In humans,blood passes from the post caval to the diastolic right atrium of the heart due to:

Foramen ovale

$A$: 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.

The trilobed valve present between the right atrium and the right ventricle in the mammalian heart is known as:

Define / Explain the following terms:
$(1)$ Pacemaker
$(2)$ Erythroblastosis

Heartbeat originates from