I had a campfire discussion not long ago with a man that got pulmonary edema while attempting the summit of Aconcagua, and we got into the discussion of the effects of high elevation. I had always assumed that after you adapted to the thin air, the risk to your body was how thick your blood got with the production of extra red blood cells to compensate for the lack of oxygen. The thicker blood making frostbite and edema more likely because of poor blood flow, and heart attacks and strokes more likely because of increased chances of forming blood clots. But my friend said it had less to do with thick blood and more to do with the lack of air pressure...

What does low pressure do to you? What exactly happens to your body at high altitude?

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    There is a variety of conditions that can result from being at high altitude, through a bunch of different physiological mechanisms. en.wikipedia.org/wiki/Altitude_sickness Two really bas ones are high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE). Re HAPE, which you asked about, see en.wikipedia.org/wiki/High-altitude_pulmonary_edema
    – user2169
    Commented Nov 10, 2014 at 1:26
  • I'm aware of what altitude sickness is and what the risks of being at high altitude are. What I'm trying to understand is why they happen at high altitude. The article you linked states, "The mechanisms by which this shortage of oxygen causes HAPE are poorly understood..." adding to the mystery of what low pressure is actually doing to your body that causes all of these conditions.
    – ShemSeger
    Commented Nov 10, 2014 at 5:42
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    Related - Is “altitude” an absolute measure of air “thinness”?
    – ShemSeger
    Commented Nov 16, 2014 at 5:33
  • Maybe related?: Brain.Mic
    – ShemSeger
    Commented Nov 18, 2014 at 22:04
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    @WedaPashi - It's a known fact that larger lung volume is advantageous at high altitude, like the barrel-chested Peruvians of the high Andes, but even they will suffer effects at altitude. The mountain town I was brought up in is the same elevation as Kathmandu. As a result, I preform much better at higher elevations than most people. But I think that despite evolution/adaptation, the effects of altitude are the same, some people's bodies are just better at dealing with the effects, and can last a bit longer at dangerously high altitude.
    – ShemSeger
    Commented Nov 25, 2014 at 17:11

3 Answers 3


I had always assumed that after you adapted to the thin air, the risk to your body was how thick your blood got with the production of extra red blood cells to compensate for the lack of oxygen.

an increase of red blood cells will only happen after a long period of time. This is why athletes often train at high altitude. this is a very gradual process though and is not to be confused with what happens to you on a high altitude trek. On a trek you body will not produce any significantly greater number of red blood cells (dependent on time spent at altitude obviously).

Also, more red blood cells does not equal increased blood clotting. Blood clotting is performed by platelets not blood cells. Totally different mechanism.

The thicker blood making frostbite and edema more likely because of poor blood flow, and heart attacks and strokes more likely because of increased chances of forming blood clots

See above. More Red blood cells does not make your blood thicker. It increases its ability to carry oxygen. For "thicker", don't think "syrup"; think "marbles moving though a smooth tube". More marbles doesn't make it more likely they will get stuck.

I think you're getting confused with Deep vein thrombosis which is caused by inactivity in a pressurised cabin (i.e. plane). High altitude does not increase your likelihood of getting blood clots.

But my friend said it had less to do with thick blood and more to do with the lack of air pressure...

He's right. The higher you go the less air pressure there is (oxygen remains static as a percentage of air regardless of height). Less air pressure means less air, which means less oxygen. So every breath contains less oxygen the higher you go. So you blood has less oxygen in it because your body cannot receive enough oxygen from its surroundings.


Frostbite is caused by tissue not receiving enough oxygen, i.e. it's so cold your body diverts blood to your core to keep you alive, potentially sacrificing a finger in the process. If your blood has less oxygen because of the above, you're more prone to frostbite because it takes a lower amount of blood starvation to reduce the amount of qxygen in the tissue to the point where frostbite occurs


HAPE and HACE are slightly different to frostbite. They are still caused by the lower amount of air pressure, but no one's 100% clear. You can have two people climb the same mountain at the same rate and be of the same fitness, one may get HAPE and the other won't, obviously something about the person is involved. It is known that lower air pressure causes HAPE and HACE. It's thought that it's to do with the pressure imbalance in the air. Put overly simplistically: Your lungs are designed to work at x pressure, they have difficulty at x -10 pressure.

Having increased blood cells in your blood may or may not increase you're chance of getting HAPE or HACE. It's been known for Sherpas to get these issues and they have a much greater ability to handle altitude than your average western climber.

  • I think you misread what I said when you make your first point. I said, "I assumed that after you adapt" not, "I assumed that you adapt."
    – ShemSeger
    Commented Nov 16, 2014 at 5:41
  • I slightly para-phrased to make the point. I'm trying to say, that adaptation is not an important mechanism, not in the way that your expect it to be anyway.
    – user2766
    Commented Nov 17, 2014 at 10:04

I'll try and be brief but specific towards answering your question: What exactly happens to your body at high altitude?
Disclaimer: A lot of data is from Wiki Pages and definitions from Human Anatomy and Physiology Books.

