Pulmonary vein three and five in the healthy population. The two left lobar veins may be united as a single pulmonary vein in about 25% of people; the two right veins may be united in about 3%. The pulmonary veins play an essential role in respiration, by receiving blood that has been oxygenated in the alveoli and returning it to the left atrium. As part of the pulmonary circulation they carry oxygenated blood back to the heart, as opposed to the veins of the systemic circulation which carry deoxygenated blood. A rare genetic defect of the pulmonary veins can cause them to
Superior mesenteric vein Superior mesenteric vein The superior mesenteric vein (SMV) is a blood vessel that drains blood from the small intestine (jejunum and ileum). At its termination behind the neck of the pancreas, the SMV combines with the splenic vein to form the hepatic portal vein. The SMV lies to the right of the similarly named artery, the superior mesenteric artery, which originates from the abdominal aorta. Tributaries of the superior mesenteric vein drain the small intestine, large intestine, stomach, pancreas and appendix and include: Thrombosis of the superior mesenteric vein is quite rare, but a significant cause of mesenteric ischemia and