Why does blood vessels swell in the brain? what causes this?

Swell?: If we find a swollen blood vessel, it is either due to aneurysm or arteriovenous malformation. Blood vessels may dilate minimally due to increased acidic reaction associated with carbon dioxide increases, or metabolic issues, and of course, caliber varies due to blood pressure, relative hypoxia, and even migraine. Inflammatory arteritis more usually causes narrowing.