Which of the following chambers of the human heart has the thickest wall?

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

When the heart of a frog is removed from the body,it continues to beat for some time. Select the correct statement$(s)$ from the following:
$(i)$ The frog is a cold-blooded animal.
$(ii)$ The frog does not have a heart-based circulatory system.
$(iii)$ The heart has myogenic properties.
$(iv)$ The heart is self-excitable.

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

Explain in short: $Lub$ and $Dub$ sounds.

Impulse of heart beat originates from