The largest vein in the human body is the inferior vena cava, the vein that returns blood from the lower half of the body back to the heart.

Why does blood in the veins look blue?

Since venous blood is oxygen-poor blood, it is not as bright red as arterial blood. It appears as a deep, dark-red, almost purplish color. Seeing “blue blood” in veins through the skin is a combination of light passing through the skin and the oxygen-poor blood.