Why does the affinity of haemoglobin for oxygen decrease at high altitudes?

Why does the affinity of haemoglobin for oxygen decrease at high altitudes?

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My class 12 NCERT book says, Pg 226

The body compensates low oxygen availability by increasing red blood cell production, decreasing the binding affinity of haemoglobin and by increasing breathing rate.

Why should the haemoglobin binding capacity decrease at high altitude?

I think it should increase for better oxygen transfer and uptake from air. The concentrtion of oxygen in the atmosphere decreases with height. Hence, if the haemoglobin binding increases, we will be able to draw more oxygen from the air and transport it to the cells.

Haemoglobin's job is to transport oxygen and not store it. Therefore it should also be able to release oxygen effectively. When the differences in partial pressure of oxygen between the tissues and blood are low then oxygen will not be transported to the tissues from blood, leading to hypoxia.

2,3 Bisphosphoglycerate (2,3-BPG) stabilizes the T- (taut; oxygen unbound) form of haemoglobin thereby reducing its affinity to bind to oxygen. 2,3-BPG is found to be elevated in people living at high altitudes. The production of 2,3-BPG is controlled by a negative feedback (Mulquiney et al., 1999; also see wikipedia) so that it does not overdo its job.

David's answer explains the dynamics in details and addresses the non-linear (sigmoidal) nature of Hb-Oxygen binding which is a critical point for understanding how the effect of 2,3-BPG is actually favourable.

However, haemoglobin content also increases so that higher amounts of oxygen can be captured. This is a fine balance and as you may notice (even if you do not read it to great depth) in the linked paper, there are many mechanisms that are working towards this.

The answer to this question is yes, a decrease in oxygen affinity will decrease the oxygen taken up by the haemoglobin (Hb), but it is an appropriate response because it will have a greater effect in increasing the release of oxygen to the tissues.

This is not intrinsically obvious. It happens because of the (sigmoid) shape of the oxygen binding curve, and can only really be appreciated if you examine the curves for the normal situation and that in which there is increased 2,3 Bisphosphoglycerate (2,3-BPG) producing the change in oxygen affinity. I have devised a figure to demonstrate this, although it is not based on actual data, so should only be regarded as illustratative.

  • At sea level the lungs pick up oxygen at 1 (say 100% saturation of Hb) and when the oxygen pressure drops to 40 mm Hg in the tissues (2) the Hb will be 55% saturated. Hence in this example they have released 45% of a full complement of oxygen.

  • If we look at the same curve for high altitude, in this example the oxygen pressure at the lungs (1') will be such that the Hb is only 80% saturated. Thus at 40 mm Hg in the tissues (2) when Hb is only 55% saturated it will only have released and made available 25% of a full complement of oxygen.

  • Now consider the effect of high altitude acclimatization with increased 2,3-BPG production changing the equilibrium and the oxygen binding curve (red). At the lungs (3) the Hb will be less charged with oxygen - only 70% saturation - but at 40mm Hg in the tissues (4) it will be much less saturated than on the black curve - 30%. Thus it will have made available 40% of a full complement of oxygen.

This is not a perfect solution, which is why over a longer term there is increased production of red blood cells to provide more haemoglobin to compensate for the smaller amount of oxygen it can bind.

