Although the respiratory system is most commonly associated with the lungs, the breathing process involving both the circulatory and the pulmonary systems must work in harmony to be effective. When we breathe in, we oxygenate blood as it moves through the lungs to the arteries that crisscross the body. Once it is drained of oxygen, the blood returns through the veins to the heart then to the lungs, where it transfers carbon dioxide into the lungs for exhalation. Given the partnership between the respiratory and circulatory systems, neither works well without the other.