Like every organ of our body, the brain has a system of draining veins by which oxygen-depleted blood is channeled back to the lungs, where it can once again be re-oxygenated.  This system of draining veins begins as a collection of small channels which fuse with one another as you move away from the brain to form larger and larger veins that finally gather between the brain and the skull bone to form what is called the "dural sinuses".  The dural sinuses are the largest canals through which blood flows out of the brain, on its way back to the lungs.