2.6 Intestines, Pancreas, Liver, Biliary Ducts, Hepatic Circulation & Kidney/Ureters

Small Intestine

The small intestine extends from the pylorus to the ileocecal valve (~6–7 m in vivo) and is divided into duodenum, jejunum, and ileum.

Duodenum

The duodenum is mostly retroperitoneal (except the first 2 cm, which is intraperitoneal) and forms a C-loop around the head of the pancreas. It has four parts:

• First (superior) — includes the duodenal bulb (intraperitoneal), the classic site of anterior duodenal ulcers that can perforate into the peritoneal cavity.
• Second (descending) — receives the major duodenal papilla (ampulla of Vater), where the common bile duct and main pancreatic duct empty. The minor papilla nearby drains the accessory pancreatic duct.
• Third (horizontal) — crosses the midline between the superior mesenteric artery (anterior) and the aorta and IVC (posterior); SMA syndrome results when this part is compressed between the SMA and aorta, classically after rapid weight loss that reduces the aortomesenteric fat pad.
• Fourth (ascending) — joins the jejunum at the duodenojejunal flexure, suspended by the ligament of Treitz (the landmark dividing upper from lower GI bleeding).

Jejunum and Ileum

The jejunum (proximal two-fifths) lies in the left upper quadrant, has a thick wall, prominent plicae circulares (valves of Kerckring), and few arcades with long vasa recta. The ileum (distal three-fifths) lies in the right lower quadrant, has a thinner wall, shorter plicae, more arterial arcades with short vasa recta, and contains Peyer's patches (lymphoid aggregates, especially in the terminal ileum).

Vascular Supply of the Small Intestine

The superior mesenteric artery (SMA) supplies the entire small intestine distal to the major duodenal papilla:

• Inferior pancreaticoduodenal — to the distal duodenum and pancreatic head (anastomoses with the superior pancreaticoduodenal from the gastroduodenal artery, completing the celiac–SMA anastomosis).
• Jejunal and ileal arteries — 15–18 branches in arcades supplying the jejunum and ileum.
• Ileocolic, right colic, middle colic — to the ascending and transverse colon.

The proximal duodenum (above the papilla) is supplied by branches of the celiac trunk (via the gastroduodenal and superior pancreaticoduodenal arteries). This dual supply creates the celiac–SMA anastomosis that can compensate for stenosis of either vessel.

Large Intestine

The large intestine extends from the ileocecal junction to the anus (~1.5 m) and includes the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. Distinguishing features: teniae coli (three longitudinal muscle bands), haustra, appendices epiploicae (fat-filled tags), and a larger caliber than small bowel.

Vascular Supply of the Large Intestine — the SMA/IMA Boundary

The arterial supply of the colon is the PA-CAT's classic map of the SMA/IMA territorial boundary:

The splenic flexure sits at the SMA/IMA boundary — a watershed region (Griffith's point) vulnerable to ischemia in low-flow states, producing ischemic colitis.

The rectum has a three-way anastomosis between the superior rectal (IMA), middle rectal (internal iliac), and inferior rectal (internal pudendal) arteries. This dual systemic–splanchnic supply makes the rectum relatively resistant to ischemia and is also the site of portosystemic anastomosis (superior rectal vein → portal; middle and inferior rectal veins → systemic via internal iliac) that produces rectal varices in portal hypertension.

Pancreas

The pancreas is a retroperitoneal organ crossing the upper abdomen at L1–L2. It has four parts:

• Head — nestled in the C-loop of the duodenum; includes the uncinate process that hooks behind the SMA and SMV.
• Neck — overlying the superior mesenteric vessels; the portal vein forms behind the neck (union of splenic and superior mesenteric veins).
• Body — crosses the midline anterior to the aorta, left kidney, and left adrenal gland; the splenic artery runs along its superior border.
• Tail — reaches the splenic hilum in the splenorenal ligament.

Pancreatic Ducts

• Main pancreatic duct (of Wirsung) — runs the length of the pancreas and joins the common bile duct at the hepatopancreatic ampulla (of Vater), which empties into the major duodenal papilla guarded by the sphincter of Oddi.
• Accessory pancreatic duct (of Santorini) — drains the upper anterior head and opens separately at the minor duodenal papilla; usually communicates with the main duct.

Pancreatic Vascular Supply

• Head — superior and inferior pancreaticoduodenal arteries (from the gastroduodenal and SMA, respectively).
• Body and tail — dorsal pancreatic artery and branches of the splenic artery.

Clinical Correlates

• Pancreatic cancer — most often arises in the head; obstructs the distal bile duct, causing painless jaundice and a palpable, non-tender gallbladder (Courvoisier's sign).
• Pancreatitis — inflammation with elevated amylase and lipase; can arise from a posterior gastric ulcer eroding into the pancreas (the PA-CAT sample item).
• Annular pancreas — a congenital band of pancreatic tissue encircling the duodenum, causing duodenal obstruction in neonates.

Liver

The liver is the largest internal organ (~1.5 kg), occupying the right upper quadrant beneath the right hemidiaphragm. It is divided anatomically into right, left, caudate, and quadrate lobes by surface landmarks:

• The falciform ligament separates the right and left lobes anteriorly and carries the ligamentum teres (obliterated umbilical vein) in its free edge.
• The ligamentum venosum (obliterated ductus venosus) separates the caudate lobe posteriorly.
• The porta hepatis is the hilum transmitting the portal triad (hepatic artery proper, portal vein, and bile ducts), plus lymphatics and nerves.

