Saturday, February 27, 2010

BOHR effect

It has been a while since my last post, and today yeah, about BOHR effect.

Generally, we know that haemoglobin and oxygen are always in pairs. They are like the rhythm and melody. But, we must also take notice that haemoglobin do combines with carbon dioxide too, forming carbaminohaemoglobin.

In the usual oxygen dissociation curves, the shape of the curve is the normal S-shape. Easily to say, when the oxygen concentration is high, or oxygen is saturated, meaning the affinity of haemoglobin for oxygen is therefore higher. This result in the binding of oxygen to haemoglobin forming oxyhaemoglobin. When this happens, the curve moves to the left, showing higher percentage of saturation of haemoglobin to oxygen. even when the partial pressure of oxygen is relatively low.

Reversibly, when concentration of Oxygen is low, the oxyhaemoglobin dissociates into haemoglobin and oxygen, thus releasing the oxygen to the targeted area, where they are needed. Example, tissues.

higher oxygen, curve moves to the left and vice versa :)

Ok, as in BOHR effect, in respiring tissues, oxygen is lower since it has been used up in respiration, ventilation and etc but carbon dioxide is in higher concentration in contrast. 

To be exact the presence of carbon dioxide is crucial to meet the demand for oxygen in the continually respiring tissues. Why? Let us go and see together:)

When there is carbon dioxide, Bohr effect appears. Carbon dioxide helps the haemoglobin to dissociates oxygen, and thus providing oxygen for the oxygen deprived tissues. 

Here, we are still using the same concept. Higher carbon dioxide concentration, lower affinity of haemoglobin for oxygen thus curve shifts to the right, showing lower saturation percentage of haemoglobin to oxygen since carbon dioxide is in higher concentration compared to oxygen. 

Do you see the above diagram. Ok, so it is not a Doughnut, rather, it is our beloved Red Blood Cell. Referring to the above diagram, there are 6 important sequences that can explain briefly why carbon dioxide actually helps in the dissociation of oxygen.

In respiring tissues:-

1. Carbon dioxide diffuses into the red blood cells from the plasma

2. Carbon dioxide that enters the RBC will then combines with water as in the following equation 

H2O + CO2  <-->  H2CO3

--> so carbon dioxide present combines with water in the erythrocytes to form carbonic acid with the aid of enzyme carbonic anhydrase. This is a reversible process.


3.  Carbonic acid then dissociates into hydrogen ions and hydrogen carbonate ions. Here, the hydrogen carbonate ions will diffuse into the plasma where carbon dioxide is most transported in this form. 

H2CO3  <-->  H+ + HCO3-

When hydrogen carbonate ions diffuse from the cytoplasm into the plasma, the reaction is balanced by the diffusion of chloride ion, Cl-. This maintains the negative and the positive balance on either side. This is known as chloride shift.

4. The dissociation of carbonic acid increases the acidity in the blood as what is left is the hydrogen ions. Hydrogen ions then react with the oxyhaemoglobin to release the bound oxygen and at the same time reducing the acidity in the blood. 

Hb.4O2 + H+  <-->  HHb+ + 4O2
The HHb is haemoglobinic acid. It is this reversible action which accounts for Bohr's effect. The carbon dioxide is higher due to respiration and ventilation of living cells and tissues and so it is here where haemoglobin releases the oxygen to the respiring tissues. 

5. At last, the oxygen releases will be diffused out from the RBC into the plasma.

In the Lungs.

Oxygen concentration is high in the lung and the oxygen diffuses into the RBC. Here, the affinity of haemoglobin for oxygen is high so

  1. Hb + O2 --> HbO (releasing H+)
  3. H+ + HCO3- --> H2CO3 
  5. H2CO3 --> H2O + CO2 (with the aid of carbonic anhydrase as enzyme)
The CO2 is then diffused from RBC into lungs and removed from the body through exhalation. 



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