How are the respiratory and circulatory systems connected apex?

The respiratory system and the circulatory system is connected through the lungs, specifically the ALVEOLI.  The alveoli is the site of gas exchange. This is where blood gets oxygenated. 

The circulatory system's main role is to deliver oxygen and nutrients and at the same bring out waste. The oxygen that goes around the body comes from the lungs.The respiratory system is in charge of bringing in the oxygen from the atmosphere. 

Blood leaves the heart and enters the lungs via the pulmonary arteries which branches out into the alveoli of the lungs. This is where the oxygen gets picked up and it goes back into the heart. 

The answer is then LUNGS, specifically the ALVEOLI.