The Couinaud classification divides the liver into eight functional segments based on the branching of the portal and hepatic veins. Each segment has its own portal triad and venous drainage, which permits segmental resection. The caudate lobe (segment I) drains directly into the IVC, which is why it hypertrophies in Budd-Chiari syndrome (hepatic vein obstruction) when the rest of the liver congests.

Biliary Ducts

The biliary tree conducts bile from the liver to the duodenum:

1. Right and left hepatic ducts — drain the right and left lobes; unite to form the common hepatic duct.
2. Cystic duct — drains the gallbladder; joins the common hepatic duct to form the common bile duct (CBD).
3. Common bile duct — ~8 cm long, runs in the hepatoduodenal ligament (free edge of the lesser omentum) with the hepatic artery proper and portal vein, then passes behind the head of the pancreas to join the main pancreatic duct at the ampulla of Vater.
4. The sphincter of Oddi surrounds the ampulla and regulates bile and pancreatic juice flow into the major duodenal papilla.

The gallbladder stores and concentrates bile. It has a fundus, body, and neck; the neck tapers into the cystic duct. Hartmann's pouch is a dilation at the gallbladder neck where gallstones commonly lodge. Calot's triangle (bounded by the cystic duct, common hepatic duct, and inferior edge of the liver) contains the cystic artery (usually a branch of the right hepatic artery) and is the key landmark for cholecystectomy.

Clinical Correlates

• Gallstones — lodge at the cystic duct (biliary colic), Hartmann's pouch, or the CBD (obstructive jaundice). A stone impacted at the ampulla can cause gallstone pancreatitis by blocking pancreatic juice outflow.
• Courvoisier's law — a palpable gallbladder with painless jaundice is more likely due to malignancy (e.g., pancreatic head) than to stones, because chronic stone disease causes a fibrotic, contracted gallbladder.

Hepatic Circulation — Dual Blood Supply

The liver has a unique dual blood supply:

• Hepatic artery proper (continuation of the common hepatic artery from the celiac trunk) — contributes ~25–30% of hepatic blood flow; oxygen-rich.
• Portal vein (formed by the union of the superior mesenteric and splenic veins behind the pancreatic neck) — contributes ~70–75% of hepatic blood flow; nutrient-rich, partially deoxygenated blood from the gut, spleen, and pancreas.

These two vessels, together with a bile ductule, travel in the portal triad within the portal tracts at the periphery of each hepatic lobule. Blood from both sources perfuses the hepatic sinusoids and drains into the central vein, then the hepatic veins, and finally the inferior vena cava (IVC).

The portal triad in the hepatoduodenal ligament is arranged with the bile duct anterolateral, the hepatic artery anteromedial, and the portal vein posterior (the mnemonic Mickey Mouse sign on transverse ultrasound shows the portal vein as the head, with the artery and duct as the two ears).

Portosystemic Anastomoses

Because portal venous blood normally flows to the liver, obstruction of that flow (cirrhosis, portal hypertension) diverts blood into systemic veins at four principal anastomoses:

1. Esophageal — left gastric (portal) to esophageal (azygos, systemic); produces esophageal varices.
2. Rectal — superior rectal (portal) to middle/inferior rectal (systemic); produces rectal varices (distinct from hemorrhoids, which arise from the inferior rectal veins).
3. Periumbilical — paraumbilical veins (portal) to superficial epigastric veins (systemic); produces a caput medusae.
4. Retroperitoneal — colic veins (portal) to lumbar veins (systemic).

Kidneys and Ureters

The kidneys are paired retroperitoneal organs at the level of T12–L3. The right kidney is slightly lower than the left because of the liver. Each kidney lies anterior to the quadratus lumborum and psoas major, behind the parietal peritoneum. The renal hilum transmits (from anterior to posterior) the renal vein, renal artery, and renal pelvis (V-A-P), plus lymphatics.

The kidneys are supplied by the renal arteries (branches of the abdominal aorta at L1–L2). The right renal artery is longer and passes behind the IVC. The left renal vein is longer and crosses anterior to the aorta below the SMA; it receives the left gonadal and left suprarenal veins.

Ureters

The ureters are ~25–30 cm muscular tubes carrying urine from the renal pelvis to the bladder. They descend retroperitoneally on the anterior surface of the psoas major, cross the pelvic brim near the bifurcation of the common iliac arteries, and enter the bladder at the trigone. They have three natural constrictions where stones lodge:

1. At the ureteropelvic junction (where the renal pelvis narrows into the ureter).
2. At the pelvic brim (where the ureter crosses the common iliac bifurcation).
3. At the ureterovesical junction (where the ureter enters the bladder — the narrowest point).

A ureteral stone produces colicky pain that radiates from the flank to the groin, following the T11–L2 dermatomal distribution. The PA-CAT samples the pelvic-brim crossing because it is both an anatomical landmark and a clinical site of impaction.

Summary Table of Abdominal Viscera and Supply

This map is the high-yield capstone of the PA-CAT back-and-abdomen anatomy: it ties together every organ in this chapter and shows how the celiac trunk, SMA, and IMA partition the abdominal viscera — a question pattern the exam samples repeatedly.