A vein is a type of blood vessel that collects blood from various parts of the circulatory system and returns it toward the heart. Unlike arteries, veins typically carry blood under lower pressure and are responsible for returning deoxygenated blood to the right side of the heart, where it will then be pumped to the lungs for oxygenation. The internal structure of a vein includes smooth muscle, connective tissue, and an inner layer of endothelium. To counteract the low pressure and prevent blood from pooling, many veins contain one-way valves that ensure the blood flows unidirectionally.