Every time you breathe in air, your diaphragm tightens, moving downward to make space in your chest. Your lungs expand, pulling air in through your nose and/or mouth. That air then moves down your trachea, through your bronchi and into the bronchioles, where it enters your alveoli. Oxygen passes from the alveoli to the hemoglobin in the surrounding capillaries, while carbon dioxide passes from the capillaries into the alveoli. The carbon dioxide is exhaled when your diaphragm relaxes and moves upward into the chest cavity, forcing the air out of your lungs. The oxygen-rich blood in your capillaries is carried to the pulmonary vein, where it travels to the left side of the heart and is then pumped throughout the rest of the body.