Normally, arteries deliver oxygen rich blood to the brain. These blood vessels get smaller in size until they become capillaries. The brain extracts the oxygen and sends the blood back to the heart via the veins. During embryonic development the brain capillaries do not form, and the arteries dump blood directly into the veins. Unlike arteries, the veins do not have strong walls, and because of the high arterial pressure, they dilate and get engorged. This congenital condition (present at birth) develops during the fetal stages, and is neither hereditary nor familial.