Which statement about closed-chest drainage systems is correct?

The statement that wet suction creates suction by changing the water level is accurate because closed-chest drainage systems often utilize a water seal and suction mechanism to facilitate the removal of air and fluid from the pleural space. In these systems, the height of the water in the suction chamber generates the necessary negative pressure. As fluid levels change, this directly affects the amount of suction pressure applied to the pleural cavity. The water level can be adjusted to increase or decrease the suction, making this mechanism reliable for managing pleural drainage effectively.

The other options do not accurately portray the functionality of closed-chest drainage systems. For instance, while the underwater seal does limit air entry, it does not prevent it entirely under all conditions, particularly if there is a leak or if negative pressure exceeds the seal's capability. Gravity is also not solely responsible for creating negative pressure; it may aid in drainage but does not replace the need for suction. Lastly, closed systems are specifically designed to be effective for drainage; suggesting they are ineffective contradicts their established purpose in clinical practice.