The Bain circuit is a modification of the Mapleson D system. This is a coaxial system where fresh gas flows through a narrow inner tube inserted into the outer corrugated tube.
Essentially, the Bain circuit works in the same way as the T-fitting, with the difference that the tube that supplies fresh gas to the patient is located inside the tank tube.
Inspiration: The patient inhales fresh gas from the outer tank tube.
Exhalation - The patient exhales into the spare tube. Although fresh gas continues to flow into the system right now, this is wasted because it is contaminated with the exhausted gas.
Exhalation pause: Fresh gas from the inner tube washes away the gas exhaled from the spare tube, filling it with fresh gas for next inhalation.