Why do you need two different circuits of blood vessels in your body?

Consider the red cells. They carry oxygen. They get the oxygen from the lungs.

They take the oxygen to another part of the body.  They have performed their duty.

Now what?

Now they must get back somehow to do the same thing again. If there were not 2 pathways, one set of red cells would be fighting to get back for more oxygen, and the other set would be fighting to deliver what they had.

Such chaos could not long endure. The body would eventually collapse from such an insane competition. Hence, two pathways.

ia. mver.y..sor .r..yinee..i am.ina.rush.pleas eforg.ive   meiknowiama. nas.s holei apalo.gize