Altitude Hypoxia

What actually happens as you go up

The single fact that drives everything about altitude hypoxia is this: the percentage of oxygen in air never changes. From the beach to the summit of Everest, dry air is 20.9% oxygen, 78% nitrogen, and trace gases. What changes is the total barometric pressure — the weight of the atmosphere above you. As you climb, there is less air overhead, so the pressure falls roughly exponentially, halving about every 5,500 m.

Gases diffuse down gradients of partial pressure, not concentration. The partial pressure of a gas is simply its fraction multiplied by the total pressure. At sea level the partial pressure of oxygen in dry air is about 0.209 × 760 ≈ 159 mmHg. By the time air reaches the alveoli it has been warmed, fully humidified (water vapor contributes a fixed 47 mmHg at body temperature), and diluted by carbon dioxide, so the alveolar oxygen tension (P_AO₂) sits near 100 mmHg at sea level. At 5,500 m, with barometric pressure halved, that same calculation collapses the alveolar oxygen tension toward 50 mmHg.

The alveolar gas equation makes this precise: P_AO₂ = F_IO₂ × (P_barometric − 47) − P_ACO₂ / R. As barometric pressure falls, the first term shrinks; the only physiological lever the body has on the right side is to lower alveolar carbon dioxide by breathing harder. That is why hyperventilation is the first and most important defense against altitude hypoxia.

The oxygen cascade and where it breaks

Oxygen flows from atmosphere to mitochondria down a staircase of falling partial pressures known as the oxygen cascade: inspired air (≈150 mmHg humidified) → alveoli (≈100 mmHg) → arterial blood (≈95 mmHg) → tissue capillaries (≈40 mmHg) → mitochondria (1-5 mmHg). Each step is a diffusion gradient. At altitude the top of the staircase drops, so every downstream step starts lower, and the gradient driving oxygen into the mitochondria narrows dangerously.

The relationship between arterial oxygen tension and hemoglobin saturation is not linear — it is the sigmoid oxygen-hemoglobin dissociation curve. Above an arterial oxygen tension of about 60 mmHg the curve is flat, so saturation stays near 90% even as tension falls; this plateau protects you on a modest mountain. But below 60 mmHg the curve plunges down its steep limb, and small further drops in tension cause large losses of saturation. This is exactly why altitude becomes abruptly dangerous past a certain point rather than gradually: you fall off the shoulder of the curve.

Acclimatization: three clocks running at once

The body defends oxygen delivery on three different timescales, and understanding altitude hypoxia means tracking all three at once.

• Seconds to minutes — the ventilatory response. Peripheral chemoreceptors in the carotid bodies detect the low arterial oxygen tension and drive the respiratory center to increase ventilation. This raises alveolar oxygen but blows off carbon dioxide, producing a respiratory alkalosis that paradoxically restrains further breathing.
• Hours to days — renal compensation. The kidneys excrete bicarbonate to bring blood pH back toward normal, releasing the brake on ventilation so breathing can climb higher. This is the basis of staged ascent and of acetazolamide, a carbonic anhydrase inhibitor that forces bicarbonate excretion and speeds this step.
• Days to weeks — hematological adaptation. Sustained tissue hypoxia stabilizes hypoxia-inducible factor (HIF), a transcription factor normally degraded in the presence of oxygen. Stabilized HIF turns on the gene for erythropoietin (EPO) in the kidney. EPO rises within hours, peaks at 24-48 h, and stimulates the marrow to expand red-cell production, raising hemoglobin and oxygen-carrying capacity over the following weeks.

There is also a fast plasma-volume contraction in the first days that raises hemoglobin concentration before any new red cells are made — a quick way to boost oxygen content while the slower erythropoietic machinery spins up. The 2019 Nobel Prize in Physiology recognized the discovery of the HIF oxygen-sensing pathway, the molecular heart of this entire response.

Altitude hypoxia versus other low-oxygen states

Clinicians classify hypoxia by where in the oxygen cascade it fails. Altitude hypoxia is the textbook example of hypoxic hypoxia — a low inspired oxygen tension — and it behaves very differently from anemia or carbon-monoxide poisoning, even though all leave tissues short of oxygen.

