Why is there no mixing of oxygen rich and oxygen poor blood in a four chambered heart and why is it

There is no mixing of oxygen rich blood and oxygen poor blood because the chambers keep the blood from mixing. The four chambers are the right atrium, the left atrium, the right ventricle, and the left vetricle (here's a diagram, it should help you visualize my expalantion: http://en.wikipedia.org/wiki/image:heart.

The process starts when the right ventricle pumps oxygen poor blood to the lungs, where it picks up oxygen. The blood then goes back to the left atrium. The blood in the left atrium is transfered to the left ventricle by way of a one-eay valve in the heart.

The blood in the left ventricle is pumped throughout the rest of the body. (head, arms, legs), where it drops off its oxygen, and the oxygen poor blood returns to the right atrium. This oxygen poor blood is then transfered to the right ventricle by way of a one-way valve, where the process begins again.

As you can see, there is no mixing of the oxygen richa and poor blood because the valves in the heart (and in the arteries and veins!!) keep the blood from moving backward in the cycle. This is important because if the oxygen rich and poor blood mixed, there wouldn't be as much oxygen getting to the body cells, which is inefficient. Some cells might not get any oxygen and could die if the blood mixed.