Red cell 2,3-diphosphoglycerate and oxygen affinity

The ease with which haemoglobin releases oxygen to the tissues is controlled by erythrocytic 2,3-diphosphoglycerate (2,3-DPG) such that an increase in the concentration of 2,3-DPG decreases oxygen affinity and vice versa. This review article describes the synthesis and breakdown of 2,3-DPG in the Embden-Meyerof pathway in red cells and briefly explains the molecular basis for its effect on oxygen affinity. Interaction of the effects of pH, Pco2, temperature and 2,3-DPG on the oxyhaemoglobin dissociation curve are discussed. The role of 2,3-DPG in the intraerythrocytic adaptation to various types of hypoxaemia is described. The increased oxygen affinity of blood stored in acid-citrate-dextrose (ACD) solution has been shown to be due to the decrease in the concentration of 2,3-DPG which occurs during storage. Methods of maintaining the concentration of 2,3-DPG in stored blood are described. The clinical implication of transfusion of elderly people, anaemic or pregnant patients with ACD stored blood to anaesthetically and surgically acceptable haemoglobin concentrations are discussed. Hypophosphataemia in association with parenteral feeding reduces 2,3-DPG concentration and so increases oxygen affinity. Since post-operative use of intravenous fluids such as dextrose or dextrose/saline also lead to hypophosphataemia, the addition of inorganic phosphorus to routine post-operative intravenous fluid may be advisable. Disorders of acid-base balance effect oxygen affinity not only by the direct effect of pH on the oxyhaemoglobin dissociation curve but by its control of 2,3-DPG metabolism. Management of acid-base disorders and pre-operative aklalinization of patients with sickle cell disease whould take account of this. It is known that anaesthesia alters the position of the oxyhaemoglobin dissociation curve, but it is thought that this is independent of any effects which anaesthetic agents may have on 2,3-DPG concentration. In vitro manipulation of 2,3-DPG concentration with steroids has already been carried out. Elucidation of the role of 2,3-DPG in the control of oxygen affinity may ultimately lead to iatrogenic manipulation of oxygen affinity in vivo.

Understanding the Oxygen Dissociation Curve

The oxygen dissociation curve is a graph that plots the proportion of haemoglobin in its oxygen-laden saturated form on the vertical axis against the partial pressure of oxygen on the horizontal axis. The curve is a valuable aid in understanding how the blood carries and releases oxygen and it is a common theme that is tested on in many medical examinations.

At high partial pressures of oxygen, haemoglobin binds to oxygen to form oxyhaemoglobin. All of the red blood cells are in the form of oxyhaemoglobin when the blood is fully saturated with oxygen. Each gram of haemoglobin can combine with 1.34 mL of oxygen. At low partial pressures of oxygen (e.g. within tissues that are deprived of oxygen), oxyhaemoglobin releases the oxygen to form haemoglobin.

The oxygen dissociation curve has a sigmoid shape because of the co-operative binding of oxygen to the 4 polypeptide chains. Co-operative binding means that haemoglobin has a greater ability to bind oxygen after a subunit has already bound oxygen. Haemoglobin is, therefore, most attracted to oxygen when 3 of the 4 polypeptide chains are bound to oxygen.

There is often a P50 value expressed on the curve, which is the value that tells us the partial pressure of oxygen at which the red blood cells are 50% saturated with oxygen. At an oxygen saturation of 50%, the PaO2 is approximately 25 mmHg (3.5k Pa).

The oxygen dissociation curve and the factors affecting it.

Which factors affect the oxygen dissociation curve?

The oxygen dissociation curve can be shifted right or left by a variety of factors. A right shift indicates decreased oxygen affinity of haemoglobin allowing more oxygen to be available to the tissues. A left shift indicates increased oxygen affinity of haemoglobin allowing less oxygen to be available to the tissues.

A decrease in the pH shifts the curve to the right, while an increase in pH shifts the curve to the left. This occurs because a higher hydrogen ion concentration causes an alteration in amino acid residues that stabilises deoxyhaemoglobin in a state (the T state) that has a lower affinity for oxygen. This rightward shift is referred to as the Bohr effect.

Carbon dioxide (CO2):

A decrease in CO2 shifts the curve to the left, while an increase in CO2 shifts the curve to the right. CO2 affects the curve in two ways. Firstly, the accumulation of CO2 causes carbamino compounds to be generated, which bind to oxygen and form carbaminohaemoglobin. Carbaminohaemoglobin stabilizes deoxyhaemoglobin in the T state. Secondly, the accumulation of CO2 causes an increase in H+ ion concentrations and a decrease in the pH, which will shift the curve to the right as explained above.

An increase in temperature shifts the curve to the right, whilst a decrease in temperature shifts the curve to the left. Increasing the temperature denatures the bond between oxygen and haemoglobin, which increases the amount of oxygen and haemoglobin and decreases the concentration of oxyhaemoglobin. Temperature does not have a dramatic effect but the effects are noticeable in cases of hypothermia and hyperthermia.

2,3-Diphosphoglycerate (2,3-DPG) is the main primary organic phosphate. An increase in 2,3-DPG shifts the curve to the right, whilst a decrease in 2,3-DPG shifts the curve to the left. 2,3-DPG binds to haemoglobin and rearranges it into the T state, which decreases its affinity for oxygen.

