How Exactly Does Your Body Adjust To High Altitude?

When you find yourself at high altitude, your body has to adjust to the lack of oxygen, relative to that available at sea level. Otherwise you would not be able to survive or thrive in that type of environment. But how exactly does your body do just that?

Indeed it performs an entire set of physiological changes to try to maintain its cells’ normal oxygenation. Some of which are immediate effects, once it realizes you are in a low oxygen environment, and some of which take days to weeks to fully execute. But before we get into the details, let’s first define what ‘high altitude’ actually is.

What is high altitude?

High altitude generally refers to elevations at or over 2,500 meters or a little more than 8000 feet. However different references have their own definitions.

Medscape.com, a learning website for healthcare professionals, provides the following definitions:

altitude	corresponding label
anything over 4,800 feet (1,500 meters)	generally refers to ‘high altitude environment’
6,400 – 11,200 feet (2,000 – 3,500 meters)	moderate altitude
11,200 – 18,000 feet (3,500 – 5,600 meters)	very high altitude
18,000 feet or more (5,600 meters or more)	extreme altitude

Medscape altitude definitions from the article ‘Altitude Illness – Cerebral Syndromes’ (mobile scroll left and right)

Cleveland Clinic’s article on high altitude illnesses has similar but not exactly the same definitions:

altitude	corresponding label
8,000 – 12,000 feet (2,400 – 3,600 meters)	high altitude
12,000 – 18,000 feet (3,600 – 5,600 meters)	very high altitude
18,000 feet or more (5,600 meters or more)	extreme altitude

Cleveland Clinic’s altitude definitions from their article ‘Altitude Sickness‘ (mobile scroll left and right)

Note, though, that both references define extreme altitude at the exact same 18,000 foot cut off. This is the point where humans cannot adapt to the very low levels of oxygen in the air. In fact they start to perish at that level, without supplemental oxygen.

Here’s one more medical reference from the American Heart Association that again shows these definitions are a bit fuzzy:

Altitude definitions from AHA. VO₂ Max is the maximum amount of O₂ your body can use when exercising as hard as possible.

How does your body know there is less oxygen than normal at altitude?

Right off the bat, when you find yourself at altitude, the carotid bodies – clusters of cells in the carotid arteries in your neck – notice your oxygen levels are lower. They then initiate the hypoxic ventilatory response detailed below. The carotid bodies can also sense differences in carbon dioxide levels, pH levels, and temperature.

The chemical machinery inside your individual cells too can tell there less oxygen, and responds to such. But let’s get into the big adjustments your body makes at high altitude.

What are the immediate changes your body makes to adapt to high altitude?

Hypoxic Ventilatory Response (HVR)

The top immediate change your body makes to a lower oxygen environment is to increase the number of breaths you take per minute. You may have noticed this when flying on a commercial flight, where the cabin pressure is usually around that found in nature at 8,000 feet above sea level. It’s a pretty straight forward approach: breath more to get more oxygen.

This particular response is directly triggered by the oxygen sensing carotid bodies I mentioned in the last section.

Cardiovascular changes

Your body also tries to supercharge its oxygen delivery system: your cardiovascular system. Sympathetic activity, i.e. fight or flight type activity, occurs right when your body first notices the dip in oxygen in its systems. Your heart rate, and amount of blood pumped per heart beat increases. Your blood pressure too goes up a bit.

In order to provide oxygen for your brain, cerebral blood flow increases generously. This ultimately leads to the altitude sickness headache you may experience in lower O₂ environments. Your brain swells from this increase in blood flow, and you can develop acute mountain sickness then high altitude cerebral edema if your blood vessel capillaries become too leaky.

Also within minutes of arriving at altitude, your lung’s vascular system constricts, raising its local blood pressure. This initially helps improve the O₂ and CO₂ gas exchange at the alveoli. However, it can eventually lead to fluid from the blood stream being deposited in the alveolar sacs. You don’t want to be in that situation, as High Altitude Pulmonary Edema can be fatal (see below).

Hemoglobin concentration from reduced blood plasma volume

Once your body finds itself at altitude, it tries to reduce the fluid volume of your blood plasma in your arteries and veins. You would start to urinate more to get rid of this fluid. Or that extra fluid may find its way to other areas of your body, like in and around your brain.

But once there’s less fluid in your blood, the amount of hemoglobin per fluid volume of it consequently goes up. It’s kind of like if New York City banned all vehicles but taxis on the road. All of a sudden the number of taxis that could fit on a given block goes up, and are able to pick up and drop off more people. Same deal, except the hemoglobin taxis are just picking up and dropping off oxygen.

Sleep disturbance

When you are asleep, your breathing and heart rate slow down. While this is of little consequence in your normal environment, at an unfamiliar altitude this means you’re taking in and distributing even less oxygen to your body’s cells and organs. In fact, for a given novel altitude, sleeping overnight there puts you at higher risk of developing altitude sickness, due to this period of taking in less oxygen.

Not only do you experience a greater degree of sympathetic output at altitude, which again, raises your heart rate, and how much blood your hear pumps with each beat, making it harder to sleep. But when you are getting even less oxygen during slumber, your body naturally wants to rouse you, to bring levels at least up to what is available.

What changes to the body at altitude take days to weeks to happen?

