Abdomen & Pelvis

The anterior abdominal wall is built from skin, superficial fascia, three flat muscle layers laterally, rectus abdominis anteriorly, associated aponeuroses and deep fascia/peritoneal lining. These layers protect viscera, allow trunk movement and help generate intra-abdominal pressure. The rectus sheath changes composition with level because the aponeuroses of the lateral muscles pass around rectus differently above and below the arcuate line.

The inguinal canal is an oblique passage through the lower abdominal wall. Its normal development and contents differ by sex, but in both cases its boundaries create potential sites of weakness. Direct and indirect inguinal hernias are distinguished by their relationship to the inferior epigastric vessels and by the route through the abdominal wall.

The peritoneum is a serous membrane with parietal and visceral components. Organs relate to it in different ways: some are suspended within peritoneal folds, while others lie primarily behind the peritoneum. Mesenteries connect organs to the body wall and provide pathways for vessels, lymphatics and nerves. Omenta and peritoneal ligaments connect viscera to one another or to the abdominal wall.

Peritoneal spaces and recesses matter because fluid, blood or infection can collect and spread along them. Position changes where free fluid accumulates. 'Intraperitoneal' does not mean an organ floats freely; it describes its serosal covering and mesenteric relationship. Likewise, retroperitoneal organs occupy a different anatomical compartment with different surgical and pathological implications.

The abdominal gastrointestinal tract is organized around foregut, midgut and hindgut developmental territories that correspond broadly with characteristic arterial supplies. The stomach acts as a reservoir and mixing chamber; the small intestine provides most nutrient absorption; the large intestine reclaims water and forms fecal material. Organ position, mesenteric attachment and vascular arcades determine mobility and surgical access.

Surface anatomy gives approximate locations, but organs move with respiration, filling and body position. Therefore, clinical examination must be interpreted together with deeper relationships. The duodenum and pancreas are largely retroperitoneal after development, while much of the jejunum and ileum remains mobile on a mesentery. These differences explain why disease processes can present differently.

The liver occupies much of the right upper abdomen and receives dual blood inflow from the hepatic artery and portal vein. The portal system delivers nutrient-rich venous blood from much of the gastrointestinal tract and spleen to the liver before it returns to systemic circulation through hepatic veins. Bile produced by hepatocytes drains through progressively larger ducts, while the gallbladder stores and concentrates bile between meals.

The pancreas lies largely retroperitoneally and has close relationships with the duodenum, stomach, spleen and major vessels. Its ductal system joins or approaches the biliary pathway near the duodenum. This close arrangement explains why obstruction at different levels can affect bile flow, pancreatic drainage or both. Anatomy should be learned as connected vascular and ductal pathways.

The posterior abdominal wall contains the lumbar vertebrae, psoas and quadratus lumborum muscles, major vessels, sympathetic structures, lumbar plexus and retroperitoneal organs including kidneys and ureters. The kidneys lie high on the posterior wall, with the right usually slightly lower because of the liver. At the renal hilum, vessels and renal pelvis have organized relationships that continue into the renal sinus.

The ureter descends retroperitoneally toward the bladder and narrows at predictable sites, which helps explain common locations where stones may lodge. The abdominal aorta and inferior vena cava lie close to the vertebral column, and their branches or tributaries create important surgical relationships. Deep anatomy is therefore best learned from cross-sectional views as well as anterior diagrams.

The bony pelvis transmits body weight to the lower limbs and encloses pelvic viscera. The pelvic floor, especially the levator ani complex and associated fascia, supports pelvic organs while allowing controlled passage of urinary, gastrointestinal and reproductive structures. Pelvic vessels arise mainly from internal iliac branches, while autonomic and somatic nerves have distinct functions.

The perineum lies inferior to the pelvic diaphragm and is divided conceptually into urogenital and anal triangles. Fascial spaces and pouches organize muscles, erectile tissues, glands, vessels and nerves. Pelvic anatomy differs between sexes in organ arrangement but shares the same fundamental principles of support, neurovascular pathways and confined spaces. Damage to the pelvic floor or nerves can affect continence and organ position.