Breathing at High Altitude

What Happens to Breathing at High Altitudes?

At high altitudes you breathe faster and more deeply, because each breath brings less oxygen pressure into the lungs and the body tries to make up for it with more air. This happens within minutes of arriving, before any other change in the body.

Physiologists usually divide altitude into bands. High altitude is about 1,500 to 3,500 meters (roughly 5,000 to 11,500 feet), very high altitude is 3,500 to 5,500 meters, and extreme altitude is anything above that. The effects on breathing grow steadily through these bands.

The response follows a clear chain:

1. Lower air pressure means a lower partial pressure of oxygen in the air you inhale.
2. Less oxygen moves across the alveolar wall during gas exchange, so blood oxygen falls.
3. Sensors in the neck detect the drop and signal the brainstem.
4. Breathing becomes deeper and faster, which raises oxygen but also blows off carbon dioxide.
5. Over days and weeks, the kidneys, blood and blood vessels adjust so the body copes better.

This chain links almost every topic in breathing physiology, from partial pressure of gases to oxygen transport in the blood.

Why Is It Harder to Breathe at High Altitudes?

It is harder to breathe at high altitudes because there is less air pressure pushing oxygen into the lungs and blood, not because the air contains a smaller percentage of oxygen. Oxygen makes up about 21 percent of dry air everywhere in the lower atmosphere, at sea level and on the highest peaks.

Air pressure comes from the weight of the air above you. The higher you go, the less air is stacked overhead, so the total pressure falls. Because oxygen's share stays the same, its partial pressure, the part of the total pressure due to oxygen, falls in step. Gas diffusion depends on partial pressure, so a lower oxygen pressure in the alveoli means a weaker push of oxygen into the blood.

There is one more loss. Air is warmed and fully moistened as it enters the airways, and water vapor takes up a fixed pressure of about 47 mmHg at body temperature. At sea level that hardly matters, but at extreme altitude it takes a much bigger share of the shrinking total.

Explained simply: the air on a mountain is "thinner." Every breath you take holds fewer molecules of everything, including oxygen, so your lungs have to work harder to collect the same amount.

Partial Pressure of Oxygen vs Altitude: How Fast Does It Fall?

The partial pressure of oxygen falls by about half between sea level and roughly 5,500 meters (18,000 feet), and to under a third of its sea-level value at the summit of Mount Everest. The drop is gradual at first and is not a straight line, because air pressure falls in a curve with height.

What matters for the body is the oxygen pressure in the alveoli, which is lower again than in the inhaled air because the alveoli also hold carbon dioxide. The link between that pressure and the oxygen carried by hemoglobin is shown by the oxygen-hemoglobin dissociation curve. Its flat upper part means that at moderate altitude, blood oxygen saturation falls only a little. Once alveolar oxygen pressure drops toward about 60 mmHg, the curve becomes steep and saturation falls quickly.

This is why many people notice little at rest at moderate altitude, while the same small climb higher up can make a large difference.

How Does the Body Sense Low Oxygen at High Altitude?

The body senses low oxygen mainly through the carotid bodies, small clusters of sensor cells at the fork of each carotid artery in the neck. They are the most important of the peripheral respiratory chemoreceptors, and they fire faster when the oxygen pressure of arterial blood falls.

Their signals travel to the breathing centers in the brainstem, which increase the depth and rate of breathing. This is called the hypoxic ventilatory response, and it starts within seconds to minutes of exposure to low oxygen. The full picture of how these centers work is on our control of breathing page.

The central chemoreceptors in the medulla, which respond mainly to carbon dioxide and acidity rather than oxygen, act as a brake at first. As faster breathing lowers carbon dioxide, they reduce their drive, so the first rise in breathing is smaller than the oxygen sensors alone would produce. This brake is gradually released over the following days.

Why Does High Altitude Cause Respiratory Alkalosis?

High altitude causes respiratory alkalosis because the extra breathing removes carbon dioxide from the blood faster than the body produces it, and less carbon dioxide means less carbonic acid, so blood pH rises. This is a form of hyperventilation driven by low oxygen, not by anxiety.

The body corrects it in steps:

• Kidneys: over several days they excrete more bicarbonate in the urine, which brings blood pH back toward normal.
• Brain fluid: bicarbonate in the fluid around the brain also falls, so the central chemoreceptors stop braking breathing.
• Breathing: with the brake released, ventilation rises further. This slow climb over days is called ventilatory acclimatization.

