Dr Brown's bottlea twopiece internal vent system that works unlike any other bottle designThe vent system fully vents the bottle for vacuumfree feeding which is called positivepressure flow similar to breastfeeding As the baby feeds air is channeled from the nipple collar through the vent system bypassing the breastmilk or formula to the back of the bottle Air never mixes with the breastmilk or formula which minimizes oxidation and helps preserve essential nutrients