Feature	Altitude (hypoxic) hypoxia	Anemic hypoxia	Carbon monoxide poisoning
Primary defect	Low inspired and alveolar PO₂	Low hemoglobin quantity	Hemoglobin bound by CO, can't carry O₂
Arterial PO₂	Low	Normal	Normal
Hemoglobin saturation (true)	Low	Normal % (but few cells)	Low (occupied by CO)
Pulse oximeter reading	Low (accurate)	Normal (misleading)	Falsely normal/high
Oxygen content of blood	Reduced	Markedly reduced	Markedly reduced
Response to supplemental O₂	Excellent	Limited	Helpful (speeds CO washout)
Diss. curve shift	Right (2,3-BPG) over days	Right (chronic)	Left shift, impairs unloading

The pulse-oximeter column is the clinically dangerous one. At altitude the oximeter honestly reports the falling saturation, which is why it is a useful field tool for trekkers and rescue teams. In carbon-monoxide poisoning the device cannot distinguish carboxyhemoglobin from oxyhemoglobin and reads falsely reassuring — a co-oximeter is required. Recognizing these distinctions changes management entirely: descent and oxygen for altitude, transfusion for anemia, high-flow or hyperbaric oxygen for CO.

Clinical syndromes of altitude

When ascent outpaces acclimatization, three overlapping illnesses appear. Acute mountain sickness (AMS) is the mild, common form: headache plus nausea, fatigue, dizziness, or disturbed sleep, affecting up to a quarter of visitors at 2,500 m and more with faster ascent. It usually resolves with rest, hydration, and avoiding further climb, and acetazolamide both prevents and treats it.

High-altitude pulmonary edema (HAPE) is more sinister. Low alveolar oxygen triggers hypoxic pulmonary vasoconstriction — the lung's attempt to divert blood away from poorly oxygenated regions. At altitude this constriction becomes diffuse and uneven, raising pulmonary artery pressure and forcing fluid out of overperfused capillaries into the air spaces. The result is breathlessness at rest, a dry then productive cough, pink frothy sputum, and crackles, and it can kill within hours. Unlike cardiogenic pulmonary edema, HAPE arises from a non-cardiogenic, pressure-and-permeability mechanism in an otherwise healthy heart.

High-altitude cerebral edema (HACE) is brain swelling, thought to share the vasogenic-leak mechanism, presenting with ataxia (the classic early sign — the climber cannot walk a straight line), confusion, severe headache, and declining consciousness. HAPE and HACE are emergencies. The definitive treatment for both is immediate descent; supplemental oxygen, dexamethasone (for HACE), nifedipine (for HAPE), and portable hyperbaric bags buy time but do not substitute for losing altitude.

At the chronic end, lifelong high-altitude residents can develop chronic mountain sickness (Monge disease), in which the adaptive rise in red-cell mass overshoots into excessive polycythemia. Hematocrit above 55-60% sharply increases blood viscosity, straining the right heart and raising clot risk — the adaptation itself becomes the disease, the mirror image of acute hypoxia.

Why the numbers matter

• Aviation. Cabins are pressurized to an equivalent altitude no higher than about 2,400 m precisely so passenger saturation stays on the safe plateau of the dissociation curve. A sudden decompression at cruising altitude gives only seconds of useful consciousness, which is why oxygen masks deploy immediately.
• Sports physiology. Endurance athletes use "live high, train low" to trigger EPO and red-cell expansion without the training cost of exercising in thin air. The same EPO axis is abused in blood doping.
• Critical care. The dissociation-curve shoulder explains why a patient can look stable at 92% saturation and then crash quickly once they slip below 88-90% onto the steep limb.
• Genetic adaptation. Tibetan, Andean, and Ethiopian highlanders show distinct evolved strategies; Tibetans carry EPAS1 variants that blunt the runaway rise in hemoglobin, avoiding Monge disease despite generations at altitude.

This article is educational and is not medical advice. Altitude illness can be life-threatening; if you or a companion develops worsening breathlessness, confusion, or loss of coordination at altitude, descend and seek medical care.