Arteries and veins are the parts of the circulatory system which carry blood between the heart, lungs, and all other areas of the body. While they both carry blood, they do not have much else in common. Arteries and veins are made of somewhat different tissue, each performing certain functions in a specialized way. The first and most important difference between the two is that all arteries carry blood away from the heart, and all veins carry blood to the heart from outlying areas. Most arteries carry oxygenated blood, and most veins carry deoxygenated blood; the pulmonary arteries and veins are the exceptions to this rule.
Arterial tissue is designed and specialized in a way to make it particularly suited to the quick and efficient delivery of blood, which carries the oxygen essential for the functioning of every bodily cell. The outer layer of an artery is made of connective tissue, which covers the muscular middle layer. These muscles contract between heartbeats in such a reliable way that when we take our pulse, we are not actually feeling our heartbeat per se, but arterial muscle contraction instead.
Beyond the arterial muscle is the innermost layer, made of smooth endothelial cells. These cells are specialized to provide a smooth pathway for blood to flow through. This area of cells is also what can become damaged and compromised over a person's lifetime, leading to two common causes of death, namely heart attack and stroke.
Veins have a different structure and function from arteries. They are very flexible, and collapse when they are not filled with blood. They usually carry deoxygenated blood, rich in carbon dioxide, to the heart so that it can be sent to the lungs for oxygenation. The layers of vein tissue are similar in some ways to those of arteries, although the muscle does not contract like arterial muscle does.
Unlike other arteries, the pulmonary artery carries deoxygenated blood. Once the veins have brought this blood from the body to the heart, it is pumped to the lungs. The pulmonary vein moves the oxygenated blood from the lungs back to the heart.
While the location of arteries is very similar from person to person, this is not so much the case with veins, which have greater variability. Veins, unlike arteries, are used as access points to the bloodstream in the medical field, such as when a person receives medicine or fluids directly into the bloodstream, or when blood is drawn. Because veins do not contract as arteries do, there are valves present in veins which keep blood flow going in one direction only. Without these valves, gravity would quickly cause blood to pool in the extremities, causing injury or at the very least impairing the system's efficiency.