The low carbon dioxide also affects how hemoglobin holds oxygen. Alkalosis shifts the dissociation curve to the left, helping hemoglobin load oxygen in the lungs, while a rise in a red-cell chemical called 2,3-BPG pushes it the other way and helps unloading in the tissues. These shifts relate to the Bohr effect and to how carbon dioxide transport links breathing to blood acidity.

How Does the Body Acclimatize to Breathing at Higher Altitudes?

The body acclimatizes to higher altitudes through a sequence of changes in breathing, blood and blood vessels that unfold over minutes, days and weeks. Together they raise the amount of oxygen the blood can pick up and deliver, although they never fully restore sea-level performance.

Minutes to hours

Breathing and heart rate rise, and the heart pumps more blood each minute. The kidneys begin releasing more erythropoietin (EPO), the hormone that tells the bone marrow to make red blood cells.

Days

The kidneys correct the alkalosis, breathing climbs further, and the amount of 2,3-BPG in red blood cells increases. Plasma volume falls, which raises the concentration of hemoglobin in each liter of blood even before new cells appear.

Weeks

New red blood cells enter the circulation, so the total amount of hemoglobin increases and the blood can carry more oxygen. The step-by-step timeline is shown in the table below.

All of these changes are part of how the body keeps its internal balance, a theme explored on our respiratory system homeostasis page.

What Are the High Altitude Effects on the Lungs and Pulmonary Circulation?

At high altitude the small arteries of the lungs constrict, the air reaching the lungs is colder and drier, and diffusion of oxygen becomes a limiting step during exercise. Each of these effects comes from the low oxygen pressure or the thin air.

In normal lungs, small arteries narrow in any region that has low oxygen, which sends blood toward better-ventilated areas and keeps the ventilation-perfusion ratio balanced. At altitude the whole lung has low oxygen, so this hypoxic pulmonary vasoconstriction happens everywhere at once, and the pressure in the pulmonary circulation rises.

Gas exchange also changes. At sea level, blood picks up its full load of oxygen in the first part of its short trip through a lung capillary. At altitude the pressure gradient across the respiratory membrane is smaller, so loading is slower, and during hard exercise the blood may leave the capillary before it is fully loaded.

The thin air has one advantage: it is less dense, so it flows through the airways with less resistance. On the other hand, cold, dry mountain air draws more water from the airway lining with every breath.

What Does Breathing at High Altitude Feel Like?

Breathing at high altitude feels like being out of breath more easily: climbing stairs or walking uphill makes you pant sooner, and even at rest you may notice deeper, faster breaths. Many people also feel the dry air in the throat and nose.

Exercise is affected most. The body's maximum oxygen uptake falls as altitude rises, so the same effort takes a bigger share of a person's capacity, as explained on our page on how exercise affects the respiratory system.

Sleep can feel different too. Many visitors to high altitude develop periodic breathing at night, a cycle in which breaths grow deeper, then shallower, then pause briefly before starting again. It happens because the low carbon dioxide and the sensitive oxygen sensors make the breathing control loop overshoot. This pattern is one of the breathing patterns doctors recognize, and it usually lessens as the body acclimatizes.

How Do People Who Live at High Altitude Breathe?

People whose families have lived at high altitude for many generations, such as Andean, Tibetan and Ethiopian highlanders, breathe and carry oxygen in ways that differ from lowland visitors. Their adaptations are inherited, not only acquired during a stay.

• Andean highlanders tend to have higher hemoglobin levels and large chests with big lung volumes.
• Tibetan highlanders tend to breathe more air at rest and keep hemoglobin closer to sea-level values, and many carry gene variants, such as one in the EPAS1 gene, that limit the rise in red blood cells.
• Ethiopian highlanders show yet another pattern, which suggests that human populations solved the same problem in different ways.

Animals offer more examples. Llamas have hemoglobin that binds oxygen especially well, and some geese fly over the highest mountains, helped by hemoglobin that grips oxygen tightly and the efficient one-way airflow of the bird respiratory system, part of the wide range of respiration in animals.

Common Conditions Linked to High Altitude

The main altitude-related conditions are acute mountain sickness, high-altitude pulmonary edema and high-altitude cerebral edema, all caused by low oxygen when the body has not yet adapted. They are described here only to show how they relate to breathing physiology.

• Acute mountain sickness is the most common form, a general reaction to hypoxia in the first days at altitude.
• High-altitude pulmonary edema (HAPE) is fluid leaking into the air spaces of the lungs, linked to uneven, very high pressure in the pulmonary arteries.
• High-altitude cerebral edema (HACE) is swelling of the brain, the most serious of the three.

These conditions are covered with other lung problems on our respiratory diseases page. Breathing problems or feeling unwell at altitude are a reason to see a doctor.