In human anatomy, a portal system is a unique vascular arrangement where blood flows through two consecutive capillary beds before returning to the heart.
While the body has a few minor portal systems—such as the hypophyseal portal system in the brain—the term “the portal system” almost always refers to the hepatic portal system, which routes nutrient-rich and toxin-heavy blood from the digestive tract directly to the liver for filtering. 1. The Core Structure: Formation of the Portal Vein
The main hepatic portal vein is a short, wide vessel (about 6 to 8 centimeters long). It originates right behind the neck of the pancreas and is formed by the convergence of three primary veins:
Superior Mesenteric Vein (SMV): Drains the midgut, collecting nutrient-packed blood from the small intestine, cecum, and ascending colon.
Splenic Vein: Drains the spleen and receives blood from parts of the stomach and the pancreas.
Inferior Mesenteric Vein (IMV): Drains the hindgut, including the descending colon, sigmoid colon, and upper rectum. In most people, the IMV empties into the splenic vein just before it merges with the SMV. 2. Minor Tributaries
As the main portal vein travels upward toward the liver, it picks up smaller, direct tributaries:
Left and Right Gastric Veins: Drain blood from the lesser curvature of the stomach. Cystic Veins: Drain blood from the gallbladder.
Paraumbilical Veins: Connect the portal system to the veins of the anterior abdominal wall. 3. The Liver Gate: The Portal Triad Hepatic portal vein (anatomy)
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