The lungs are respiratory organs located in our thoracic cavity, and they clean our blood of carbon dioxide and replenish it with oxygen. Oxygen-deficient blood is converted into oxygen-rich blood by the lungs. This process, called gas exchange, occurs in the alveoli. The air comes in through the bronchi, which divide into bronchioles, and eventually into alveoli. There are capillaries (small blood vessels) around the alveoli, which carry blood for the gas exchange process. When the blood reaches the lungs, the carbon dioxide is absorbed by the alveoli. The oxygen from the inhaled air is added back to the blood vessels through the alveoli. The carbon dioxide is transported through the alveoli and eventually exhaled back into the air.