The left internal jugular vein is the left of two valves that make up the internal jugular veins on each side of the neck. This is the smaller of the two internal jugular veins. The internal jugular veins return deoxygenated blood from the head to the heart. The left internal jugular vein curves forward and joins the subclavian vein, and then crosses over the left common carotid artery at the neck's root.