Which of the following are examples of closed breathing systems?

• A. Endotracheal tube, laryngeal mask airway, and King airway
• B. Simple facemask and nasal cannula
• C. Non-rebreather mask
• D. Oxygen mask with reservoir

Answer: (A)

The idea being tested is what makes a breathing system “closed.” A closed breathing system is one that forms a tight seal with the airway and connects to a breathing circuit so that most exhaled gas is retained, scrubbed of CO2, and recirculated rather than mixing with room air. Endotracheal tubes, laryngeal mask airways, and King airway devices create that sealed interface, allowing connection to a circle-type circuit where fresh gas and the patient’s exhaled gas blend in a controlled loop. This setup minimizes entrainment of ambient air and supports gas recycling during anesthesia or deep sedation.

In contrast, simple facemasks, nasal cannulas, non-rebreather masks, and oxygen masks with reservoirs do not seal the airway effectively; gases escape around the device and room air can mix in, so they operate as open or semi-open systems rather than closed.