A table summarizing these effects is shown below:

pHRight shiftLeft shift
CO2Left shiftRight shift
TemperatureLeft shiftRight shift
2,3-DPGLeft shiftRight shift

How does carbon monoxide affect the curve?

Carbon monoxide (CO) interferes with the oxygen transport function of the blood by combining with haemoglobin to form carboxyhaemoglobin (COHb). CO has approximately 240 times the affinity for haemoglobin than oxygen does and for that reason, even small amounts of CO can tie up a large proportion of the haemoglobin in the blood making it unavailable for oxygen carriage. If this happens the PO2 of the blood and haemoglobin concentration will be normal but the oxygen concentration will be grossly reduced. The presence of COHb also causes the oxygen dissociation curve to be shifted to the left, interfering with the unloading of oxygen.

How does methaemoglobin affect the curve?

Methaemoglobin is an abnormal form of haemoglobin in which the normal ferrous form is converted to the ferric state. Methaemoglobinaemia causes a left shift in the curve as methaemoglobin does not unload oxygen from haemoglobin.

The other oxygen transport molecules

There are two other oxygen transport molecules that are required knowledge and commonly asked about in medical exams, fetal haemoglobin and myoglobin:

Fetal haemoglobin

Fetal haemoglobin (HbF) is the main oxygen transport protein in the human fetus during the last 7 months of development. It persists in the newborn until roughly 6 months of age. HbF has different globin chains to adult haemoglobin (Hb). Whereas adult haemoglobin is composed of two alpha and two beta subunits, fetal haemoglobin is composed of two alpha and two gamma subunits. This change in the globin chain results in a greater affinity for oxygen and allows the fetus to extract oxygen from the maternal circulation. This increased affinity for oxygen means that the oxygen dissociation curve for fetal haemoglobin is shifted to the left of that of adult haemoglobin.

The curve for myoglobin lies even further to the left than that of fetal haemoglobin and has a hyperbolic, not sigmoidal, shape. Myoglobin has a very high affinity for oxygen and acts as an oxygen storage molecule. It only releases oxygen when the partial pressure of oxygen has fallen considerably. The function of myoglobin is to provide additional oxygen to muscles during periods of anaerobic respiration.

Header image used on licence from Shutterstock

Thank you to the joint editorial team of FRCEM Exam Prep for this ‘Exam Tips’ blog post.

Investigation of the Hereditary Haemolytic Anaemias

Oxygen dissociation curve

The oxygen dissociation curve is the expression of the relationship between the partial pressure of oxygen and oxygen saturation of haemoglobin. Details of this relationship and the physiological importance of changes in this relationship were worked out in detail at the beginning of the last century by the great physiologists Hüfner, Bohr, Barcroft, Henderson and many others. Their work was summarised by Peters and Van Slyke in Quantitative Clinical Chemistry. 58 The relevant chapters of this book have been reprinted and it would be difficult to improve their description of the importance of the oxygen dissociation curve.

The physiological value of haemoglobin as an oxygen carrier lies in the fact that its affinity for oxygen is so nicely balanced that in the lungs haemoglobin becomes 95%–96% oxygenated, whereas in the tissues and capillaries it can give up as much of the gas as is demanded. If the affinity were much less, complete oxygenation in the lungs could not be approached if it were greater, the tissues would have difficulty in removing from the blood the oxygen they need. Because the affinity is adjusted as it is, both oxyhaemoglobin and reduced haemoglobin exist in all parts of the circulation but in greatly varied proportions.

Determining the oxygen dissociation curve

Determination of the oxygen dissociation curve depends on two measurements: pO2 with which the blood is equilibrated and the proportion of haemoglobin that is saturated with oxygen. Methods for determining the dissociation curve fall into three main groups:

The pO2 is set by the experimental conditions, and the percentage saturation of haemoglobin is measured.

The percentage saturation is predetermined by mixing known proportions of oxygenated and deoxygenated blood, and the pO2 is measured.

The change in oxygen content of the blood is plotted continuously against pO2 during oxygenation or deoxygenation, and the percentage saturation is calculated.

