Alveoli have thin, porous walls that contain capillaries; these carry blood to and from the lungs and other vital organs. Oxygen passes through the alveoli into blood carried by the capillaries, and carbon dioxide passes out of the capillaries back into the alveoli. In a person with pneumonia, the alveoli become inflamed and fluid accumulates in the alveoli, causing shortness of breath, difficulty breathing, and other symptoms of pneumonia.
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