Final answer:
The endocardium is the innermost layer of the heart made up of endothelial cells and a thin layer of connective tissue, playing a critical role in lining the heart's chambers and affecting myocardial function.
Step-by-step explanation:
The endocardium is the innermost layer of the heart wall and plays a pivotal role in the function of the heart. It is composed of a type of simple squamous epithelial cells known as endothelium, which is also continuous with the endothelial lining of blood vessels. This endothelial layer is further supported by a thin layer of connective tissue that binds it to the myocardium, the muscular middle layer of the heart wall. The endocardium functions in lining the chambers and covering the valves where the blood circulates within the heart.
Not only does the endocardium serve as a barrier between the blood and the heart muscle, but it is also involved in various regulatory functions; for instance, recent evidence suggests that the endothelial cells of the endocardium may actively regulate the contraction patterns of the myocardium and influence the growth patterns of cardiac muscle cells throughout a person's life.