The Ambu has a squeezable bag, a one-way valve, and a face mask. Compressing the bag opens a valve, forcing air through a mask or artificial airway into the lungs, according to the journal "Critical Care Nurse." Releasing the bag closes the valve and exhalation occurs passively through the exhalation port. During exhalation, the bag automatically refills with room air or oxygen. The cycle continues until spontaneous breathing occurs or breathing is mechanically supported by a breathing machine.