What is altitude sickness and why does it affect climbers?

The answer

Altitude sickness (acute mountain sickness) occurs when the body cannot adapt quickly enough to reduced oxygen levels at high elevations. At sea level, air pressure is about 1013 millibars with an oxygen partial pressure of 213 millibars; at the summit of Everest (8,849 meters), air pressure drops to about 337 millibars with an oxygen partial pressure of only 70 millibars, roughly one-third of sea-level values. The body responds to hypoxia by increasing breathing rate, heart rate, and red blood cell production (through erythropoietin). When ascent outpaces acclimatization, fluid can leak into the brain (high-altitude cerebral edema) or lungs (high-altitude pulmonary edema), both potentially fatal conditions. Genetic adaptations in high-altitude populations like Tibetans, Andean Quechua, and Ethiopian highlanders involve different hemoglobin variants and hypoxia pathway genes that developed independently over thousands of years.