(A) A chest tube extends from the pleural space to a collection device. The tube is placed below the surface of the saline so that air cannot enter the pleural space. (B) Fluid may enter the pleural space as a result of injury or disease. A chest tube may drain fluid from the pleural space, but the water seal is not involved in this. (C) Chest drainage should be measured, but the water seal is not involved in this. (D) Fluctuations in the tube in the water-sealed bottle will give an indication of respiratory effort, but that is not the purpose of the water seal.