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 !