In the circulatory system, veins (from the Latin vena) are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood. They differ from arteries in structure and function; for example, arteries are more muscular than veins and they carry blood away from the heart.

Notable veins and vein systems

The greater saphenous vein is the most important superficial vein of the lower limb. First described by the

Persian physician Avicenna, Saphenous derives its name from Safina, meaning hidden. This vein is "hidden" in its own fascial compartment in the thigh and only exits the fascia near the knee. Incompetence of this vein is an important cause of varicose veins of lower limbs.

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.

A portal venous system is a series of veins or venules that directly connect two capillary beds. Examples of such systems include the hepatic portal vein and hypophyseal portal system.

Color

Vein color 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 off.

This results from a phenomenon called Rayleigh scattering.

Laser Vein Therapy