Complete information on the Process of Transport of Oxygen and Carbon Dioxide in Blood

Blood Transports O₂ from the lungs to tissues and CO₂ from tissues to lungs.

Transport of O₂:

(i) By physical solution:

Oxygen is relatively insoluble in plasma. So very little amount of O2 i.e. about 0.3ml/100 ml of blood is carried in physical solution.

(ii) Transport of Oxygen by Gaemoglobin:

Haemoglobin is a respiratory pigment in RBCs. It is tetrameric conjugated protein. It consists of two parts: haem and globin. Haem is the non-protein part and made up of iron or ferrous (Fe⁺⁺) containing perphyrins. The globin is the protein part and made up of 4 polypeptide chains. Oxygen diffuses into RBCs and combines with Fe⁺⁺ of haemoglobin to form oxyhaemoglobin. It is the iron that actually binds O₂.

Thus each haemoglobin molecule can carry 4 molecules of O₂ by its 4 sub-units. The binding oxygen to one sub-unit, induces the remaining subunits to change their shapes slightly so that their affinity for oxygen increase.This is called positive cooperatively binding. About 97% of oxygen is carried by haemoglobin to different tissues of the body in the form of oxyhaemoglobin. One gram of haemoglobin carries 1.34 ml of oxygen which is called respiratory capacity of blood.

Hb₄ — + 4O2 = Hb4O₈

(Haemoglobin) – (Oxygen) – (Oxyhaemoglobin)

Bohr Effect – At tissue level CO₂ released by respiring cells reacts with H2o to form carbonic acid. The carbonic acid decreases pH of the surrounding medium. The low pH lowers the affinity of haemoglobin with O₂. Therefore, O₂ is released from haemoglobin. This effect of CO₂ on O₂ carrying capacity of haemoglobin is called Bohr Effect. In this condition the oxygen dissociation curve moves towards right.

Transport of CO2:

(i) Absent Physical Solution:

About 5-7% of CO₂ dissolves in blood plasma and forms carbonic acid and is carried by blood to lungs.

H2O + CO₂ — H₂CO₃

(ii) As Carbamino Compound:

About 10% of CO₂ is transported absent carbamino compound .CO₂ combines with NH₂ group of globin of haemoglobin and other plasma proteins to form carbamino compounds.

HbNH₂ + CO₂ → HbNHCOOH (Carbamino compound)

At lungs carbamino compounds are dissociated to release O₂ in alveoli.

(iii) Absent Bicarbonates:

Nearly 80-85% of CO₂ is carried in the form of bicarbonates of sodium and potassium. Most of CO₂ produced by tissue enters the RBCs by diffusion. The RBCs are rich in zinc-enzyme called carbonic enhydrase. This enzyme converts reversible CO₂ into carbonic acid which dissociates into H⁺ and HCO₃^– ions with RBCs.

CO₂ + H₂O — Carbonic enhydrase — H₂CO₃ (carbonic acid)

H₂CO₃ — Carbonic enhydrase — H+ + HCO₃

The hydrogen ions produced in this process combine with the protein ions of blood. Some of the bicarbonate ions bind to K⁺ and form KHCO₃. The rest of HCO₃^– ions diffuse out of RBCs into the plasma. In plasma HCO₃^– ions bind to sodium ions (Na⁺) to form sodium bicarbonate. The entry of HCO₃^– ions into plasma changes the electro neutrality. This is balanced by the shift of an equal number of chloride ions (Cl^–) from the plasma into RBC. The shift of chloride ions between plasma and RBC helps in transport of CO₂ in blood. This process is called chloride shift or hamburger phenomenon.

Releases of CO2 at Lungs:

Carbonic acids, bicarbonates of sodium and potassium and carbamino compounds are carried to the lungs where they break down under the influence of various factors and liberate free CO₂.