Which layer of the ventricles is closest to the blood within the chamber?

The endocardium is the layer that lines the interior surface of the heart chambers, including the ventricles. It is a thin, smooth membrane composed of endothelial cells, which provides a non-thrombogenic surface for blood flow and ensures that the blood stays in direct contact with a smooth layer, minimizing turbulence and potential clot formation.

This layer also plays a crucial role in maintaining the electrical properties necessary for efficient cardiac contraction. The proximity of the endocardium to the blood within the chamber makes it vital for processes such as ion exchange and transmission of signals that regulate heart rhythm.

In contrast, the myocardium is the thick muscular layer responsible for the contraction of the heart, the epicardium acts as a protective outer layer (visceral pericardium), and the pericardium is the fibrous sac surrounding the heart. None of these layers comes in direct contact with the blood within the heart's chambers, which is why they do not fulfill the role of the endocardium in this context.