Question

Please discuss oxygen transport from the lungs to the tissues by hemoglobin, including the uptake and expulsion of protons and carbon dioxide. A detailed description of binding with the respective ligands is important.

Answer 1

Ans> Oxygen is transported through the red blood cells present in the blood by the virtue of a pigment called haemoglobin. Each molecule of haemoglobin can bind to four molecules of oxygen. Binding of haemoglobin to oxygen is a reversible reaction. The oxygen molecule binds to the haemoglobin at the lungs and are carried throughout and released at the tissues for their uptake. Hb + 4O₂ Hb^.4O₂  

This binding and detaching of O₂ from the haemoglobin is intimately regulated by the CO₂ concentrations and the proton concentrations at different sites of gaseous exchange. At higher oxygen concentrations there is low haemoglobin and mostly oxyhaemoglobin but at lower oxygen concentrations we find oxygen dissociated from haemoglobin. The CO₂ concentrations affect the % saturation of haemoglobin with oxygen. The same partial pressure of O₂ results in a lower % saturation of O₂ in the presence of CO₂ which accounts for the bohr effect.

At the tissues site where the CO₂ concentrations are relatively higher, CO₂ reacts with water in the presence of a catalyst carbonic anhydrase in the erythrocytes to from carbonic acid and the following equillibrium is established: H₂O + CO₂ H₂CO₃

This carbonic acid again dissociates into H⁺ and HCO₃^- and is proposed to form another equillibrium with the undissociated carbonic acid: H₂CO₃ H⁺ + HCO₃^- The bicarbonate ion diffuses out of the erythrocyte and this is coupled with a transfer of cl^- ion into the erythrocyte form the plasma and constituting what is know as chloride shift.

The dissociation of carbonic acid increases the acidity of the blood and H⁺ then reacts with oxyhaemoglobin to release the bound oxygen.and reduce the acidity of blood. Hb^.4O₂ + H⁺ HHb + 4O₂

Carbon dioxide is a waste product of respiration and its concentration is higher at the tissue sites and it is where oxyhaemoglobin dissociates to release oxygen and free haemoglobin. This free haemoglobin then binds to CO₂ and carries it all to the lungs where again the O₂ concentrations are high and here the opposite is true. This is how oxygen is transported from the lungs to the tissues by haemoglobin