endocardium

n. membrane that lines the heart (Anatomy)

In the heart, the endocardium is the innermost layer of tissue that lines the chambers of the heart. Its cells, embryologically and biologically, are similar to the endothelial cells that line blood vessels.The endocardium overlies the much more voluminous myocardium, the muscular tissue responsible for the contraction of the heart. The outer layer of the heart is termed epicardium and the heart is surrounded by a small amount of fluid enclosed by a fibrous sac called the pericardium.
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Endocardium /en·do·car·di·um/ the endothelial lining membrane of the cavities of the heart and the connective tissue bed on which it lies. [more]Endocardium - Community and Resources

Noun
1. the membrane that lines the cavities of the heart and forms part of the heart valves
(hypernym) serous membrane, serosa

(n.)
The membrane lining the cavities of the heart.