counterlungs

Counterlungs are flexible gas-filled reservoirs used in breathing equipment to store inspired air and to moderate pressure and volume in a breathing circuit. They act as artificial lungs within the system, expanding during inhalation and contracting during exhalation to smooth tidal flow and reduce work of breathing.

In underwater breathing systems such as closed-circuit rebreathers, counterlungs are integral parts of the breathing loop. They typically consist of one or more collapsible bags that expand as the diver inhales and compress on exhalation, helping to regulate flow, maintain stable loop pressure, and reduce the effort required to breathe.

In buoyancy control devices used on scuba gear, counterlungs are internal air bladders that contribute to buoyancy. When air is added from the regulator, the counterlungs inflate and increase buoyancy; they deflate as gas is vented or as the diver adjusts depth, altering overall volume and resulting buoyancy.

In medical and clinical contexts, the term counterlung describes the patient-side reservoir in ventilator circuits and manual resuscitation bags. The counterlung expands with delivered gas, stores tidal volume, and influences resistance and delivered pressures during ventilation.

Design and maintenance considerations include material choice (such as silicone or polyurethane), elasticity and puff, storage volume, cleaning and disinfection, and regular inspection for tears or leaks to ensure safe operation.