Which type of hypoxia is caused by low ambient oxygen pressure, such as at altitude?

• A. Hypoxic hypoxia
• B. Hypemic hypoxia
• C. Stagnant hypoxia
• D. Histotoxic hypoxia

Answer: (A)

Low ambient oxygen pressure reduces the amount of oxygen available to load onto hemoglobin in the lungs. Even though hemoglobin function and blood flow are normal, the alveolar PO2 falls with altitude, so arterial oxygen tension drops and tissues receive less oxygen. This situation is called hypoxic hypoxia because the primary problem is insufficient oxygen transfer from the air to the blood due to low inspired oxygen, not a defect in the blood’s carrying capacity, circulation, or cellular use of oxygen. By contrast, hypemic hypoxia involves reduced carrying capacity (anemia or carbon monoxide poisoning), stagnant hypoxia involves impaired circulation, and histotoxic hypoxia involves tissues unable to use oxygen.