Which layer of the heart wall is the muscular middle layer?

• A. Epicardium
• B. Myocardium
• C. Endocardium
• D. Pericardium

Answer: (B)

The heart wall has three distinct layers, with the muscular middle layer doing the pumping. This layer, the myocardium, is made of cardiac muscle arranged in spiraling bundles that contract in a coordinated way to squeeze blood out of the chambers. Its thickness varies by chamber, being thickest in the left ventricle because it must generate high pressure to push blood through the systemic circulation. Surrounding the myocardium is the epicardium, the outer serous layer that covers the heart’s surface, while lining the heart chambers and valves is the endocardium, a smooth endothelial layer. Outside the heart, the pericardium forms a fibrous sac with a small amount of lubricating fluid that reduces friction as the heart beats. In short, the muscular middle layer responsible for contraction and force generation is the myocardium.