When we breathe, air enters our bodies through our nose and mouth and makes its way down the trachea. The trachea branches into two smaller tubes, called bronchi, which send air to each lung. The bronchi divide into even smaller tubes called bronchioles which finally send the air into air-sacs called alveoli. The alveoli look like a bunch of grapes and it’s this structure where gas exchange takes place. Oxygen diffuses from a region of high concentration in the alveoli to a region of low concentration in the bloodstream, where it travels to different tissues of the body and is used for respiration. Carbon dioxide travels in the other direction, from a region of high concentration in the bloodstream to a region of low concentration in the alveoli, where it travels up the trachea and is breathed out.