The Respiratory system controls the intake of oxygen and the exhale of waste products such as carbon dioxide from our circulatory system. The lungs control the gas exchange between our respiratory system and our circulatory system. Once the nutrients are in the blood stream, the circulatory system, red blood cells and white blood cells and other cells floating through arteries, all controlled by the pumping of the heart. Once the nutrients inhaled by the respiratory system are brought to the cells by the circulatory system (from the pulmonary vein to the heart to the arteries) through the much smaller capillaries, they are used in cellular respiration. The oxygen is used to help the products of glycolysis enter the mitochondria to produce more ATP.
The waste products are then put back into the circulatory system through the capillaries. The circulatory system then takes the nutrients through veins back to the heart where it is then put through the pulmonary artery to the lungs. Once in the lungs gas exchange occurs, expelling the waste products and reoxygenating the blood.
Source: https://sites.google.com/a/student.oprfhs.org/ap-biology-interdependence/interdependence/interdependence-of-the-circulatory-system
Hopefully this helps you with your confusion!
