The portal vein (in light blue) and its tributaries. It is formed by the superior mesenteric vein, inferior mesenteric vein, and splenic vein. Lienal vein is an old term for splenic vein.
The portal vein is not a true vein, because it conducts blood to capillary beds in the liver and not directly to the heart. It is a major component of the hepatic portal system, one of three portal venous systems in the human body; the others being the hypophyseal and renal portal systems.
Measuring approximately 8cm (3 inches) long in adults,[3] the portal vein is located in the right upper quadrant of the abdomen, originating behind the neck of the pancreas.[4]
In most individuals, the portal vein is formed by the union of the superior mesenteric vein and the splenic vein.[5] For this reason, the portal vein is occasionally called the splenic-mesenteric confluence.[4] Occasionally, the portal vein also directly communicates with the inferior mesenteric vein, although this is highly variable. Other tributaries of the portal vein include the cystic and the left and right gastric veins.[6] and also pararumbilical vein and prepyloric vein.
Immediately before reaching the liver, the portal vein divides into right and left. It ramifies further, forming smaller venous branches and ultimately portal venules. Each portal venule courses alongside a hepatic arteriole and the two vessels form the vascular components of the portal triad. These vessels ultimately empty into the hepatic sinusoids to supply blood to the liver.[6]
Accessory hepatic portal veins are those veins that drain directly into the liver without joining the hepatic portal vein. These include the paraumbilical veins as well as veins of the lesser omentum, falciform ligament, and those draining the gallbladder wall.[4]
Function
The portal vein and hepatic arteries form the liver's dual blood supply. Approximately 75% of hepatic blood flow is derived from the portal vein, while the remainder is from the hepatic arteries.[4]
Unlike most veins, the portal vein does not drain into the heart. Rather, it is part of a portal venous system that delivers venous blood into another capillary system, the hepatic sinusoids of the liver. In carrying venous blood from the gastrointestinal tract to the liver, the portal vein accomplishes two tasks: it supplies the liver with metabolic substrates and it ensures that substances ingested are first processed by the liver before reaching the systemic circulation. This accomplishes two things. First, possible toxins that may be ingested can be detoxified by the hepatocytes before they are released into the systemic circulation. Second, the liver is the first organ to absorb nutrients just taken in by the intestines. After draining into the liver sinusoids, blood from the liver is drained by the hepatic vein.
Increased blood pressure in the portal vein, called portal hypertension, is a major complication of liver disease, most commonly cirrhosis.[7] A dilated portal vein (diameter of greater than 13 or 15mm) is a sign of portal hypertension, with a sensitivity estimated at 12.5% or 40%.[8] On Doppler ultrasonography, the main portal vein (MPV) peak systolic velocity normally ranges between 20cm/s and 40cm/s.[9] A slow velocity of <16cm/s in addition to dilatation in the MPV are diagnostic of portal hypertension.[9]
Portal vein pulsatility can be measured by Doppler ultrasonography. An increased pulsatility may be caused by cirrhosis, as well as increased right atrial pressure (which in turn may be caused by right heart failure or tricuspid regurgitation).[9] Portal vein pulsatility can be quantified by pulsatility indices (PI), where an index above a certain cutoff indicates pathology:
Hepatic portal venous gas is a rare finding on radiological exams. Gas is shown to enter the portal venous system. It is most commonly caused by intestinal ischemia but has also been associated with colon cancer.[15]
Additional images
Human embryo with heart and anterior body-wall removed to show the sinus venosus and its tributaries
Harold M Chung; Chung, Kyung Won (2008). Gross anatomy. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. pp.208. ISBN978-0-7817-7174-0.
Page 367 in: Henryk Dancygier (2009). Clinical Hepatology: Principles and Practice of Hepatobiliary Diseases. Vol.1. Springer Science & Business Media. ISBN9783540938422.
Plemmons RM, Dooley DP, Longfield RN (November 1995). "Septic thrombophlebitis of the portal vein (pylephlebitis): diagnosis and management in the modern era". Clin. Infect. Dis. 21 (5): 1114–20. doi:10.1093/clinids/21.5.1114. PMID8589130.
This article uses material from the Wikipedia article Portal_vein, and is written by contributors.
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