The word “vein” pops up in a variety of contexts. When used as a figure of speech, vein means similar or alike, for example “in the same vein as.” But in the body, vein refers to a specific type of blood vessel.
Encyclopedia Britannica defines blood vessel as “a vessel in the human or animal body in which blood circulates.” Though they may be structurally similar to the two other types of blood vessels—arteries and capillaries—veins have a unique and important role in circulation.
When the heart contracts, it pumps out oxygen-rich blood that first passes through arteries, then into capillaries—the location where oxygen is sent out to the body and carbon dioxide and other waste is picked up. The deoxygenated blood then travels through the veins back to the heart. The whole process takes just 20 seconds to complete. (WATCH this cool real-time animation of the red blood cell cycle.)
While veins are primarily responsible for moving oxygen-depleted blood back to the heart, this is not always the case. The amount of oxygen in the blood carried in the veins varies depending on whether they service the lungs (as part of pulmonary circulation) or the rest of the body (as part of systemic circulation). In systemic circulation, arteries carry oxygenated blood to the organs, and muscles and veins carry deoxygenated blood back to the heart. In pulmonary circulation, the process is reversed. The arteries carry oxygen-poor blood to the lungs, while the veins deliver newly oxygenated blood to the heart.
Veins and arteries also look different. Arteries control blood flow and pressure, so they are muscular to narrow and widen (dilate) effectively. Veins are less muscular than arteries and have thin, stretchy walls, which is why you can see them bulge out of your arms and hands. Because the blood pressure in the veins is lower and the blood is often moving against gravity, most veins also have valves to ensure blood flows in only one direction.
So when it comes to blood vessels, differences in their look and function demonstrate that they are not in the same vein.