The multiplicity of methods available for measuring the oxygen dissociation curve suggests that no method is ideal. The advantages and disadvantages of the various techniques have been reviewed. 59, 60 The standard method with which new methods are compared is the gasometric method of Van Slyke and Neill. 61 This method is slow, demands considerable expertise and is not suitable for most haematology laboratories. Commercial instruments are now available for performing the test and drawing the complete oxygen dissociation curve, for example, Hemox Analyzer ( ). Such analysers are extremely quick and accurate and are therefore ideal for laboratories performing multiple determinations. Approximate measurement of oxygen saturation of haemoglobin can also be obtained at the bedside by noninvasive pulse oximetry.


Figure 12-6 shows the sigmoid nature of the oxygen dissociation curve of haemoglobin A and the effect of hydrogen ions on the position of the curve. A shift of the curve to the right indicates decreased affinity of the haemoglobin for oxygen and hence an increased tendency to give up oxygen to the tissues. A shift to the left indicates increased affinity and so an increased tendency for haemoglobin to take up and retain oxygen. Hydrogen ions, 2,3-DPG and some other organic phosphates such as ATP shift the curve to the right. The amount by which the curve is shifted may be expressed by the p50O2 (i.e. the partial pressure of oxygen at which the haemoglobin is 50% saturated).

The oxygen affinity, as represented by the p50O2, is related to compensation in haemolytic anaemias 62 1 g of Hb can carry about 1.34 ml of O2. Figure 12-7 shows the O2 dissociation curves of Hb A and Hb S plotted according to the volume of oxygen contained in 1 litre of blood when the Hb is 146 g/l and 80 g/l, respectively. The p50O2 of haemoglobin A is given as 26.5 mmHg (3.5 kPa) and of haemoglobin S as 36.5 mmHg (4.8 kPa). It will be seen that in the change from arterial to venous saturation, the same volume of oxygen is given up despite the difference in Hb. Patients with a high p50O2 achieve a stable Hb at a lower level than normal and this should be taken into account when planning transfusion for these patients.

Bohr effect

An increase in CO2 concentration produces a shift to the right (i.e. a decrease in oxygen affinity). This effect, originally described by C. Bohr, 63 is mainly a result of changes in pH, although CO2 itself has some direct effect. The Bohr effect is given a numeric value, Δlog p50O2/ΔpH, where Δlog p50O2 is the change in p50O2 produced by a change in pH (ΔpH). The normal value of the Bohr effect at physiological pH and temperature is about 0.45.

Hill constant (‘n’)

The Hill constant or coefficient (‘n’) represents the number of molecules of oxygen that combine with one molecule of haemoglobin. 64 Experiments showed that the value was 2.8–3.0 rather than the expected 4. The explanation for this lies in the effect of binding one molecule of oxygen by haem on the affinity for oxygen of further haem groups, the so-called allosteric (cooperativity) effect of haem–haem interaction: ‘n’ is a measure of this effect and the calculation of the ‘n’ value helps in identifying abnormal haemoglobins, the molecular abnormality of which leads to abnormal haem–haem interaction. 65

Why does the affinity of haemoglobin for oxygen decrease at high altitudes? - Biology

The Effect of Carbon Monoxide on Hemoglobin-Oxygen Equilibrium
What is Haemoglobin?

Haemoglobin, a protein that contains iron, is the material in red blood cells that is responsible for the transportation of oxygen to the cells. The equilibrium conditions of the haemoglobin-oxygen interactions can be expressed through the following equation:

Hb (aq) + 4O 2 (g) <–> Hb(O 2 ) 4 (aq)

“Hb” stands for haemoglobin and each haemoglobin molecule will attach to four oxygen atoms and as long as there is sufficient oxygen in the air, equilibrium is maintained.

Effect of pressure on Hemoglobin-Oxygen Equilibrium:
This equilibrium is however easily affected by high altitudes due to the fact that the air pressure is lowered and thus it is difficult to obtain the oxygen that is needed to keep the equation equal. In accordance to Le Châtelier’s principle, the reaction shifts to the left (1)(2).

Hb (aq) + 4O 2 (g) <– Hb(O 2 ) 4 (aq)

The reaction shifts away from the oxygenated haemoglobin because there is not an adequate oxygen supply to feed the body’s cells and tissues which results in a person feeling light-headed and in some cases pressurized oxygen from an oxygen tank is required which would then shift the equilibrium to the right to produce more oxygenated haemoglobin which would be transported to the cells (1).

