Abstract: The present invention provides for a closed circuit breathing unit comprising a reservoir bag, oxygen source, CO2 scrubber, and actuation device. The oxygen source and an exit from the scrubber may be fluidly connected to the reservoir bag. The reservoir bag may be attached to a mouthpiece to provide inhalation air. Expired air may be exhaled through the mouthpiece and directed to an inlet for the scrubber. As a user breathes normally, expired air is scrubbed of excess CO2, mixed with generated oxygen, and delivered to the user for inhalation within a closed circuit. Additionally, the oxygen source and the scrubber may be replaced and/or replenished without compromising or interrupting a breathing cycle.