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The portal venous system is responsible for directing blood from parts of the gastrointestinal tract to the liver. Things absorbed in the small intestine, for example, would be taken to the liver for processing before sent to the heart. 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.
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. This explains why certain drugs can only be taken via certain routes. For example, nitro-glycerin cannot be swallowed, but it can be taken under the tongue. The detoxification powers of the liver are strong enough to inactivate the medication before it reaches the heart. Therefore, nitroglycerin is given 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, portal blood has lower pO2 and perfusion pressure than the 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 into 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: