Gastrointestinal system · USMLE notes

The gallbladder is a pear-shaped reservoir nestled in a fossa on the underside of the liver, to the right of the quadrate lobe. Its principal role is to store and concentrate bile produced by the liver, releasing it during digestion.

• Bile flow: Bile enters and leaves via the cystic duct. The cystic duct merges with the common hepatic duct to form the common bile duct, which descends in the hepatoduodenal ligament, passes behind the first part of the duodenum, and penetrates the head of the pancreas.
• Ampulla of Vater: the common bile duct joins the main pancreatic duct to form the hepatopancreatic ampulla, which opens into the second part of the duodenum at the major duodenal papilla.

This retroperitoneal gland spans the posterior abdominal wall at the transpyloric plane (L1). It has four parts: head (nestled in the duodenal C‑loop), neck, body (anterior to the aorta and left kidney), and tail (the only intraperitoneal portion, entering the splenorenal ligament).

• Ducts: the main pancreatic duct (Wirsung) courses through the body and tail, joining the common bile duct at the ampulla of Vater. An accessory duct (Santorini) may persist.
• Vascular supply: the head is supplied by pancreaticoduodenal arteries (from gastroduodenal and superior mesenteric arteries), providing collateral circulation between the celiac trunk and SMA. The body and tail are supplied by the splenic artery.

A peritoneal organ in the left upper quadrant, deep to ribs 9–11. Its visceral surface contacts the stomach, left kidney, and splenic flexure of the colon. The splenic artery and vein traverse the splenorenal ligament to reach the hilum.

J‑shaped organ with a lesser curvature (attached to the liver by the lesser omentum) and a greater curvature (suspends the greater omentum). Regions: cardia (esophageal junction), fundus (air‑filled dome), body (main portion), and pyloric part (thick muscular wall, narrow lumen) which empties into the duodenum at the transpyloric plane (L1).

C‑shaped retroperitoneal structure (except for the first part). Four parts:

• 1st part (duodenal cap/bulb): the gastroduodenal artery and common bile duct descend posteriorly.
• 2nd part (descending): receives the common bile duct and main pancreatic duct at the ampulla of Vater; the sphincter of Oddi controls flow.
• Foregut–midgut boundary: the foregut terminates at the entry of the common bile duct; the rest of the duodenum is midgut.

The jejunum begins at the duodenojejunal flexure and comprises the proximal 2/5 of the small intestine; the ileum is the distal 3/5. Suspended by the mesentery proper.

• Jejunum: thicker wall, more plicae circulares, fewer lymphoid aggregates.
• Ileum: prominent Peyer’s patches (GALT), thinner wall.

Cecum (blind pouch, often with mesentery) gives rise to the appendix (mesoappendix). The ascending and descending colon are retroperitoneal; the transverse colon has its own mesocolon; the sigmoid colon is suspended by the sigmoid mesocolon. The midgut ends at the junction of the proximal 2/3 and distal 1/3 of the transverse colon.

Rectum: the terminal hindgut; the superior 1/3 is covered by peritoneum anteriorly and laterally.

Anal canal: ~1.5 inches, begins at the pelvic diaphragm (anorectal flexure). The puborectalis maintains continence. The internal anal sphincter (smooth muscle, sympathetic tone ↑, parasympathetic relaxation) and external anal sphincter (skeletal, pudendal nerve) control defecation.

The alimentary wall is composed of four concentric layers:

Serosa (visceral peritoneum) covers intraperitoneal organs, with mesothelium and connective tissue.

Branched tubuloalveolar glands producing ~1.5 L saliva/day. Parasympathetic stimulation → watery secretion; sympathetic → viscous secretion.

• Parotid: serous (25% volume); Stensen’s duct opens near upper second molar.
• Submandibular: mixed, mostly serous (70%); Wharton’s duct opens in the floor of mouth.
• Sublingual: mixed, mostly mucous (5%); multiple small ducts.

Branched tubuloacinar gland with serous acini. Acinar cells have basophilic RER and apical zymogen granules (proteases, lipases, amylases). Unlike salivary glands, the pancreas lacks myoepithelial cells and striated ducts; centroacinar cells are present. The islets of Langerhans (endocrine) are scattered among acini.

The largest gland, with dual blood supply: ~75% from the portal vein (nutrient‑rich, deoxygenated) and ~25% from the hepatic artery (oxygenated). Venous drainage occurs via hepatic veins into the inferior vena cava.

• Portal triad: branches of hepatic artery, portal vein, and bile duct travel together in connective tissue sheaths.
• Hepatocytes: polarized cells with bile canaliculi (apical) and sinusoidal surfaces (basal) facing the space of Disse.
• Kupffer cells: hepatic macrophages in the sinusoids.
• Ito cells (stellate): store vitamin A; can produce collagen in cirrhosis.

Lobular architecture

• Classic lobule: hexagon with portal triads at corners and a central vein.
• Portal lobule: triangular, with a portal triad at the center.
• Hepatic acinus: zones 1 (periportal, most oxygenated) → 3 (pericentral, most susceptible to ischemia/toxic injury). Zone 1: glycogen synthesis, albumin; zone 3: lipid metabolism, detoxification.