Which structure lines the lungs?

The structure that lines the lungs is the visceral pleura. This is a delicate membrane that closely adheres to the surface of the lungs, serving as the protective layer that covers the lung tissue. The visceral pleura is crucial because it creates a smooth surface that allows the lungs to expand and contract without friction during breathing.

The parietal pleura, while related, does not directly line the lungs; instead, it lines the chest cavity and provides a protective layer for other structures in the thoracic cavity. The diaphragm is a muscle that plays a vital role in respiration but is not a membrane lining the lungs. The mediastinum is the central compartment of the thoracic cavity, situated between the lungs, containing vital structures such as the heart and major blood vessels, but it does not line the lungs.