Figure 1: Mapleson B system
The Mapleson B system features the fresh gas inlet near the patient, distal to the expiratory valve. The expiratory valve opens when pressure in the circuit increases, and a mixture of alveolar gas and fresh gas is discharged. During the next inspiration, a mixture of retained fresh gas and alveolar gas is inhaled. Rebreathing is avoided with fresh gas flow rates of greater than twice the minute ventilation for both spontaneous and controlled ventilation.