What is the primary characteristic of an open breathing system?

An open breathing system is characterized by the absence of rebreathing and a reservoir. In such systems, gases are continuously vented to the environment, which prevents the patient from inhaling any exhaled gases. This setup allows for a fresh gas flow that meets or exceeds the patient's metabolic needs, ensuring that the patient receives only fresh anesthetic gases.

The lack of a reservoir means that there is no collection of exhaled gases, further supporting the open nature of this system. Consequently, this design is primarily used in situations where there is a need for minimal resistance to breathing and a free flow of gases.

In contrast, systems that involve complete rebreathing or partial rebreathing would incorporate mechanisms to allow for the inhalation of previously exhaled gases, thus not aligning with the definition of an open system. Therefore, the core feature of an open breathing system is the absence of any form of gas rebreathing and the continuous introduction of fresh gas to the patient.