Which of the following regulates the heart's contractility?

The regulation of the heart's contractility is primarily associated with the myocardium, which is the muscular layer of the heart. The myocardium consists of cardiac muscle tissue responsible for contracting and pumping blood throughout the body. This layer is equipped with specialized cardiac myocytes that respond to nervous and hormonal signals, which can enhance or decrease the strength of heart contractions.

The endocardium, while important for lining the heart chambers and ensuring smooth blood flow, does not play a direct role in regulating the contractility of the heart muscle. Therefore, in terms of the contractile function of the heart, the myocardium is where the actual contraction occurs and is influenced by various factors such as calcium ion availability and neural input.

Focusing on the structural components, the epicardium is the outer layer of the heart, and the pericardium is the fibrous sac surrounding the heart; both serve protective and supportive functions rather than being directly involved in the heart's contractility.