How is a rebreather system different from an open-circuit system?

There are a few basic differences between rebreather systems and open-circuit systems and these differences equate to a rebreather diver staying warmer and more hydrated during a dive.  The rebreather air is moist, as it is exhaled from the diver, and is warm as the chemical reaction that removes the carbon dioxide produces heat.  That means that the diver does not lose core heat warming up the air that is breathed in the way it happens in an open-circuit system.  In an open-circuit system, once the air is breathed out, it is gone and the new air supplied by the tanks is cold and dry by the time it reaches the diver.