Altitude and oxygen

Your body’s ability to absorb oxygen depends on the partial pressure of oxygen. As air is inhaled, it inflates the lungs with oxygen-rich air. The inhalation inflates the alveoli in the lungs, which are like microscopic balloons. The air in the alveoli is one cell layer from the blood in the tiny capillaries that traverse the lung tissue. It is pressure differences that drive oxygen from the air to your lungs, then into your blood through the alveoli, and finally to your tissues. Oxygen is transported into the capillaries by the oxygen pressure gradient, affected by ambient barometric pressure and capillary oxygen pressure. The millions of alveoli provide an exceptionally large surface area for oxygen to diffuse through. Since the oxygen level is lower in the venous blood in the lung, the oxygen-rich air inhaled “wants” to pass through the alveoli into the bloodstream just like any high concentration of gas wants to spread to an area of low concentration.

There are other factors that affect the diffusion of oxygen, such as pneumonia or less severe forms of congestion, which fill the alveoli with fluid, preventing the transfer of gas molecules. Also, when we sit in the cockpit, we tend not to take deep breaths, which means some alveoli are not cycling with air. Ventilation is also decreased by those who have large abdomens.

Oxygen saturation is not directly affected by density altitude because the percentage of oxygen in the air remains roughly 21 percent, depending on some inconsequential variations influenced by where you are located on Earth. Although the number of oxygen molecules that are inhaled with higher density altitude decreases with each breath, it does not significantly affect the amount of oxygen transferred to the blood in the lung tissue because it is not the controlling variable.

Humans tend to compensate to some extent for the decreased partial pressure of oxygen at altitude by increasing breathing rates. However, some people decrease their breathing rate in response to increasing altitude, referred to as Cheyne-Stokes or periodic breathing, characterized by cyclic increases and decreases in air movement volume with brief pauses in breath. This has been described in mountain climbers while they sleep, and I documented this in pilots when I tested the first use of pulse oximeters in aircraft. Physical exersion supersedes an altitude effect, so it was never evalutated in mountain climbers during day activities.

Adding oxygen by mask or nasal cannula helps alleviate the effects of altitude by dramatically increasing the inhaled percentage of oxygen in the alveoli to diffuse into the capillaries.

Issues with the circulatory system can also affect oxygen being delivered to the tissues and can impact your flying. Anemia, which is a decrease in the number of oxygen-carrying red blood cells or the decrease in the amount of oxygen a cell can carry, can affect the amount of oxygen delivered to the brain and retina, which are the most oxygen-sensitive areas of the body. Iron deficiency can affect the production of red blood cells, leading to anemia as well as imperceptible blood loss that can occur with a bleeding ulcer.I urge all pilots, whether they fly at high altitude or not, to take pressure chamber training.

Importantly, carbon monoxide can severely affect the blood’s ability to carry oxygen by changing hemoglobin into carboxyhemoglobin, which prevents the cell from carrying oxygen. Since carbon monoxide binds hemoglobin about eight times more “tightly” than oxygen, long, drawn-out periods of low-level exposure can slowly disable a pilot. Carbon monoxide creates an adverse synergistic hypoxic effect with lower oxygen saturation levels at higher altitudes.

Also, a significant altitude effect is dehydration. As altitude increases, humidity decreases. When a person takes a breath, the inhaled air is essentially humidified in the lungs to about 100 percent. Thus, with each breath, the difference between inhaled humidity and 100 percent is lost. This is easy to visualize when you see your breath in freezing weather. If it is hot in the aircraft, perceptible and imperceptible sweat loss adds to the fluid deficit.

I urge all pilots, whether they fly at high altitude or not, to take pressure chamber training. You’ll learn a lot.

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