The circulatory system relation with the respiratory system. The main organ of the respiratory system is the lungs. The diaphragm is a muscle located underneath the heart and lungs that contracts to pull air into the lungs and relaxes to force air out of the lungs. When the air enters the lungs, it travels through the bronchi to tiny air sacs called alveoli. In the alveoli, oxygen diffuses into the blood stream through capillaries that surround each alveoli. The newly oxygen rich blood then travels back towards the heart, where it will then get pumped to the rest of the body.