Hepatic portal system
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In human anatomy, the hepatic portal system is the system of veins that comprises the hepatic portal vein and its tributaries. It is also called the portal venous system, although it is not the only example of a portal venous system.
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[edit] Function
The portal venous system is responsible for directing blood from parts of the gastrointestinal tract to the liver. Substances absorbed in the small intestine travel first to the liver for processing before continuing to the heart. Not all of the gastrointestinal tract is part of this system. The system extends from about the lower portion of the esophagus to the upper part of the anal canal. It also includes venous drainage from the spleen and pancreas.
Many drugs that are absorbed through the GI tract are substantially metabolized by the liver before reaching general circulation. This is known as the first pass effect. As a consequence, certain drugs can only be taken via certain routes. For example, nitroglycerin cannot be swallowed because the the liver would inactivate the medication, but it can be taken under the tongue and thus is absorbed in a way that bypasses the portal venous system.
Blood flow to the liver is unique in that it receives both oxygenated and deoxygenated blood. As a result, the partial pressure of oxygen (pO2) and perfusion pressure of portal blood are lower than in other organs of the body. Blood passes from branches of the portal vein through cavities between "plates" of hepatocytes called sinusoids. Blood also flows from branches of the hepatic artery and mixes in the sinusoids to supply the hepatocytes with oxygen. This mixture percolates through the sinusoids and collects in a central vein which drains into the hepatic vein. The hepatic vein subsequently drains into the inferior vena cava.
Large veins that are considered part of the portal venous system are the:
Roughly, the portal venous system corresponds to areas supplied by the celiac trunk, the superior mesenteric artery, and the inferior mesenteric artery.
[edit] Pathology
Portal hypertension is a condition in which the blood pressure of the portal venous system is too high. It is often the result of cirrhosis of the liver.
[edit] See also
[edit] References
- William F. Ganong. "Review of Medical Physiology 22nd Edition."
- Hepatic portal system
- Kimball Pages, Hepatic portal system
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