Hb (aq) + 4O 2 (g) –> Hb(O 2 ) 4 (aq) (1)

What is Carbon Monoxide?
Carbon monoxide is a colourless, odourless, and tasteless gas that is lighter than air and can be fatal to humans and animals when exposed in great concentrations.
What is Carbon Monoxide poisoning?
“a toxic condition that results from inhaling and absorbing carbon monoxide gas” (3)

How does carbon monoxide poisoning occur?
Carbon monoxide gas can cause a frightening variation of the normal hemoglobin-oxygen equilibrium when somebody is exposed to it due to the fact that the carbon monoxide tricks the hemoglobin into mistaking it for oxygen and bonds to the hemoglobin in groups of four, and the equilibrium expression becomes:

Hb (aq) + 4CO (g) ⇋ Hb(CO) 4 (aq). (1)

Carboxyhemoglobin is produced opposed to hemoglobin which appears redder thus one symptom of carbon monoxide poisoning is a red face. Hemoglobin and oxygen have weaker bonds compared to carbon monoxide and hemoglobin where the bonds are approximately 300 times stronger which means that according to the equation above, the equilibrium shifts to the right towards the carboxyhemoglobin side:

Hb (aq) + 4CO (g) –> Hb(CO) 4 (aq). (1)

The equilibrium constant, K, is much stronger in haemoglobin-carbon monoxide reaction compared to the haemoglobin-oxygen reaction which means that the haemoglobin puts priority on carbon monoxide bonds, therefore the haemoglobin that has bonded with carbon monoxide is no longer available for oxygen transportation.

How can the effects be reversed?
To reverse the effects of carbon monoxide, pure oxygen is needed to be introduced to the body which will then react with carboxyhemoglobin to produce the properly oxygenated haemoglobin alongside carbon monoxide thus the gaseous carbon monoxide produced dissipates when the person exhales.

Hb(CO) 4 (aq) + 4O 2 (g) ⇋ Hb(O 2 ) 4 (aq) + 4CO (g) (1)(2)

The Loading and Unloading Reactions

Deoxyhemoglobin and oxygen combine to form oxyhemoglo-bin this is called the loading reaction. Oxyhemoglobin, in turn, dissociates to yield deoxyhemoglobin and free oxygen molecules this is the unloading reaction. The loading reaction occurs in the lungs and the unloading reaction occurs in the systemic capillaries.

Loading and unloading can thus be shown as a reversible reaction:

Deoxyhemoglobin + O2 < z>: Oxyhemoglobin (tissues)

The extent to which the reaction will go in each direction depends on two factors: (1) the PO2 of the environment and (2) the affinity, or bond strength, between hemoglobin and oxygen. High PO2 drives the equation to the right (favors the loading reaction) at the high PO2 of the pulmonary capillaries, almost all

Table 16.7 Relationship Between Percent Oxyhemoglobin Saturation and Po2 (at pH of 7.40 and Temperature of 37° C)

PO2 (mmHg) 100 80 61 45 40 36 30 26 23

Percent Oxyhemoglobin 97 95 90 80 75 70 60 50 40

Arterial Blood Venous Blood

■ Figure 16.34 The oxyhemoglobin dissociation curve. The percentage of oxyhemoglobin saturation and the blood oxygen content are shown at different values of PO2. Notice that the percent oxyhemoglobin decreases by about 25% as the blood passes through the tissue from arteries to veins, resulting in the unloading of approximately 5 ml of O2 per 100 ml of blood to the tissues.

the deoxyhemoglobin molecules combine with oxygen. Low PO2 in the systemic capillaries drives the reaction in the opposite direction to promote unloading. The extent of this unloading depends on how low the PO2 values are.

The affinity between hemoglobin and oxygen also influences the loading and unloading reactions. A very strong bond would favor loading but inhibit unloading a weak bond would hinder loading but improve unloading. The bond strength between hemoglobin and oxygen is normally strong enough so that 97% of the hemoglobin leaving the lungs is in the form of oxyhemoglo-bin, yet the bond is sufficiently weak so that adequate amounts of oxygen are unloaded to sustain aerobic respiration in the tissues.

Haemoglobin dissociation curve help

Hi, I am a bit confused in regards to why animals living at high altitudes/with a low partial pressure of oxygen have their curves shifted further left/why this is an advantage? Surely this will mean that Hb has a higher affinity for oxygen, so less dissociation occurs, so muscles get less o2?

