A co-axial system in which the fresh gas flows through a narrow inner tube within the outer corrugated tubing. The Bain circuit is a modification of the Mapleson D system.

Functions in the same way as the T-piece, except that the tube supplying fresh gas to the patient is located inside the reservoir tube.

• Inspiration - The patient inspires fresh gas from the outer reservoir tube.
• Expiration - The patient expires into the reservoir tube.

Although fresh gas is still flowing into the system at this time, it is wasted, as it is contaminated by expired gas.

• Expiratory pause - Fresh gas from the inner tube washes the expired gas out of the reservoir tube, filling it with fresh gas for the next inspiration.