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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.
The Alveolus. 4. An alveolus is composed of a single layer of epithelial cells surrounding a lumen, and surrounded by a basket network of myoepithelial cells (their cell bodies are in green in this diagram). This is surrounded by a basement membrane (made of connective tissue proteins). Blood vessels and capillaries run through the stroma (inter-alveolar space). The stroma also contains fibroblasts, leukocytes (plasma cells) and other connective tissue cells. The series of images below will help better define an alveolus.