More than 25 percent of all people have an opening in the wall of the heart that separates the upper chambers of the heart. The hole typically closes as soon as a baby takes her first breath, but in some individuals it does not close completely. With holes remaining, small amounts of blood can bypass the lungs, which as well as oxygenating the blood, filter out clots or air bubbles. Clots in the bloodstream increase a person's risk for stroke. The lungs also filter out chemicals that can cause blood vessels to expand and contract. This may provide a relationship with severe migraines.