Oxygen plays a vital role in human physiology. Oxygen gets into the body from the outer environment like plants, and other aerobic microbes and get into the lungs and control our respiratory system.

Oxygen firstly gets into the alveolar layer and then capillary endothelial tissues and finally into the blood circulation. Now once the oxygen reaches into the bloodstream it then gets distributed into different regions of the rest of ghe tissues of the human body.

Now the big question is who carries this oxygen into our body? And how oxygen is transported in human body?


As Oxyhaemoglobin– Heme + Oxygen




  • When the oxygen gets into the bloodstream from the lungs it is taken up by the Haemoglobinwhich is present in the red blood cell.
  • Hemoglobin has four subunits. And each heme fraction is linked up with a polypeptide.
  • Each heme molecule contains four iron attached to a tetrapyrrole ring.
  • Each ferrous iron binds with oxygen reversibly.
  • Hence, one hemoglobin fragment can bind with four oxygen at a time.



  • Heme- heme interaction is that interaction in which affinity of Hemoglobin increases with the successive numbers of oxygen.
  • That is when the hemoglobin combines with the first oxygen molecule the affinity of Hb increases for the second oxygen molecule, likewise, the second binding of Hb with oxygen increases the affinity for the third molecule of oxygen and so on till the fourth molecule of oxygen.
  • And because of this heme-heme interaction, the oxygen-dissociation curve is in sigmoid shape.
  • Explanation:- When the hemoglobin is in the deoxygenated form it is in a tensed state or T state and the globin units are tightly packed. When one oxygen molecule binds with the deoxygenated Hb, the bonds between globin units are broken and make the oxygenated Hb in a relaxed state or R state. And further addition makes the Hb in more R state.



  • The sigmoid curve is obtained by plotting the percentage saturation of Hb in the y-axis against partial pressure of oxygen in the x-axis.
  • The curve is divided into two parts: i) at a lower partial pressure of oxygen (<60mm of Hg) the curve is steep, and ii) when the partial pressure of oxygen is higher than 60mm of Hg then the curve is a plateau-like flat portion.
  • When the partial pressure of oxygen is 60mm of Hg that means Hb is saturated about 90%. From this point, the curve remains horizontal i.e. the percentage of Hb saturation increases with an increase in the partial pressure of oxygen. So, the oxygen supply in the tissues is not that affected when partial pressure oxygen drops from 100mm of Hg to 60mm of Hg. And this explains the term hypoxia.
  • When the partial pressure of oxygen fall below 60mm of Hg then the saturation percentage of Hb drops drastically and the curve becomes steep. This indicates that with the modest drop of the partial pressure of oxygen it results in desaturation and leads in the release of oxygen in the tissues. So this part of the curve unleashes the oxygen from the bloodstream into the tissues.

So this is how oxygen is transported in human body with the help of Haemoglobin.



There are various factors which affect the oxygen-binding affinity with the Heme part. So the factors affecting the affinity of oxygen are followings:


  • pH:- Have you heard of Bohr’s effect?

Oxygen binding affinity with hemoglobin depends upon the concentration of the hydrogen ions. Because when hemoglobin is in T state it has a higher affinity towards hydrogen ions than oxygen. That means with increase ⬆ H+ concentration, the pH level goes down ⬇ and Hb is in T state and oxygen binding affinity is also decreased.

So, to get the maximum percentage of Hb saturation we need more oxygen. And this is called Bohr’s effect, where there is increase ⬆ oxygen dissociation at a lower pH level. And this leads to the curve shift towards the right. And lower the pH will be better will be the cellular respiration.


  • 2,3-diphosphoglycerate (2,3-DPG):-

2,3-DPG increases ⬆ when hemoglobin is poorly saturated with oxygen this happens at higher altitudes just to compensate for the low atmospheric oxygen. So, 2,3-DPG decreases ⬇ the Hb affinity towards oxygen to increase the oxygen accessibility in the oxygen-depleted tissues. And this results in the right shift of the oxygen-Hb dissociation curve.


  • Temperature:-

Temperature affects the kinetic energy of hemoglobin and oxygen. Higher ⬆ the temperature, the more the oxygen will dissociate into the tissues due to higher kinetic energy. So more oxygen gets released from the hemoglobin into the tissues resulting in the production of more heat. And this leads to the curve shifting towards right.


  • Carbon dioxide:-

Increased CO² ⬆ shifts the oxygen-Hb dissociation curve towards right. Because the ⬆CO² level decreases the pH and this leads to acidosis. And in acidosis H+ ions bind with histidine residues of Hb. And this leads to a decrease ⬇ in Hb affinity towards oxygen. And this results in a huge release of oxygen into the tissues. Again Bohr’s effect us being applied over here also.


  • Effect of exercise:-

During exercise, CO², temperature, lactic acid, and 2,3-DPG levels increase. And all these results in the right shift of the oxygen-Hb dissociation curve.


  • Fetal Hemoglobin:-

Fetal hemoglobin has gamma chains that do not get bind with 2,3-DPG. That is why fetal hemoglobin has a higher affinity towards oxygen and this leads to the left shift of the oxygen-Hb dissociation curve.


  • Carbon Monoxide:-

Carbon monoxide has 210x times greater affinity towards hemoglobin than oxygen. So when iron of the hemoglobin binds with CO it leads to the formation of Carboxyhemoglobin. And after that, the ferrous binding site gets blocked for the remaining RBC life cycle. And with the increase ⬆ in CO level, severe hypoxia state occurs in the human tissues. And this leads to the left shift of the oxygen-Hb dissociation curve.


Hence, these are some factors that affect the binding the oxygen to hemoglobin.

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