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 !
I am not really sure about the city, but if we look at the country, its BORDERS would be the same analogy for the cell membrane. I am saying this because cities dont really have a border .
