<P> Veins are translucent, so the color a vein appears from an organism's exterior is determined in large part by the color of venous blood, which is usually dark red as a result of its low oxygen content . Veins appear blue because the subcutaneous fat absorbs low - frequency light, permitting only the highly energetic blue wavelengths to penetrate through to the dark vein and reflect back to the viewer . The colour of a vein can be affected by the characteristics of a person's skin, how much oxygen is being carried in the blood, and how big and deep the vessels are . When a vein is drained of blood and removed from an organism, it appears grey - white . </P> <P> The largest veins in the human body are the venae cavae . These are two large veins which enter the right atrium of the heart from above and below . The superior vena cava carries blood from the arms and head to the right atrium of the heart, while the inferior vena cava carries blood from the legs and abdomen to the heart . The inferior vena cava is retroperitoneal and runs to the right and roughly parallel to the abdominal aorta along the spine . Large veins feed into these two veins, and smaller veins into these . Together this forms the venous system . </P> <P> Whilst the main veins hold a relatively constant position, the position of veins person to person can display quite a lot of variation . </P> <P> The pulmonary veins carry relatively oxygenated blood from the lungs to the heart . The superior and inferior venae cavae carry relatively deoxygenated blood from the upper and lower systemic circulations, respectively . </P>

Location of artery and vein in the body