Why is the ventricle a stronger muscle than the atria? When blood leaves the ventricle and goes to the body, why isn't the blood 100% oxygenated (in a frog) ?

First answer is, as the work of ventricle is to push blood to he whole body, so it has to be strong or it can not pump blood to head to the toe!

as in frog (an amphibian) it has only three chambers as two artia and one ventricle, so the oxygenated blood from left artia and deoxygenated blood from right, combines and thus partial oxygenated blood is circulated!