What Happens to Your Body at Altitude?

As you ascend, atmospheric pressure drops and the air contains fewer oxygen molecules per breath. Your body responds immediately: breathing rate increases, heart rate rises, and blood thickens as your kidneys excrete bicarbonate to compensate for altered blood chemistry. Over days and weeks, your body produces more red blood cells to carry oxygen more efficiently — this is acclimatization.

The problem is that these adaptations take time. Ascend too fast, and your body can't keep up. The result is Acute Mountain Sickness (AMS) — and in severe cases, life-threatening conditions like High Altitude Pulmonary Edema (HAPE) or High Altitude Cerebral Edema (HACE).

Altitude Zones

  • Low altitude: Below 1,500 m — no significant physiological effects for most people.
  • Moderate altitude: 1,500–2,500 m — some people notice mild symptoms.
  • High altitude: 2,500–3,500 m — AMS can occur; acclimatization needed.
  • Very high altitude: 3,500–5,500 m — significant risk; careful staging required.
  • Extreme altitude: Above 5,500 m — no true acclimatization possible; deterioration is continuous.

The Golden Rule: Climb High, Sleep Low

The most important principle in high-altitude mountaineering is climb high, sleep low. During the day you can ascend to a higher camp or altitude to encourage adaptation, but you descend to a lower sleeping elevation. This lets your body experience the stimulus of altitude while recovering during rest at a safer height.

This is why Himalayan expeditions establish multiple camps and rotate between them for weeks before a summit push.

Recommended Ascent Rates

Above 3,000 m, the widely accepted guideline is:

  • Increase sleeping altitude by no more than 300–500 m per day.
  • For every 1,000 m gained in sleeping altitude, take a rest day.
  • If symptoms of AMS develop, do not ascend further until they fully resolve.

Individual responses to altitude vary considerably. Age, fitness level, and prior altitude experience are all factors, but none reliably predict susceptibility to AMS. Even highly trained athletes can suffer while less-fit companions do fine.

Recognizing Acute Mountain Sickness

AMS typically begins within 6–12 hours of arrival at a new altitude. Watch for:

  • Headache (the hallmark symptom)
  • Fatigue and weakness
  • Nausea or loss of appetite
  • Dizziness or lightheadedness
  • Difficulty sleeping

Red flag symptoms requiring immediate descent: confusion, loss of coordination (ataxia), persistent dry cough, breathlessness at rest, or coughing pink/frothy sputum. These may indicate HACE or HAPE — both can be fatal without rapid descent.

Strategies to Aid Acclimatization

Hydration

Dehydration worsens altitude symptoms. Drink enough to produce pale, clear urine — typically more than you'd drink at sea level because dry mountain air increases fluid loss through breathing.

Acetazolamide (Diamox)

Acetazolamide is a prescription medication that speeds acclimatization by stimulating increased breathing. It is widely used as both a preventive and treatment for AMS. Consult a doctor before your trip — it's not suitable for everyone (especially those with sulfa allergies).

Avoid Alcohol and Sedatives

Both suppress breathing and worsen the oxygen deficit at altitude. Save the celebratory drinks for base camp or lower elevations.

Eat Carbohydrate-Rich Foods

Carbohydrates require less oxygen to metabolize than fats or proteins, giving a marginal but real advantage at extreme altitudes. Many high-altitude mountaineers increase their carbohydrate intake above normal levels during summit pushes.

When to Descend

The rule is simple: if in doubt, go down. Descent of even 300–500 m can produce rapid, dramatic improvement in symptoms. No summit is worth risking your life. The mountain will still be there next season.