If you were to be teleported from sea level to the peak of Mount Everest, your body would experience significant problems. At 8,848 meters, the barometric pressure is around 33% of what it is at sea level, meaning there is far less oxygen in the air, and you would likely suffocate within minutes.
However, for those who make the journey over the course of a month, it is possible to survive at the summit for hours. What changes occur in our bodies during this time that allow us to endure such an incredible altitude?
If you live less than 500 meters above sea level, when you breathe in, your lungs fill up with air containing numerous gases and compounds, with the most important being oxygen molecules that bind to the hemoglobin in your red blood cells. Blood then circulates throughout your body, bringing vital oxygen to all your cells. As altitude increases, however, the air becomes thinner, and there is less oxygen for our bodies to absorb.
When you ascend to altitudes above 2,500 meters, the resulting oxygen deprivation can cause altitude sickness, known as AMS, which often causes headaches, fatigue, and nausea. Fortunately, AMS only happens when you ascend too quickly, as our bodies have various ways of adapting to high altitudes.
Within minutes of reaching altitudes of 1,500 meters or more, carotid chemoreceptors in your neck sense your blood’s low oxygen pressure. This triggers a response that increases the rate and depth of your breathing to counteract the lack of oxygen. Your heart rate also increases, and your heart contracts more tightly to pump additional oxygenated blood around your body.
If you were to keep ascending, your heart rate and breathing would speed up accordingly. However, if you were to remain at this altitude for several weeks, you could benefit from longer-term adaptations. Within the first few days above 1,500 meters, the volume of plasma in your blood decreases, which increases the concentration of hemoglobin. Over the next two weeks, your hemoglobin levels continue to rise, allowing your blood to carry even more oxygen per milliliter.