Which structure initiates the heartbeat?

Which one has the thickest wall?

$A$: Endocardium provides a smooth lining for the inside of the heart and covers the valves of the heart.
$R$: Endocardium is continuous with the endothelial lining of the large blood vessels associated with the heart and the rest of the cardiovascular system.

What are the values of stroke volume and cardiac output under normal conditions?
Stroke Volume $\quad$ Cardiac Output

In a patient,the cardiac pacemaker stops functioning. The doctor decides to implant an artificial pacemaker. Its function is similar to which of the following?

The cardiac impulse,which causes the heartbeat,is delayed in which of the following?