Portal Venous System, Portocaval Anastomoses, Aorta, and Inferior Vena Cava Study Guide

Overview of the Portal Venous System and Portocaval Anastomoses

• Definition of Porto-systemic (Portocaval) Anastomosis: This is the collateral communication between the portal venous system and the systemic venous system.

• Function and Pathway of the Portal Venous System:     * Transmits deoxygenated blood from most of the gastrointestinal (GI) tract and GI organs to the liver.     * The path proceeds through the portal vein into a second capillary network known as the liver sinusoids.     * Finally, blood reaches the heart via the inferior vena cava (systemic circulation) through the hepatic veins.

Anatomy of the Portal Vein

• Formation: It is formed by the union of the superior mesenteric vein and the splenic vein.

• Verbatim Level of Origin: It forms at the level of the second lumbar vertebrae ($L2$).

• Physical Dimensions: The portal vein is approximately $8\,cm$ long.

• Anatomical Course and Relations:     * It runs upwards, situated behind the bile duct and the hepatic artery.     * It lies anterior to the inferior vena cava ($IVC$).     * It is posterior (at the back) to the neck of the pancreas.     * It penetrates the right border of the lesser omentum.     * It continues upwards anterior to the epiploic foramen to reach the porta hepatis (the transverse fissure on the liver).

• Termination: Upon reaching the porta hepatis, it bifurcates into a right branch and a left branch which penetrate the liver parenchyma.

Tributaries of the Portal Venous System

Various veins drain into the portal vein or its formative vessels:

• Superior Mesenteric Vein: Primarily drains blood from the small intestine.

• Splenic Vein: Receives blood from the short gastric veins, left gastroepiploic vein, inferior mesenteric vein, and pancreatic veins.

• Right and Left Gastric Veins: Drain blood from the stomach and the esophagus.

• Superior Pancreaticoduodenal Veins: Drain blood from the pancreas and the duodenum.

• Cystic Veins: Drain blood from the gallbladder.

• Paraumbilical Vein: Drains into the portal system.

• Other Specific Veins Identified in Diagrams:     * Right gastroepiploic vein     * Gastrocolic trunk     * Middle colic vein     * Right colic vein     * Ileocolic vein     * Appendicular vein     * Left colic veins     * Jejunal and ileal veins     * Sigmoid veins     * Superior rectal vein

Portocaval Anastomoses Sites and Components

These anastomoses provide alternative routes of circulation if the liver or portal vein is blocked, ensuring GI venous blood reaches the heart via the $IVC$.

• Lower Esophagus:     * Portal Side: Left gastric veins.     * Systemic Side: Lower branches of esophageal veins draining into the azygos and hemiazygos veins.

• Upper Part of the Anal Canal:     * Portal Side: Superior rectal veins.     * Systemic Side: Inferior and middle rectal veins.

• Umbilicus:     * Portal Side: Paraumbilical veins (running in the ligamentum teres).     * Systemic Side: Small epigastric veins.

• Bare Area of the Liver:     * Portal Side: Intraparenchymal branches of the right division of the portal vein.     * Systemic Side: Retroperitoneal veins draining into the azygos, hemiazygos, and lumbar veins.

• Hepatic and Splenic Flexures:     * Portal Side: Omental and colonic veins.     * Systemic Side: Retroperitoneal veins.

• Patent Ductus Venosus (Rare):     * Portal Side: Ductus venosus.     * Systemic Side: Inferior vena cava.

The Aorta

• General Characteristics: The largest artery in the body, with a diameter of approximately $2.5\,cm$ ($1\,inch$) at its origin.

• Function: Receives blood from the left ventricle and distributes oxygenated blood via systemic circulation.

• Major Divisions:     * Ascending Aorta: Gives rise to the right and left coronary arteries to supply the heart.     * Aortic Arch: Gives rise to the brachiocephalic trunk, left common carotid artery, and left subclavian artery; supplies the head, neck, and arms.     * Descending Aorta: Divided into the Thoracic aorta and Abdominal aorta.     * Termination: Terminates at the level of the $L4$ vertebra, bifurcating into the right and left common iliac arteries.

The Abdominal Aorta

• Course: Passes through the aortic hiatus of the diaphragm at the level of $T12$, descends anterior to the vertebral bodies, and bifurcates at $L4$.

• Anatomical Relations:     * Anteriorly: Pancreas, splenic vein, left renal vein, and the third part of the duodenum.     * Posteriorly: Several lumbar veins passing toward the $IVC$.     * Right Side: Cisterna chyli, thoracic duct, azygos vein, right crus of the diaphragm, and the $IVC$.     * Left Side: Left crus of the diaphragm.

