How does the chest tube collect fluid or air, and what prevents backflow?

• A. The chest tube actively pumps drainage
• B. Backflow is prevented by gravity alone
• C. There is no backflow prevention mechanism
• D. Fluid or air drains into the collection chamber via the water-seal; the one-way valve of the water-seal prevents backflow into the pleural space

Answer: (D)

The main idea is that drainage is driven by pressure differences between the pleural space and the outside world, with the water-seal acting as a one-way barrier. The chest tube carries fluid or air from the pleural space into the collection chamber of the drainage system. The water-seal creates a barrier that only allows flow outward toward the collection chamber (and, if suction is used, outward with the added pull of negative pressure). If a pressure trying to push back toward the pleural space were to occur, the water column blocks it, preventing backflow into the chest. So, drainage is not done by an active pump or gravity alone; it relies on the pressure gradient and the unidirectional action of the water-seal to keep fluid or air from returning to the pleural space.