What is the function of unidirectional valves in the breathing system?

• A. Ensure one-way gas flow and prevent backflow
• B. Regulate fresh gas concentrations
• C. Measure pressure
• D. Vent gas to scavenging

Answer: (A)

Unidirectional valves ensure one-way flow of gases in the breathing circuit, keeping inspiratory gas moving toward the patient and expiratory gas moving away from the patient. In a circle system, the inspiratory valve opens during inhalation to deliver fresh gas to the patient while the expiratory valve closes, and during exhalation the expiratory valve opens to route the exhaled gas through the absorber and toward the scavenging system while the inspiratory path closes. This separation prevents backflow and, importantly, prevents rebreathing of CO2-rich exhaled gas and cross-contamination between the fresh gas supply and the patient.

Other choices describe functions handled by other parts of the system: regulating fresh gas concentrations is controlled by flowmeters and vaporizers, measuring pressure is done with pressure gauges/transducers, and venting to scavenging occurs as part of the expiratory flow pathway rather than the primary role of the valves.