Circulatory and respiratory system is connected to each other, they help to provide the need of our body. First the air will enter the nose, the nasal cavities will moisten the air, then it will go to the trachea, to the bronchi, to the small cells the brochioles, and to the alveoli, it is the time that the blood comes to get the oxygen and exchange the carbon dioxide to exhale by the lungs, if the exchange of gases is done,the blood will distribute the oxygen  to cells to break down sugar to convert energy to sustain us energy to perform activities. The blood will comes in the left atrium and goes out to the left ventricle, to the arteries, to the capillaries, in capillaries happen a gas exchange, the blood will collect the carbon dioxide end leave the oxygen, then to the viens, goes inside the right atrium, and goes out to the right ventricle, the  carbon dioxide will go out to our body by exhaling it, if it is you will be poisoned because the carbon dioxide is a toxic.