Oxidation of carbohydrates in body cells produces CO2. The CO2 produced enters the lungs through the blood and is exhaled. CO2 is transported in the blood in three ways. namely-
(i) As physical solution: 5% CO2 is transported in blood as physical solution. CO2 dissolves in the water of the blood plasma to form carbonic acid and is transported.
CO2 + H2O → H2CO2)
(ii) As carbamino compounds: 10% of CO2 is transported in blood as carbamino compounds. Carbon dioxide reacts with hemoglobin in body cells to form carbaminohemoglobin compound. Carbon dioxide is transported as a carbaminohemoglobin compound.
CO2 + HbNH2 → HbNHCOOH
(iii) As bicarbonate compound: 85% of CO2 is transported in blood as bicarbonate compound. It is transported in two ways. as NaHCO3 and KHCO3.
In the diffusion process CO2 enters the red blood cells and reacts with water with the help of carbonic anhydrase enzyme to produce HCO3. Most carbonic acid breaks down into H+ and HCO3- ions.
CO2 + H2O → H2CO3
H2CO3 → H+ + HCO3-
K+ and HCO3- combine inside red blood cells to form KHCO3. Some HCO3- moves from the red blood cells into the plasma and combines with Na+ to form NaHCO3. As much HCO3- enters the plasma from the red blood cells as Cl- enters the red blood cells from the plasma. This is called chloride shift reaction or Hamburger reaction.
In red blood cells: K+ + HCO3 →KHCO3
In plasma : Na+ + HCO3 → NaHCO3
About 25-33% oxygen enters the collars under normal conditions. 100 ml blood transports 19-20 ml oxygen in 100 ml blood 14-15 ml CO2.