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
Vein directly connect two capillary beds. Examples of such systems include the hepatic portal vein and hypophyseal portal system. The peripheral veins carry blood from the limbs and hands and feet. Microscopically, veins have a thick outer layer made of connective tissue, called the tunica externa or "tunica adventitia". There is a middle layer of bands of smooth muscle called tunica media, which are, in general, much thinner than those of arteries, as veins do not function primarily in a contractile manner and are not subject to the high pressures of systole, as arteries are. The interior is lined with endothelial