What adds O2 to the breathing bag as required when the breathing bag is emptied on inhalation?

• A. the demand valve
• B. the oxygen sensor
• C. the regulator
• D. the bypass knob

Answer: (A)

The main idea here is how oxygen is replenished in a closed-circuit breathing loop when you take a breath. The device that adds O2 to the breathing bag on inhalation is the demand valve. It responds to your inhalation by opening and delivering a measured amount of oxygen from the supply into the bag, so the partial pressure of oxygen in the loop stays within the safe range as you breathe.

The other components don’t perform this inhalation-driven delivery. The oxygen sensor simply measures the loop’s O2 level and informs the system; it doesn’t inject gas itself. The regulator controls the pressure of the gas supply but isn’t the device that automatically adds O2 during inhalation. The bypass knob is used to alter the flow path or purge in certain situations, not to continuously add O2 in response to a breath.