Ventilatory acclimatization

During the hypoxic ventilatory response, as you take in more breaths per minute, you’re also getting rid of carbon dioxide at a greater rate. CO₂ in its dissolved form in the blood has an acidic effect there. Thus if its leaving the body at a faster rate, your blood’s pH begins to raise. This breathing induced alkalosis then has to be dealt with by the kidneys, who normalize your blood’s pH by getting rid of the basic compound bicarbonate, at a matched rate that your CO₂ is leaving. Your kidneys force the bicarbonate into the urine to dispense with it.

The lower carbon dioxide from breathing faster initially tells your brain to stop breathing so fast. But through this pH compensatory mechanism, you eventually can breath faster without the brain pumping the brakes to avoid the respiratory alkalosis explained above. This ventilatory acclimatization usually takes about 4 days to pull off.

Increased red blood cell production

Over days to weeks at altitude you produce additional red blood cells to increase the carrying capacity of oxygen in the blood. This then takes some of the pressure off the other physiological mechanisms compensating for less oxygen in your environment. Pretty straight forward.

Increased oxygen carrying capacity of hemoglobin

If you find yourself at extreme altitude, you’re more likely to have a higher blood pH than you normally would, due to the mechanisms previously explained. But when you have this more basic pH, it’s chemically easier for your hemoglobin to grab hold of O₂ molecules in your pulmonary capillary beds.

One of the way your body adjusts to high altitude. Hemoglobin attaches to oxygen easier, when the pH of your blood is higher. — At extreme altitude, the higher pH of your blood pushes this hemoglobin saturation curve to the left, meaning it gets saturated with oxygen easier, even at lower partial pressures found at such altitude. Credit: MedSchool.co, Oxyhaemoglobin

Internal cellular adaptation

From the point of view of your cells’ internal machinery, less oxygen means less energy. Thus these mechanics get to work compensating for the lower O₂ delivery to their doorsteps. Such involves altered protein synthesis, energy/carbon and fat metabolism, mitochondrial respiration, and further, alterations in the grabbing of nutrients from around the cell.

But not to worry, these effects last only as long as the low O₂ levels do. Once your back at home, at normal altitude, your cell mechanics go back to business as usual.

What are the negative consequences of your body trying to acclimatize to altitude too fast?

Acute Mountain Sickness (AMS)

As the blood flow to your brain increases, it swells, and its capillaries even become a little leaky, allowing fluid to push in and around the brain. The mild version of this is part of AMS. In order to be diagnosed with AMS you firstly have to have the headache caused by this swelling and fluid retention. But for it to be legit AMS you need at least one more symptom like lightheadedness, dizziness, fatigue from lack of oxygen, or nausea, vomiting, and lack of appetite as the swelling in your brain takes its toll.

The Cleveland Clinic further classifies AMS into mild, moderate, and severe categories:

Different forms of altitude sickness (acute mountain sickness)

Mild acute mountain sickness – light headache, light fatigue; it’s not advisable to continue to ascend higher, but you’re likely okay hanging out at your present altitude a few days, until your symptoms resolve.

Moderate acute mountain sickness – stronger headache, stronger fatigue, gastrointestinal effects previously mentioned, coordination problems; it’s advisable that you go back to a lower altitude to allow your symptoms to resolve.

Severe acute mountain sickness – continued symptoms from previous phases, but you’re out of breath even at rest, and you’re having trouble walking from both fatigue and lack of coordination. You should descend immediately and go for emergency care.

I go into more details about altitude sickness (acute mountain sickness) in my article on the subject built for healthy day hikers looking to summit local US peaks. It goes over quantified risks, time to symptom onset, and further, quantified risks for high altitude cerebral edema.

Altitude Illnesses that are medical emergencies since they can be fatal

High Altitude Cerebral Edema (HACE)

This is the point in AMS where so much fluid has leaked out of the blood vessel capillaries in the brain that it’s of grave concern. The pressure from all that extra fluid in your head is so strong it can push parts of your brain out of place, damaging them and their surrounding tissue. It usually happens over 8,200 to 9,800 feet and starts within 1 to 4 days.

A famous case of HACE occurred in 2006 on Mount Everest. The mountaineer in question was Lincoln Hall, who after reaching Everest’s summit was too fatigued to return to camp. He was abandoned by his team and Sherpas at 8,700 meters (28,500 feet), but found the next day in a confused state by another team, and rescued.

High Altitude Pulmonary Edema (HAPE)

This is quite similar to HACE except the fluid leakage happens in the lungs, and such deteriorates your ability to breath in O₂ and breath out CO₂. This one is way less frequent than AMS, but usually occurs due to a quick ascent to over 8,200 feet, and presents within 2 to 4 days at that altitude. Untreated it’s fatal in 50% of cases.

What are the 3 stages of acclimatization to high altitude?

The 3 stages of acclimatization to high altitude are staying between 8,800 and 11,800 feet for 6 days, then staying between 11,800 and 14,800 feet for 4 days, and finally staying at over 14,800 feet for 4 days. This is the protocol established by India’s army, in order to acclimate troops to high altitude stations. If personnel need to leave their high altitude station for between 10 and 30 days, they must repeat each stage, but only spend 4 days a piece on them, to return once more. However if personnel are gone longer than 30 days, they must repeat the stages, in full, using the longer initial stage.