Surviving the Summit

If someone were to teleport from sea level to the top of Mt. Everest, the situation would quickly turn dire. At an altitude of 8,848 meters, the barometric pressure is approximately 33% of what it is at sea level. This drastic change means there is significantly less oxygen in the air, and the individual would likely suffocate within minutes. However, for those who gradually ascend over the course of a month, it becomes possible to survive at the peak for hours. So, what happens to our bodies within this month that enables us to endure the incredible altitude?

Let's imagine you are 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 with air composed of various gases and compounds. Among them, oxygen molecules play a vital role as they bind to the hemoglobin in your red blood cells. Subsequently, your blood circulates throughout your body, delivering essential oxygen to all cells. However, as altitude increases, the air becomes thinner, resulting in reduced oxygen absorption for our bodies. Ascending above 2,500 meters can lead to oxygen deprivation and a form of altitude sickness known as Acute Mountain Sickness (AMS), characterized by symptoms like headaches, fatigue, and nausea. Fortunately, AMS primarily occurs when ascending too rapidly, as our bodies possess several mechanisms to adapt to high altitudes.

Within minutes or even seconds of reaching altitudes around 1,500 meters, carotid chemoreceptors in your neck sense the low oxygen pressure in your blood. This triggers a response that increases the rate and depth of your breathing, compensating for the lack of oxygen. Additionally, your heart rate rises, and your heart contracts more forcefully, pumping extra oxygenated blood with each beat, effectively circulating oxygen throughout your body. These changes happen relatively quickly, and if you continue ascending, your heart rate and breathing will adjust accordingly. However, if you spend several weeks at this altitude, you can benefit from long-term adaptations.

Within the first few days above 1,500 meters, the volume of plasma in your blood decreases, leading to an increased concentration of hemoglobin. Over the next two weeks, your hemoglobin levels continue to rise, allowing your blood to carry even more oxygen per milliliter. Combined with the elevated heart rate, this hemoglobin-rich blood efficiently distributes oxygen throughout your body, potentially returning the volume of blood being pumped with each heartbeat to normal levels. Simultaneously, your breathing further increases through a process known as ventilatory acclimatization. After several weeks of extended acclimatization, your body undergoes significant changes, enabling you to climb even higher. Nonetheless, additional time for acclimatization is necessary along the way, often requiring descents to lower altitudes for recovery before ascending further. This process is crucial because the summit of Everest is not just high—it's the highest place on Earth.

At altitudes above 3,500 meters, our bodies face immense stress. Arteries and veins in the brain dilate to accelerate blood flow, while capillaries, the smallest blood vessels, remain the same size. This increased pressure can cause blood vessels to leak, resulting in fluid buildup in the brain. Similar issues can arise in the lungs, where low oxygen levels cause blood vessels to constrict, leading to more leakage and fluid accumulation. These conditions, known as High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE) respectively, are extremely rare but can be life-threatening if not promptly addressed. Although some Tibetans and South Americans with a genetic predisposition to high altitude living have certain advantages that may prevent minor altitude sickness, even they are not immune to these severe conditions.

Despite the risks involved, climbers over the past century have demonstrated that humans can surpass the altitudes scientists once thought were impossible. Pushing beyond the limitations of their bodies, these climbers have redefined the extent to which humanity can adapt.

Henrik Leandro