How the atmosphere changes with altitude
Air composition (by fraction) stays roughly the same with height: oxygen is ~20.9% of dry air at sea level and at 6,000m. What changes is the barometric pressure, so the number of oxygen molecules per breath reduces as you climb. The useful quantity partial pressure of oxygen (PO₂) drops roughly in proportion to barometric pressure, so breathing at 4,000m or 6,000m delivers far fewer oxygen molecules per litre of air than at sea level.
Because of that, from sea‐level up to 6,000 m, the percent by volume of O₂, N₂, CO₂ etc remains effectively the same in “dry air” (aside from water vapor changes). What does change is the total number of molecules per litre of air (because pressure and density decline with altitude). That means the absolute quantity of O₂ molecules decreases even though the fraction remains ~21%. This is why available oxygen to the body declines at altitude despite composition staying “21% O₂.”
How the body responds
- Minutes, hours: The first response is hyperventilation (you breathe faster and deeper) to raise alveolar oxygen; this lowers CO₂ and produces a respiratory alkalosis. Heart rate increases to maintain oxygen delivery. These fast changes partially compensate for lower PO₂ but do not fully restore sea-level oxygenation.
- Hours, days: Kidneys begin to excrete bicarbonate to offset respiratory alkalosis (renal compensation), allowing ventilation to stay high without extreme alkalosis. Oxygen saturation (SaO₂) falls with altitude in unacclimatized people; arterial PO₂ typically falls by about 1.5–1.6 kPa for each 1,000m gain.
- Days, weeks: The body increases erythropoietin production from the kidneys creating more red blood cells (polycythemia) which raises the blood’s oxygen-carrying capacity. Microvascular and muscular changes (capillary growth, mitochondrial adjustments) also improve tissue oxygen extraction over time.
- Local circulatory changes: Low oxygen triggers hypoxic pulmonary vasoconstriction (higher pulmonary artery pressure) and can worsen in susceptible people into high-altitude pulmonary edema; some people get acute mountain sickness (AMS) or, rarely, high-altitude cerebral/pulmonary edema without gradual acclimatization. Prevention is slow ascent and, when appropriate, medications and monitoring.
Because the challenge of altitude is fundamentally about the reduced volume of air—and therefore fewer oxygen molecules—entering your lungs with each breath, effective breathing becomes an important tools for hikers and mountaineers. Techniques such as steady nasal breathing, controlled deep inhalations, rhythmic pacing, and conscious ventilation help maximize the oxygen you can extract from thinner air.
Adventure awaits, together!
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