Define / Explain the following terms:
$(1)$ Pacemaker
$(2)$ Erythroblastosis

(N/A) $(1)$ The $SA$ node initiates the heartbeat and sends stimulating impulses every $0.80$ seconds to the atria and ventricles. Therefore,the $SA$ node is known as the pacemaker of the heart.
$(2)$ Erythroblastosis fetalis occurs due to $Rh$ incompatibility. If an $Rh$-negative mother carries an $Rh$-positive fetus for the second time,the maternal antibodies produced against the $Rh$ antigen cross the placenta and destroy the fetal $RBCs$. This condition is known as erythroblastosis fetalis.