What structure appears as an air-filled space on a chest x-ray?

On a chest x-ray, the lungs are the structures that appear as air-filled spaces. The lungs contain air-filled alveoli, which allow for the exchange of oxygen and carbon dioxide. This air presence is what contributes to the dark appearance of the lungs on the radiograph.

The contrast between the lungs and other structures is significant; for instance, the heart and diaphragm are denser and will appear more radiopaque on the x-ray. The pleura, being a thin membrane around the lungs, won't present as an air-filled space either but rather as a thin line that may not be distinctly visible. Thus, the lungs are the primary structures that exhibit the characteristics of filled air spaces, providing the radiologist and clinicians with critical information regarding lung health and functionality.