Branches of the Abdominal Aorta

Anterior Group (Unpaired)

• Celiac Trunk ($T12$):     * Arises just inferior to the aortic orifice of the diaphragm.     * Three branches: Left gastric artery, common hepatic artery, and splenic artery.     * Supplies the Foregut: Abdominal esophagus, stomach, proximal duodenum, liver, gallbladder, pancreas, and spleen.

• Superior Mesenteric Artery ($L1$):     * Arises about $1\,cm$ below the celiac trunk.     * Supplies the Midgut: Proximal duodenum to the distal $\frac{1}{3}$ of the transverse colon (includes small intestine, cecum, appendix, ascending colon, and first $\frac{2}{3}$ of the transverse colon).

• Inferior Mesenteric Artery ($L3$):     * Arises about $4\,cm$ superior to the aortic bifurcation.     * Supplies the Hindgut: Distal $\frac{1}{3}$ of the transverse colon, descending colon, sigmoid colon, rectum, and superior portion of the anal canal.

Lateral Group (Paired)

• Middle Suprarenal Arteries: Arise from the aorta and run laterally on the crura of the diaphragm.

• Renal Arteries:     * Arise inferior to the superior mesenteric artery.     * The Right Renal Artery is longer, slightly lower, and passes posterior to the $IVC$.     * The Left Renal Artery passes posterior to the left renal vein.     * Branches: Inferior suprarenal, ureteric, and segmental branches (superior, anterosuperior, anteroinferior, inferior, and posterior).

• Gonadal (Testicular or Ovarian) Arteries:     * Descend retroperitoneally on the psoas major muscle and across the ureter.     * Testicular: Accompanies ductus deferens to the scrotum.     * Ovarian: Enters the suspensory ligament of the ovary and anastomoses with the uterine artery.

Dorsal Group

• Inferior Phrenic Arteries:     * Arise immediately below the aortic hiatus to supply the diaphragm.     * Give rise to the superior suprarenal arteries.     * The left passes posterior to the esophagus; the right passes posterior to the $IVC$.

• Lumbar Arteries:     * Four or five pairs arising from the posterior aorta.     * Run posterior to the sympathetic trunk, $IVC$ (on the right), psoas major, lumbar plexus, and quadratus lumborum.     * Divide into anterior (muscular) and posterior (spinal/muscular) branches.

• Median Sacral Artery:     * Arises just above the bifurcation; descends on the front of the sacrum to the coccygeal body.     * Supplies the rectum and anal canal.

Terminal Branches

• Left and Right Common Iliac Arteries:     * Arise at $L4$, approximately $1.25\,cm$ to the left of the median plane.     * Supply lower limbs, gluteal region, and pelvic viscera.

Inferior Vena Cava ($IVC$)

• Function: Drains all structures below the diaphragm to the right atrium.

• Formation: Formed by the union of two common iliac veins at the level of the $5th$ lumbar vertebra ($L5$), just right of the midline.

• Course: Ascends posterior to the abdomen, right of the aorta, and pierces the central tendon of the diaphragm at the level of the $8th$ thoracic vertebra ($T8$).

• Crossing Structures (Below to Above):     1. Right common iliac artery     2. Root of the mesentery     3. Right testicular or ovarian artery     4. $3rd$ part of the duodenum     5. Head of the pancreas     6. $1st$ part of the duodenum     7. Bile duct     8. Portal vein     9. Liver (which may overlap or surround it)

Tributaries of the Inferior Vena Cava

Note: There are no direct tributaries from the abdominal GI tract, spleen, pancreas, or gallbladder as they drain via the portal system first.

• Common Iliac Veins: Formed by external and internal iliac veins; drain lower limbs and gluteal region.

• Lumbar Veins:     * Drain the posterior abdominal wall.     * $3rd$ and $4th$ lumbar veins drain directly into the $IVC$.     * $5th$ lumbar vein drains into the iliolumbar vein.     * $1st$ and $2nd$ lumbar veins may drain into ascending lumbar veins.

• Renal Veins: Drain kidneys, left adrenal gland, and left gonad (testis/ovary).     * The left renal vein passes posterior to the superior mesenteric artery and anterior to the aorta.

• Right Gonadal (Testicular/Ovarian) Vein: Drains directly into the $IVC$ (the left drains into the left renal vein).

• Right Suprarenal Vein: Drains the right adrenal gland (the left drains into the left renal vein).

• Inferior Phrenic Veins: Drain the diaphragm (right drains to $IVC$, left usually to renal vein).

• Hepatic Veins: Three veins (left, middle, and right) draining the liver. The middle and left often unite briefly before entering the $IVC$.