The nurse is caring for a client with a chest tube connected to gravity drainage. Which of the following statements would the nurse identify as the purpose of the water seal?

A. Facilitates the drainage of secretions from chest cavity is incorrect because the drainage of fluid and air from the pleural space is primarily accomplished by the chest tube itself, not the water seal. The water seal’s role is to maintain a one-way barrier, not actively move fluid. B. Provides for more effective re-inflation of the lung is incorrect because lung re-expansion depends on removing air or fluid from the pleural space and restoring negative pressure. The water seal indirectly supports this by preventing backflow, but its primary function is not to directly re-inflate the lung. C. Prevents air from moving back into chest cavity is correct because the water seal acts as a one-way valve. It allows air and fluid to exit the pleural space while preventing air from re-entering the chest during inspiration. This is critical for maintaining negative intrapleural pressure and supporting lung expansion. D. Is necessary when suction is added to the drainage system is incorrect because the water seal functions regardless of whether suction is applied. Suction may be added to enhance drainage, but the water seal’s purpose is independent of suction—it always prevents backflow of air into the pleural space.