Final answer:
The endocardium is the thin layer of tissue lining the interior of the heart's chambers. It ensures a smooth blood flow within the heart and is composed of simple squamous epithelium called endothelium.
Step-by-step explanation:
The thin layer of tissue lining the interior of the heart chambers is called the endocardium. The endocardium is the innermost layer of the heart wall and is joined to the myocardium with a thin layer of connective tissue. This layer lines the chambers of the heart where the blood circulates and also covers the heart valves. It is composed of a simple squamous epithelium known as the endothelium, which is continuous with the endothelial lining of the blood vessels. The endocardium's main function is to provide a smooth lining for the heart chambers and to facilitate the flow of blood within the heart.