Final answer:
The innermost layer of the heart's wall is called the endocardium, which is made of endothelium and is continuous with the lining of blood vessels.
Step-by-step explanation:
The innermost layer of the heart is known as the endocardium. This layer lines the chambers where blood circulates and covers the heart valves. It is composed of a type of simple squamous epithelium called endothelium, which is also continuous with the endothelial lining of blood vessels. The endocardium is connected to the myocardium by a thin layer of connective tissue. The heart in general is composed of three layers: from the innermost out, these are the endocardium, the myocardium (the heart muscle itself), and the epicardium (the outermost layer). The second layer out, the pericardium, is a protective membrane that envelops the heart.