What is the primary purpose of the water-seal chamber in a chest drainage system?

• A. To prevent air from escaping the pleural space.
• B. To prevent air from re-entering the pleural space while allowing air to escape.
• C. To collect blood from the pleural space.
• D. To measure pleural fluid volume.

Answer: (B)

The water-seal chamber serves as a one-way barrier that lets air escape from the pleural space but prevents it from coming back in. As air from the lung or pleural space exits through the chest tube, it bubbles through the submerged water and exits to the atmosphere. If pressure in the pleural space tends to push air back toward the chest, the water barrier stops that reverse flow, preventing re-entry of air and helping to keep the pleural space from re-accumulating air. This is distinct from the collection chamber, which holds drained fluid or air, and from the suction-control chamber, which provides regulated suction.