What is the function of the water seal in a chest drainage system?

• A. Allows air to travel back to the patient.
• B. Prevents air from going back to the patient but lets it go to suction control; Normally fluctuating & gentle bubbling upon inhalation & exhalation.
• C. Provides suction to the system.
• D. Stores drainage.

Answer: (B)

The water-seal chamber acts as a one-way barrier. It lets air escape from the pleural space as you breathe or as suction pulls air out, but it prevents air from flowing back into the chest. When suction is connected, the air from the chest is directed through the water seal toward the suction control and then to the suction source, so air doesn’t re-enter the patient’s airways. You can see this working through tidaling—the water level gently rises and falls with inhalation and exhalation—indicating the system is responding to pressure changes and the tube is patent. The water seal does not store drainage (that’s the collection chamber) and it does not provide suction by itself (that’s the suction control chamber).