Question

why do the ventricals need to be more muscular thenthe atria

Answer 1

The atria simply have to collect the blood coming from the vena cava or pulmonary veins and pump this into the ventricles (right and left, respectively). The ventricles must pump the blood to thr lungs (right ventricle) or body (left ventricle) which require stronger muscle walls (left ventricle much thicker than right).