Dr. Brown's Natural Flow BPA-Free 8 oz. Bottle- 3 packDr. Brown's bottles features the Patented Internal Vent System which eliminates the negative pressure and excess air ingestion. This system is the only vent that eliminates the vacuum and the air bubbles found in most baby bottles. It allows air to enter through the nipple collar without being incorporated into the breastmilk or formula. The air is then channeled through the internal vent system to the area above the breastmilk or formula.