Caption

Animation of gas exchange in an alveolus of the lung during normal breathing. An alveolus (yellow) is a small pouch that forms the end of the highly branched tree of airways inside a lung. Each alveolus is surrounded by capillaries (green), which carry deoxygenated blood. When air is breathed in, the alveolus expands and fills with oxygen molecules (red), raising the concentration of oxygen relative to carbon dioxide (CO2, blue and red), and causing the CO2 carried by the blood cells to diffuse through the capillary wall into the alveolus. As this occurs, the oxygen in the alveolus diffuses the other way. This leaves the air in the alveolus high in CO2, which is then exhaled. The capillaries carry the oxygen-rich blood cells to the heart, where they are pumped around the body to provide oxygen to tissues and organs. Original same as master.