By separating a portion of the wall of the artery (a layer of the tunica media or in some cases tunica intima), a tear creates two lumens or passages within the vessel, the original or true lumen, and the false lumen created by the new space within the wall of the artery.
Separating a section of the artery wall, whether it's the media or the intima, results in a tear that creates two distinct channels: the original lumen, which remains intact, and the false lumen formed by the newly opened space within the artery wall.