Which layer of the heart is responsible for contraction and relaxation?

The myocardium is the layer of the heart responsible for contraction and relaxation. It consists of specialized cardiac muscle tissue that allows the heart to pump blood effectively. This layer is thick and powerful, enabling rhythmic contractions that are necessary for maintaining blood circulation throughout the body.

The other layers serve different purposes. The endocardium lines the inner chambers of the heart and is involved in providing a smooth surface for blood flow, while the epicardium is the outer layer that covers the heart and is involved in protecting it and housing blood vessels. The pericardium is a fibrous sac that surrounds the heart, providing protection and anchoring it within the chest cavity. None of these layers contribute to the heart's ability to contract and pump blood the way the myocardium does.