Carbon dioxide transport in respiration

Carbon dioxide transport in respiration

The CO₂ produced during metabolism is a waste product and has been eliminated. The flow of CO₂ is effected under a continuous pressure gradient.

The transport of CO₂ is effected in the following ways as physically dissolved CO₂ and transport in chemical combination.

1. Physically dissolved CO2

Compared with O₂, CO₂ is 24 times more soluble in blood plasma. Only about 5 – 7 % of total CO₂ carried by blood is in a simple physical solution. The factors that determine the transport are the partial pressure of CO₂ (Henry’s law) and temperature. Both plasma and cells can transport CO₂ in a physically dissolved state. (The solubility factor per litre for CO₂ at 38⁰C is 0.03 for plasma and 0.025 for cells).

2. Transport in chemical combination

By far the greater proportion of CO₂ is transported in blood in chemical combination as HCO₃. Majority of the CO₂ diffusing into the blood combines with water forms carbonic acid, which ten rapidly dissociates into HCO₃ions and H⁺. This reversible reaction is kept moving to the right because the H⁺ is buffered by plasma proteins / Hb.

Formation of HCO3 in plasma

The Chief buffers of plasma that provides for buffering action by combining with H⁺ is protein system (Proteinate / Protein).

The PCO₂ in tissues is higher than in arterial blood causes diffusion of CO₂ into blood and forces the preceding reversible reaction to the right causing an increase in HCO₃. This increase in HCO₃ represents transport due to plasma proteins.

When venous blood reaches the lungs, CO₂ diffuses into alveolar air and is exhaled out. This causes the preceding reaction to shift to left.

The pH of arterial plasma decreases about 0.025 units in becoming venous plasma.

The transport of CO₂ as HCO₃^– by plasma proteins represents about 4% of total CO₂ transport.

Formation of HCO3 by RBCs (Isohydric Transport of CO2)

The CO₂ from tissues enter plasma then into the RBCs. H₂CO₃ formation by the reaction of CO₂ with water is too slow to less of importance in plasma but inside the RBCs, the enzyme carbonic anhydrase catalyses the reaction between CO₂ and H₂O accelerating the rate of about 5,000 fold.

H₂CO₃ rapidly and spontaneously dissociates into H⁺ and HCO₃. Since an increase in H⁺ concentration is severely detrimental to an organism, a base buffer must be available to remove the H⁺ ions. The Hb provides the buffering, driving the reaction to the right.

In the tissue Oxy-Hb delivers O₂ and becomes deoxygenated Hb. The deoxygenated Hb is a weaker acid than Oxy-Hb and function as a better buffer readily combines with H⁺ and facilitates the formation of HCO₃ form CO₂.

Hb is a major buffer in blood that removes free H⁺ from blood and an equal quantity of HCO₃ is left dissolved in fluid. The HCO₃ diffuse out of RBC into the plasma due to concentration gradient by exchanging choloride ions from the plasma across the RBC membrane referred as chloride shift or Hamburger shift.

The conversion of CO₂ via H₂CO₃ to HCO₃ ion in erythrocytes accounts for 70% for CO₂transport.

In the lungs, high PO₂ favours diffusion of O₂ into the red cells and oxygenates Hb. On oxygenation HHb becomes Oxy Hb and releases H⁺ ions. This H⁺ ion combine with HCO₃ and drives the reaction to the left.

Transport as carbamino hemoglobin

Haemoglobin contains terminal amino group (NH) reversibly react with CO₂ to from an unstable carbamino compound.

About 20% CO₂ is transported as carbamino haemoglobin (Though plasma proteins can also form carbamino compounds, this reaction in plasma is negligible because free NH group is relatively less in plasma proteins).