Analogy type questions:
$(1)$ Right atrium and right corner : $SA$ node :: Corner of right atrium : .......
$(2)$ First heart sound : $Lubb$ :: Second heart sound : ...........

$SA-$node is located in

The $lubb$ and $dubb$ heart sounds are produced due to:

Which of the following blood vessels in mammals normally carries the largest amount of urea?

$SA$ 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?

Which structure first generates an action potential?