The opening of auricles and ventricles on the right side is guarded by

The $Lubb$ sound is produced during the closure of which of the following?

All vertebrates possess a ...$A$.... Fishes have a ...$B$... chambered heart with atrium and ventricles. Amphibians and reptiles have a ...$C$... chambered heart. Birds and mammals have ...$D$... chambered heart.
Choose the correct option.

Gap junctions are characteristically found in

What is the maximum efficiency of the heart?

$SA-$node is located in