The opening of the post caval vein into the right atrium is guarded by which of the following?

In mammals,the mitral valve regulates the flow between which of the following?

What is a pacemaker?

In the heart, as the action potential reaches the $AV$ node from the $SA$ node, there is a delay of the action potential. This delay is important because:

Draw a labelled diagram showing the internal structure of the human heart.

$SA$ node (Sinoatrial node) ke baare mein sahi kathan chuniye:
$I$. Located at the right upper corner of the right atrium.
$II$. Heart of heart.
$III$. It initiates the rhythmic contractile activity of the heart.
$IV$. It is called the pace-keeper of the heart.
$V$. It is called the pacemaker of the heart.
Which of the following options contains the correct statements?