Can someone please help explain this concept to me?

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(Original post by Bertybassett)
Hi, I am a bit confused in regards to why animals living at high altitudes/with a low partial pressure of oxygen have their curves shifted further left/why this is an advantage? Surely this will mean that Hb has a higher affinity for oxygen, so less dissociation occurs, so muscles get less o2?

Can someone please help explain this concept to me?

The relevant part here is the situation in the gas exchange surface involved with take-up of O2 from the atmosphere. The shift of the dissociation curve for oxyhaemoglobin to the left means that at the low partial pressures of O2 at high altitudes, more oxygen can be taken up by the animal's [different] haemoglobin in e.g. the lungs.

The release of O2 at the tissue level is not affected greatly by this shift to the left due to the sigmoid shape of the dissociation curve, AND the curve probably also is less steep in these animals, so that at the O2 levels prevalent in the respiring tissues, the release of O2 is not reduced much.

(Original post by macpatgh-Sheldon)
The relevant part here is the situation in the gas exchange surface involved with take-up of O2 from the atmosphere. The shift of the dissociation curve for oxyhaemoglobin to the left means that at the low partial pressures of O2 at high altitudes, more oxygen can be taken up by the animal's [different] haemoglobin in e.g. the lungs.

The release of O2 at the tissue level is not affected greatly by this shift to the left due to the sigmoid shape of the dissociation curve, AND the curve probably also is less steep in these animals, so that at the O2 levels prevalent in the respiring tissues, the release of O2 is not reduced much.

What causes an increase in 2/3 DPG?

Click to read in-depth answer. Similarly, what is the role of 2/3 DPG?

oxygen transport in blood &hellip &hellipthe blood), carbon dioxide, and 2,3-diphosphoglycerate (2,3-DPG a salt in red blood cells that plays a role in liberating oxygen from hemoglobin in the peripheral circulation). These substances do not bind to hemoglobin at the oxygen-binding sites.

Beside above, what is 2/3 DPG oxygen dissociation curve? 2,3-BPG acts as a heteroallosteric effector of hemoglobin, lowering hemoglobin's affinity for oxygen by binding preferentially to deoxyhemoglobin. An increased concentration of BPG in red blood cells favours formation of the T, low-affinity state of hemoglobin and so the oxygen-binding curve will shift to the right.

In this manner, how does 2/3 DPG affect oxygen binding to hemoglobin?

The accumulation of 2,3-BPG decreases the affinity of hemoglobin for oxygen. In tissues with high energetic demands, oxygen is rapidly consumed, which increases the concentration of H + and carbon dioxide. Through the Bohr effect, hemoglobin is induced to release more oxygen to supply cells that need it.

2,3-Bisphosphoglycerate (BPG), also known as 2,3-Disphosphoglycerate (2,3-DPG), promotes hemoglobin transition from a high-oxygen-affinity state to a low-oxygen-affinity state.

Why is partial pressure of oxygen lower at high altitudes?

The partial pressure of all gases will decrease at higher altitudes because the overall pressure decreases.


The partial pressure of oxygen is calculated with the following equation:

#"partial pressure" = ("moles"(O_2))/("total moles of gas") * "partial pressure" #

Hence, it is mathematically true that when the total pressure of gas in the atmosphere decreases, so will the partial pressure.

The reason that the total pressure decreases is that pressure is essentially a measure of the weight of 'stuff' above you in this case atmospheric gas. As you go higher up, you are putting more of the atmosphere below you, leaving less of it above you. Therefore the weight of gas pressing down on you decreases because there is less gas.
As the amount of oxygen in the atmosphere remains constant in the atmosphere (21%), the only factor that can affect the partial pressure of oxygen is the atmospheric pressure change.

How are high-altitude natives different?

People who reside at altitude are known to have greater capacity for physical work at altitude. For example, the Sherpas who reside in the mountainous regions of Nepal are renowned for their mountaineering prowess.

High-altitude natives exhibit large lung volumes and greater efficiency of oxygen transport to tissues, both at rest and during exercise.

While there is debate over whether these characteristics are genetic, or the result of altitude exposure throughout life, they provide high-altitude natives with a distinct advantage over lowlanders during activities in hypoxia.

So unless you’re a sherpa, it’s best to ascend slowly to give your body more time to adjust to the challenges of a hypoxic environment.

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