About Lungs and Breathing
The lungs are the main organs in the respiratory system and control breathing. The interior of a lung resembles an upside-down tree; passages at the top of the lungs branch off repeatedly, splitting into smaller and smaller passages before ending in tiny sacs called alveoli. The alveoli let blood come into contact with air, allowing the blood to pick up oxygen and release carbon dioxide. If the passages of the lungs become inflamed, as is the case with asthma, they can swell and make it hard to breathe. Particulates in the air, whether caused by smog or cigarettes, can clog the lungs and limit the oxygen they can take in. Alveoli can also become swamped with fluids like mucus; this can be a major concern for people with cystic fibrosis or pneumonia.