en·do·car·di·um 
(ĕn′dō-kärdē-əm)
(ĕn′dō-kärdē-əm)n. pl. en·do·car·di·a (-dē-ə)
The thin serous membrane, composed of endothelial tissue, that lines the interior of the heart.
[New Latin : ENDO- + Greek kardiā, heart; see kerd- in the Appendix of Indo-European roots.]
en′do·cardi·al adj.