masks used for continuous flow oxygen systems use the rebreather bag principle. In this type of mask, oxygen flows into the bottom of the bag. When the person inhales, he takes the air from the bag into his lungs. He then exhales into the rebreather bag. That air which entered last still has a good portion of its oxygen content and is the first to enter the bag. Oxygen is continuously flowing into the bag, and this oxygen plus the air being exhaled will fill the bag. The air that was in the lungs longest is the last to be exhaled, and by this time the bag is full. This air then escapes through the valve in the face piece. The first air to enter the lungs on the next inhaling is that which was first exhaled, along with the supply of oxygen.
In essence, the rebreather bag is just a means of conserving oxygen.