If someone teleported from sea level to the top of Mt. Everest, things would go bad fast. At an altitude of 8,848 meters, barometric pressure is approximately 33% of what it is at sea level. This means there's significantly less oxygen in the air, and our teleported individual would likely suffocate in minutes. However, for people that make this same journey over the course of a month, it's possible to survive at the peak for hours. So what can happen to our bodies  in just one month that allows us to endure this incredible altitude? Let’s imagine you’re one  of the 5.8 billion people living less than 500 meters  above sea level. When you take a breath at this altitude, your lungs fill up with air composed of numerous gases and compounds. Most important among these  are oxygen molecules, which bind to the hemoglobin  in your red blood cells. Blood then circulates  throughout your body, bringing essential oxygen to all your cells. But as altitude increases,  the air starts to get thinner. The relative amounts of each compound remain the same well into the upper atmosphere, but overall, there is less oxygen  for our bodies to absorb. And if you ascend to altitudes above 2,500 meters, the resulting oxygen deprivation can cause a form of altitude sickness known as AMS, often causing headaches, fatigue and nausea. Fortunately, AMS only happens when we ascend too fast, because our bodies have numerous ways of adapting to high altitudes. Within minutes or even seconds  of reaching altitudes of 1,500 meters, 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 blood with each beat, quickly moving oxygenated blood around your body. All these changes happen relatively fast, and if you were to keep ascending, your heart rate and breathing would speed up accordingly. But if you stayed at this altitude for several weeks, you could reap the benefits  of some 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 will continue to rise, allowing your blood to carry  even more oxygen per milliliter. Paired with your high heart rate, this new hemoglobin-rich blood efficiently distributes oxygen throughout your body. So much so that the volume of blood  being pumped with each heartbeat can return to normal levels. Over this same time, your breathing  also increases even further in a process called  ventilatory acclimatization. After this several weeks  of extended acclimatization, your body has made enough significant changes to climb even higher. However, you’ll still have to spend additional time acclimating along the way, often climbing back down to recover  before ascending even higher. Because the summit of Everest isn't just high, it’s the highest place on Earth. And at altitudes above 3,500 meters,  our bodies are under incredible stress. Arteries and veins in the brain dilate to speed up blood flow, But our smallest blood vessels, called capillaries, remain the same size. This increased pressure can cause blood vessels to leak, and fluid to build up in the brain. A similar issue can occur in the lungs, where low oxygen causes blood vessels  to constrict, leading to more leaking vessels and fluid buildup. These two conditions—  known as HACE and HAPE, respectively— are incredibly rare, but can be life-threatening  if not dealt with quickly. Some Tibetans and South Americans  with family histories of living at high altitude have genetic advantages that can  prevent minor altitude sickness, but even they aren’t immune to these severe conditions. Yet despite these risks,  climbers over the last century have proved people can go higher  than scientists ever thought possible. Pushing past their body’s limitations, these climbers have redefined what humanity can adapt to.