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In the alveoli, oxygen from the air is absorbed into the blood. Carbon dioxide, a waste product of metabolism, travels from the blood to the alveoli, where it can be exhaled. Between the alveoli is a thin layer of cells called the interstitium, which contains blood vessels and cells that help support the alveoli.
At the end of the smallest bronchioles are alveoli. Alveoli are tiny sacs that are lined by a very thin layer of cells. They also have an excellent blood supply. The tiny alveoli are the place where oxygen enters the blood and where carbon dioxide (CO 2) leaves the blood. The lungs are divided into different parts by what are known as fissures. Fissures are separations of the tissue of the lung that divide the lung into lobes.