The subclavian vein is an extension of the axillary vein at the outer border of the first rib. There are two of these in the human body -- the left subclavian vein and the right subclavian vein. They run under the collar bone, or clavicle, where they connect to the internal jugular veins to form the innominate, or brachiocephalic, veins. The subclavian veins measure one to two centimeters in diameter, equivalent to a pinky finger, however, they can be smaller or larger depending on the individual. While arteries carry blood from the heart to circulate throughout the body, subclavian veins, like any other vein in the body, carry blood to the heart. The function of the subclavian vein is to empty blood from the upper extremities and carry it back to the heart. They connect with several other smaller veins in the body such as the interior jugular veins. Each subclavian vein follows its own path throughout the body connecting with its own set of veins.