Atria are Primer Pumps for the Ventricles
Ventricular Filling Occurs in Three Phases:
Rapid ventricular filling
Under Resting Condition
80% of the ventricular filling is complete by rapid ventricular filling and diastasis.
Atrial contraction contributes only 20% to ventricular filling. This is a relatively small amount.
Moreover, the heart has the capacity to pump 3 to 4 times more blood than required during resting conditions.
Because of these reasons, atrial contraction is not absolutely necessary during the resting conditions and a person can do just fine even if atria fail.
The contribution of atrial contraction in ventricular filling increases up to 40%.
This is a significant contribution and if it's not available, the ability of a person to do exercise might be compromised.