The human heart is enclosed in a double-walled sac called:

State the location of the $SA$ node and the $AV$ node.

When cardiac muscle cells are damaged by a heart attack,they are usually replaced by

Select the correct option:
$(1)$ The pericardium is a double-layered membrane surrounding the heart.
$(2)$ The $SA$ node has a higher rate of impulse generation compared to the $AV$ node.

Identify the option showing the correct labeling for $p$,$q$,$r$ and $s$ with reference to the conducting system of the human heart.

What is a pulmonary vein?