The endocardium is primarily composed of what type of cells that contact the blood?

The endocardium is indeed primarily composed of endothelial cells. This delicate layer lines the interior of the heart chambers and is in direct contact with the blood that flows through them. Endothelial cells play a crucial role in maintaining a smooth surface for blood flow, facilitating the exchange of substances, and preventing thrombosis (the formation of blood clots). They are important in the regulation of vascular tone and blood flow, contributing to overall cardiovascular health.

While muscle cells are found in the myocardium and connective tissue contributes to the structural integrity of the heart, they do not form the primary layer that directly interacts with the blood flow. Epithelial cells, which include various types of cells that line other body surfaces, are not the main types of cells found in the endocardium. Thus, recognizing that endothelial cells form this critical layer clarifies their role in the cardiovascular system.