Veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart. In contrast to veins, arteries carry blood away from the heart.
Veins are less muscular than arteries and are often closer to the skin. There are valves in most veins to prevent backflow.
Three main types of blood vessels
1. Arteries. They carry oxygen-rich blood away from the heart to all of the body’s tissues. They branch into smaller and smaller arteries as they carry blood farther from the heart and into organs.
2. Capillaries. These small blood vessels connect the arteries and the veins. Their thin walls allow oxygen, nutrients, carbon dioxide, and other waste to pass to and from cells.
3. Veins. These are blood vessels that take blood back to the heart. They get become larger as they get closer to the heart. The superior vena cava is the large vein that brings blood from the head and arms to the heart, and the inferior vena cava brings blood from the abdomen and legs into the heart.