The "Bundle of His" is considered to be a part of which of the following organs?

$A$: 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 is meant by cardiac output?

Papillary muscles are found in mammalian

Assertion : Muscle fibres of $SA$ node possess the lowest rhythmicity among all cardiac muscles.
Reason : Due to this fact,it can initiate excitatory waves at the highest rate.

$A$ patch of nodal tissue responsible for initiating the rhythmic contractile activity of the heart is present in: