endocardium

https://doi.org/10.1351/goldbook.10655

Endothelial lining of the heart chambers, also containing small blood vessels and a few bundles of smooth muscle, continuous with the endothelium of the great blood vessels.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 748 [Terms] [Paper]