Healthy lungs look pink and somewhat like a sponge from the outside, according to KidsHealth. Inside the lungs is a system of hollow tubes that look like the branches of a tree. The largest tubes, the bronchi, connect to the trachea.
The bronchi connect to the bronchioles, reports Kids Health. At the ends of the bronchioles are the tiny air sacs known as the alveoli. Capillaries cover these air sacs. These tiny blood vessels allow the blood to exchange the waste carbon dioxide from the body for fresh oxygen from the air. Each time a person inhales, he brings fresh oxygen through the system to the capillaries. Exhaling expels the waste gases.