When you breathe, air travels through your nose, down the trachea, and into smaller and smaller airways called bronchi. These bronchi branch into smaller passages called bronchioles and finally into small, thin, fragile sacs called alveoli. During inspiration, the alveoli in the lungs are filled with air. It is here that oxygen is exchanged for carbon dioxide. Blood cells absorb oxygen from the capillaries in the alveoli as carbon dioxide, a waste product, is released back into the lungs from the veins. During expiration, the carbon dioxide is expelled from the body. Oxygen-rich blood then travels to the heart so it can be pumped back to the body where it is needed.