The endocardium is the inner layer of which structure?

• A. The lungs
• B. The mediastinum
• C. The heart
• D. The pericardial sac

Answer: (C)

Endocardium is the innermost lining of the heart. It lines the interior of the heart chambers and covers the heart valves, forming a smooth endothelial surface that helps reduce resistance to blood flow. This layer sits inside the heart alongside the myocardium (muscular wall) and the epicardium (outer layer). The endocardium is continuous with the endothelial lining of the great vessels as they connect to the heart.

It’s not the lining of the lungs, which are covered by pleura; it’s not the lining of the mediastinal space, which is a central thoracic compartment; and the heart’s outer surface is the epicardium, with the pericardial sac surrounding the heart having fibrous and serous layers. So the endocardium specifically belongs to the heart as its inner lining.