior mesenteric artery (L3) — supplying the hindgut (distal transverse through upper rectum). The watershed area between SMA and IMA territory (splenic flexure of the colon) is particularly vulnerable to ischemia.