Blood vessel prevent backflow as a result of a decrease in blood pressure as the blood passes through the circulatory system. There are various kinds of blood vessels: They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing "away from" (arterial) or "toward" (venous) the heart. The term "arterial blood" is nevertheless used to indicate blood high in oxygen, although the pulmonary artery carries "venous blood" and blood flowing in the pulmonary vein is rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated. Blood
Portal vein the hepatic portal system, one of only two portal venous systems in the body – with the hypophyseal portal system being the other. The portal vein is usually formed by the confluence of the superior mesenteric and splenic veins and also receives blood from the inferior mesenteric, left and right gastric veins, and cystic veins. Conditions involving the portal vein cause considerable illness and death. An important example of such a condition is elevated blood pressure in the portal vein. This condition, called portal hypertension, is a major complication of cirrhosis. Measuring approximately 8 cm (3 inches) in adults, the