A vein consists of three main layers connected by muscles and elastic connective tissue, according to the National Cancer Institute. Walls of veins are thinner because veins have lower blood pressure.
The outer layer, or "tunica adventitia," consists of collagen fibers and connective tissue. The middle layer, known as the "tunica media," contains smooth muscle fibers and elastic fibers that change the diameter of the vein, notes Pearson Education. The middle layer is the thickest of the three main layers. The "tunica initima," or inner layer, is made of specialized cells of the endothelium, which lines the veins and the interior of the heart. A thin membrane supports this inner layer in between the middle and inner portions of a vein.
Medium and large veins also contain valves to prevent blood from flowing backwards as blood moves against the flow of gravity and towards the heart. These valves are especially important in the arms and legs, as referenced by the NCI. Arteries do not have these valves, although arteries have the same layers as veins.
Veins carry approximately 70 percent of a human body's blood volume at any given time, according to the NCI. Blood passes through capillaries and then larger venules before progressing to the largest veins in the body on the way to the heart and lungs.