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.