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Stagnant hypoxia happens when delivery of oxygen to tissues is impaired because blood flow is insufficient or stagnant. Even if the lungs are taking up oxygen normally and the blood’s oxygen content is adequate, the tissues don’t receive enough oxygen if perfusion is reduced—tissues end up hypoxic because the blood isn’t reaching them in the needed amounts and times.

This is distinct from cases where the blood’s oxygen-carrying capacity is reduced (for example, anemia or carbon monoxide poisoning), where not enough oxygen is carried in the blood to begin with. It’s also different from histotoxic hypoxia, where cells can’t use oxygen even though delivery is fine, and from diffusion-related limitations, where oxygen can’t move efficiently from one region to another either in the lungs or at the tissue level.