Why does a circulatory system need the respiratory system?

Circulatory system circulates oxygenated blood through out the body. It brings deoxygenated blood back to heart. BUT WHERE DOES THIS OXYGENATED BLOOD COME FROM WHEN THE DEOXY IS SENT TO THE HEART?

It comes from the lungs where deoxy blood goes for oxygenation and then sent to heart for recirculation again. Since oxygenation requires lungs which are a part of the respiratory system, we can say that Circulatory system needs respiratory system.

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