  1. Breathlessness and Hyperventilation:
    Does it start with one panting for breathe?
    Yes it does! We all know that Atmospheric pressure decreases with the increase in altitude, the air gets thinner, so the oxygen availability is lesser than at Sea-level. This change of oxygen level is sensed by a chemoreceptor called Carotid body. The feedback from the carotid body is received by cardio-respiratory centers in the Medulla Oblongata. This is what people call Hypoxia. These centers function to regulate breathing and blood pressure, this is how you then pant for breathe with elevated respiration/ventilation rate, which is medically called as Hyperventilation.
  2. Pinching feeling and Fatigue:
    It has been observed in some cases that the guy complains about pinching feeling in the far ends of body, and then unusual fatigue and body pain, Why?
    The hyperventilation (caused as a reaction mechanism to Hypoxia) sometimes cause Respiratory Alkalosis which is a sensation of tingling, tickling, pricking, or burning of a person's skin. Respiratory Alkalosis needs adaptation which is taken care by balancing the CO2 production out of the Respiration process. But, that said, it may lead to Hypocalcaemia which is lack of Calcium to put it in a simple way, causing joint-pains? This lowers down the rate of work that one can sustain. With the deliberate lower work rates, the body produces lesser lactate (Interesting article worth a go-through), as the energy consumed/converted is lower, so the lower is Mitochondria throughput. Speaking of the same at the Cell level, Mitochondria is responsible to convert the energy released by Glucose breaking into ATP. This vaguely can make one understand the reason behind lack of energy and Fatigue.
  3. Erythropoietin and Polycythemia: Speaking of Acclimatization medically, means that body gradually adapts in such a manner that the Haemoglobin in the blood increases. Haemoglobin is required to carry the Oxygen (dissolved into the blood) to the rest of the body parts. Higher the Haemoglobin, higher the supply of oxygenated blood to the body. What carries the Haemoglobin in the Blood? RBCs! With higher haemoglobin in blood, you have high RBC count. With higher Haemoglobin, the blood becomes viscous, which becomes harder for heart to pump. Thats what is called Chronic Mountain Sickness. Acute Mountain sickness and Chronic Mountain sickness have two distinct mechanisms.
    In Acute Mountain Sickness, Hypoxia has already made an impact on amount of work that a body can sustain. The amount of work done/exercise has got relation with production of Lactic Acid in our body. Under Hyperventilation, the lower lactate production rates trigger the mechanism that tends to demand an elevated lactate production, which leads to Polycythemia, which is increased RBC production through Erythropoietin. Under hypoxic conditions, the kidney will produce and secrete Erythropoietin to increase the production of RBCs.
  4. Reason behind Yawing and Higher CO2 production:
    For the acclimatization and to balance the Chemical Equilibrium, body is already up producing more carbon dioxide. This situation is called Hypocapnia. The wiki says,

    Hypocapnia normally triggers a reflex which increases breathing and access to oxygen, such as arousal and turning the head during sleep.

    May be reason for Yawning after being a bit exhausted? (At least I do).

  5. Constriction of Pulmonary arteries and Exhausted Lungs:
    Medically it is called Hypoxic Pulmonary Vasoconstriction. This constriction leads to redistribution of blood flow to better-ventilated areas of the lung. That takes its toll on Lungs and cause Pulmonary Hypertension which is nothing but an increase of blood pressure in the pulmonary artery, pulmonary vein, or pulmonary capillaries. (Pulmonary is whatever thats related to Lungs). Pulmonary Hypertension AND Hypoxic Pulmonary Vasoconstriction altogether makes symptoms of HAPE (High Altitude Pulmonary Edema)! Edema is a condition characterized by an excess of watery fluid collecting in the cavities or tissues of the body.

  6. HACE (High Altitude Cerebral Edema):
    As we have seen that Hyperventilation may lead to Hypocapnia which leads an immediate cerebral vasoconstriction. In a layman's word, its can be called something like a brain hemorrhage. The increased the pressure leads to leaking of endothelium. Endothelium forms the Blood-Brain Barrier, which separates the circulating blood from the brain extracellular fluid (BECF) in the central nervous system. When the blood–brain barrier is disrupted, one is said to be hit by Cerebral Edema.

P.S.: I am an Engineer and not a doctor :D

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    One minor point: While the carotid body does sense the oxygon level, breathing is actually primarly controlled by the carbondioxyde level (also measured by the carotid body).
    – imsodin
    Commented Jun 4, 2015 at 18:30

As Liam has described, most problems are caused by the lack of oxygen which messes up all kinds of vital biological processes, and what exactly happens and how is still subject of (obviously difficult and dangerous) medical research.

The most extreme form is the death zone above 7500 or 8000m (sources differ), where acclimatization becomes impossible: there is simply too little oxygen there for the human body to work, and it starts shutting down not-immediately-vital functions like digestion to operate at all.

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