The seven major veins in the body bring blood back toward the heart. From the capillaries blood enters small veins, called venules, that merge into larger and larger veins, until they finally join the body's largest vein, the vena cava, returning the oxygen-poor blood to the right atrium of the heart. The vena cava actually has two branches: the lower branch brings blood from the lower part of the body, while the upper branch carries blood from the upper part of the body and the brain.
On its journey from the heart to the tissues, blood is forced through the arteries at high pressure. But on the return journey through the veins and back to the heart, the blood flows at low pressure. It's kept moving by the muscles in the arms and legs that compress the walls of the veins, and by valves in the veins that prevent the blood from flowing backward. Because each type of blood vessel performs a different job under very different pressure, the structures of the arteries, veins, and capillaries are quite different.
Veins also have three layers, but since the blood pressure in the veins is much lower than in the arteries, the vein walls are thinner, less elastic, less muscular and weaker than arteries. The inner linings of many veins contain folds, that act as valves, ensuring that blood flows only toward the heart.