While of similar structure to ARTERIES, veins have much thinner walls, with much less muscular tissue. Furthermore, most veins have one-way VALVES to ensure that the blood flows in the right direction.

Four pulmonary veins open into the left atrium of the heart, two from each lung. The superior vena cava returns the blood from the head, neck, and arms; while the inferior vena cava returns blood from the legs and abdomen. Within the abdomen, the inferior vena cava receives branches corresponding to several branches of the aorta, its largest branches being the hepatic veins, which return not only the blood that has reached the liver in the hepatic arteries, but also blood which comes from the digestive organs in the portal vein to undergo a second capillary circulation in the liver. (See PORTAL VEIN.)

There are several connections between the superior and inferior cava, the most important being three azygos veins that lie upon the sides of the spinal column, the veins on the front of the abdomen, and some veins that emerge from the abdomen at the navel and connect the portal system with those of the inferior and superior vena cava. (See also CIRCULATORY SYSTEM OF THE BLOOD.)