- Hepatic portal system
In
human anatomy , the hepatic portal system is the system ofvein s that comprises thehepatic portal vein and its tributaries. It is also called the portal venous system, although it is not the only example of aportal venous system .Function
The portal venous system is responsible for directing blood from parts of the
gastrointestinal tract to theliver . Substances absorbed in the small intestine travel first to the liver for processing before continuing to the heart. Not all of thegastrointestinal tract is part of this system. The system extends from about the lower portion of theesophagus to the upper part of theanal canal . It also includes venous drainage from thespleen andpancreas .Many drugs that are absorbed through the
GI tract are substantially metabolized by the liver before reaching general circulation. This is known as thefirst 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 thehepatic 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 thehepatic vein . The hepatic vein subsequently drains into theinferior vena cava .Large veins that are considered part of the "portal venous system" are the:
*Hepatic portal vein
*Splenic vein Roughly, the portal venous system corresponds to areas supplied by the
celiac trunk , thesuperior mesenteric artery , and theinferior mesenteric artery .Pathology
Portal hypertension is a condition in which the blood pressure of the portal venous system is too high. It is often the result ofcirrhosis of the liver.ee also
*
Portal vein
*Hepatic portal vein
*Systemic venous system
*Pulmonary venous system References
*William F. Ganong. "Review of Medical Physiology 22nd Edition."
* [http://www.rnceus.com/ld/portsys.htm Hepatic portal system]
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GITract.html Kimball Pages, Hepatic portal system]
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