Final answer:
The physical extensions of the myocardium that are lined with endocardium include structures such as papillary muscles, trabeculae carneae, and pectinate muscles, which all play important roles in the heart's ability to pump blood.
Step-by-step explanation:
The heart consists of three layers: the epicardium, myocardium, and endocardium. The myocardium is the thickest layer, composed of cardiac muscle cells that provide the force necessary to pump blood. The endocardium is the innermost layer of the heart wall that lines the heart chambers and heart valves, made up of a special type of epithelium known as endothelium that is smooth and lines the blood vessels as well. The myocardium extends into structures like papillary muscles, which connect to the heart valves via the chordae tendineae and are also lined with endocardium. Additionally, the trabeculae carneae—muscular ridges on the walls of the ventricles, and the pectinate muscles found on the anterior surface of the right atrium, represent other physical extensions of the myocardium lined with endocardium. These structures play significant roles in the functioning of the heart, contributing to its ability to pump blood effectively.