The brain receives a constant 50ml of blood per 100g of brain tissue, per minute, no matter what the conditions. This is unlike the skeletal muscles or other organs, which have different blood flow requirements depending on how they are being used.
The circle of Willis works to distribute blood at an even pressure, even if one of the major arteries to the brain becomes occluded
With all of this blood coming in, what protects the brain from blood-borne drugs or other substances? The blood-brain barrier
The "barrier" is actually a series of tight junctions between the endothelial cells of the tunica intima. The blood vessels of the brain are the least permeable of all vessels due to the intercellular connections
Astrocytes participate in the barrier too. They cover the capillaries, and their "feet" help the endothelium to form the tight junctions.
The barrier is not absolute. Important nutrients such as glucose, amino acids and certain electrolytes move easily via facilitated diffusion. Wastes, such as urea and large proteins, drugs and other molecules cannot get through to the brain.
The barrier is not effective against smaller molecules, including fats, fatty acids, oxygen, carbon dioxide, alcohol (ethanol) and nicotine.