If the internal jugular vein is ruptured, why can it lead to air embolus?

AIR EMBOLISM: Small amounts of air often enter the circulation inadvertently during surgery or other medical procedures. An example would be an air bubble entering an IV line. Most of these air bubbles are benign and are stopped at the lungs. If a large vein such as the internal jugular is ruptered, say from trauma or a procedure, air may enter the vein.