Science, Tech, Math › Science How Veins Transport Blood Share Flipboard Email Print Science Biology Anatomy Basics Cell Biology Genetics Organisms Physiology Botany Ecology Chemistry Physics Geology Astronomy Weather & Climate By Regina Bailey Biology Expert B.A., Biology, Emory University A.S., Nursing, Chattahoochee Technical College Regina Bailey is a board-certified registered nurse, science writer and educator. Her work has been featured in "Kaplan AP Biology" and "The Internet for Cellular and Molecular Biologists." our editorial process Regina Bailey Updated October 20, 2018 A vein is an elastic blood vessel that transports blood from various regions of the body to the heart. Veins are components of the cardiovascular system, which circulates blood to provide nutrients to the cells of the body. Unlike the high pressure arterial system, the venous system is a low pressure system that relies on muscle contractions to return blood to the heart. Sometimes vein problems can occur, most commonly due to either a blood clot or a vein defect. Types of Veins Human Vascular System. Veins (blue) and Arteries (red). SEBASTIAN KAULITZK/Science Photo Library/Getty Images Veins can be categorized into four main types: pulmonary, systemic, superficial, and deep veins. Pulmonary veins carry oxygenated blood from the lungs to the left atrium of the heart.Systemic veins return oxygen-depleted blood from the rest of the body to the right atrium of the heart.Superficial veins are located close to the surface of the skin and are not located near a corresponding artery.Deep veins are located deep within muscle tissue and are typically located near a corresponding artery with the same name (for example coronary arteries and veins). Vein Size A vein can range in size from 1 millimeter to 1-1.5 centimeters in diameter. The smallest veins in the body are called venules. They receive blood from the arteries via the arterioles and capillaries. The venules branch into larger veins which eventually carry the blood to the largest veins in the body, the vena cava. Blood is then transported from the superior vena cava and inferior vena cava to the right atrium of the heart. Vein Structure MedicalRF.com / Getty Images Veins are composed of layers of thin tissue. The vein wall consists of three layers: Tunica Adventitia - the strong outer covering of arteries and veins. It is composed of connective tissue as well as collagen and elastic fibers. These fibers allow the arteries and veins to stretch to prevent over expansion due to the pressure that is exerted on the walls by blood flow.Tunica Media - the middle layer of the walls of arteries and veins. It is composed of smooth muscle and elastic fibers. This layer is thicker in arteries than in veins.Tunica Intima - the inner layer of arteries and veins. In arteries, this layer is composed of an elastic membrane lining and smooth endothelium (a special type of epithelial tissue) that is covered by elastic tissues. Veins do not contain the elastic membrane lining that is found in arteries. In some veins, the tunica intima layer also contains valves to keep blood flowing in a single direction. Vein walls are thinner and more elastic than artery walls. This allows veins to hold more blood than arteries. Vein Problems Varicose veins are veins that have become swollen due to broken valves. Clint Spencer/E+/Getty Images Vein problems are typically the result of a blockage or defect. Blockages occur due to blood clots that develop in either superficial veins or deep veins, most often in the legs or arms. Blood clots develop when blood cells known as platelets or thrombocytes become activated due to a vein injury or disorder. Blood clot formation and vein swelling in superficial veins is called superficial thrombophlebitis. In the word thrombophlebitis, thrombo refers to platelets and phlebitis means inflammation. A clot that occurs in deep veins is called deep vein thrombosis. Vein problems can also arise from a defect. Varicose veins are the result of damaged vein valves that allow blood to pool in the veins. The accumulation of blood causes inflammation and bulging in the veins located near the skin's surface. Varicose veins typically appear in pregnant women, in individuals with deep vein thrombosis or vein injuries, and in those with a genetic family history. Key Takeaways Veins are vessels that bring blood from other parts of the body to the heart. The low pressure venous system needs the contraction of muscles to return the blood to the heart.There are four major types of veins. Examples include pulmonary and systemic veins as well as superficial and deep veins.Pulmonary veins carry oxygenated blood to the heart's left atrium from the lungs, while systemic veins return non-oxygenated blood from the body to the heart's right atrium.As their respective names imply, superficial veins are located close to the skin's surface while deep veins are located much deeper in the body.Venules are the smallest veins in the body. The superior and inferior venae cavae are the largest veins.Structurally, veins have three major layers consisting of a strong outer layer, a middle layer, as well as an inner layer.