The answer is ‘They are both part of the circulatory system’. The lungs expire carbon dioxide brought in by the blood capillaries in the alveoli (carbon dioxide from tissues) and also incorporates oxygen into the blood in the same capillaries’ blood. This oxygen is taken to the tissues. This phenomenon in the lungs is called gaseous exchange. The gaseous exchange also happens in the tissues when blood in the capillaries exchange oxygen to tissue for carbon dioxide from tissue.
