What is the function of the water in the underwater seal chamber?

• A. To collect drainage
• B. To prevent reflux of air back into the chest
• C. To regulate suction pressure
• D. To cool the system

Answer: (A)

Water in the underwater seal chamber acts as a one-way valve. It lets air escape from the pleural space (bubbles rise through the water and exit to the atmosphere) but prevents air from flowing back into the chest when intrapleural pressure drops. The drainage collection is handled by a separate collection chamber, and suction control (if used) is managed by the suction-control chamber. Cooling is not a function of the seal. If you see continuous bubbling in the seal during inspiration, that suggests an air leak or fistula; absence of bubbling with good drainage can indicate a sealed system.