Why does the respiratory system need to pass oxygen to the blood?

It's need to pass oxygen to the blood to the respiratory system work properly I think that the answer

 So that the oxygenated blood can go through the circulatory system and pass on the oxygen to the cells in the tissues; without it the tissues will die.

Hope this helps !

Photon

Mitochondria is the cell organelle that produced the energy that is needed in the body system in form of atp, that is why it is called the power house of the cell. energy is needed by the living organisms to carry out the activities which sustain life.  if the mitochondria is destroyed atp will not be produced. if the mitochondria is destroyed, energy production will stop and the affected living organism will not have the energy to carry out the necessary life processes; this will lead to the death of the organism.