Oxygen and Respiration — Page 45, Lesson 69

I want to walk you through the fundamentals of oxygen and respiration as they apply to you as a pilot. Let's start with the big picture: humans operate best at sea level, but we are perfectly capable of operating at higher altitudes where the partial pressure of oxygen is lower. People who live permanently at high altitudes can adapt to the reduced amount of oxygen by producing extra red blood cells, which enables more oxygen to be carried in the blood. For healthy people without those extra cells, you can function normally up to about 10,000 to 12,000 feet, provided no strenuous exercise is undertaken. Now, as altitude increases, the overall pressure decreases, and so do the partial pressures of the various gases in the atmosphere. But here's the key point: the partial pressure of oxygen in the air is not the governing factor for your body. Why? Because your body takes its oxygen from the alveoli of the lungs, where the partial pressure is actually less than in the outside air. The body produces carbon dioxide and water vapour, which are passed into the alveoli. Since the total pressure both inside and outside the lungs remains the same, the partial pressure of oxygen inside the alveoli must be lower. Let me give you the numbers from the table so you can see exactly what's happening. At sea level, in atmospheric air, the partial pressure of oxygen is 160 mm Hg, which is 21% of the total. In the alveolar air, that drops to 103 mm Hg, or 14%. Nitrogen in atmospheric air is 600 mm Hg, and in alveolar air it's 570 mm Hg. Water vapour is not present in atmospheric air, but in alveolar air it's 47 mm Hg. Carbon dioxide is also not present in atmospheric air, but in alveolar air it's 40 mm Hg, which is 5.3%. Now, at 10,000 feet, look at the alveolar air values: oxygen drops to 55 mm Hg, nitrogen to 381 mm Hg, water vapour stays at 47 mm Hg, and carbon dioxide remains at 40 mm Hg. A partial pressure of 55 mm Hg is considered the minimum for normal operations. So, above cabin heights of 10,000 feet, oxygen needs to be added to the pilot's air supply. The oxygen added is sufficient to maintain an alveolar partial pressure of 103 mm Hg, which is equivalent to breathing air at sea level. At lower levels, less oxygen needs to be added, and as altitude increases, more oxygen is added. A stage will be reached when one hundred per cent oxygen is required to maintain that 103 mm Hg partial pressure — the equivalent to breathing air at sea level. That stage is reached at 33,700 feet. But this does not limit us to flying only to 33,700 feet when breathing 100% oxygen. We can continue to operate normally with an alveolar partial pressure of 55 mm Hg, which is equivalent to breathing air at 10,000 feet. That partial pressure is reached at 40,000 feet. Above this level, 100% oxygen must be supplied at an increased pressure — that's called pressure breathing — but this is more relevant to military crews who fly at high altitudes. Pressure breathing for long periods is tiring and requires practice to perfect the technique. Let me summarise the thresholds for oxygen requirements so you have them clear in your mind. Up to 10,000 feet, air only is sufficient. From 10,000 to 33,700 feet, you need an oxygen/air mixture. From 33,700 to 40,000 feet, you need 100% oxygen. Above 40,000 feet, you need pressure breathing with 100% oxygen, but that's primarily for military operations.