CO2 Carriage and Storage

• CO₂ is carried in the blood in 3 forms:

• Dissolved (~5% in arterial blood) - obey's Henry's law, 20x more soluble than O₂ - solubility of 0.067ml/dl/mmHg - accounting for 10% of CO₂ arriving at the lung
• Bicarbonate (~90% in arterial blood) - via the following equation (H⁺ uptake by reduced Hb lowers free H+ and shifts this equation to the right causing increased CO₂ carriage):

• The first reaction is slow in plasma but fast in red cells due to carbonic anhydrase
• The second reaction (carbonic acid ionic dissociation) is fast even without an enzyme
• When H⁺ ions are produced via this reaction, HCO₃^- diffuses out of the cells, but H⁺ cannot easily therefore Cl^- ions move into the cell (the chloride shift)
• Liberated H⁺ ions bind Hb, deoxidizing it to form a reduced version, which also assists with CO₂ loading on Hb

• Haldane effect - deoxygenation of blood increases its ability to carry carbon dioxide. Conversely, oxygenated blood has a reduced capacity for carbon dioxide.

• Carbaminohemoglobin is the major contributor to the Haldane effect.
• Enhanced affinity of deoxyhemoglobin for protons enhances synthesis of bicarbonate and accordingly increases capacity of deoxygenated blood for carbon dioxide. The majority of carbon dioxide in the blood is in the form of bicarbonate. Only a very small amount is actually dissolved as carbon dioxide, and the remaining amount of carbon dioxide is bound to hemoglobin.

• Carbamino compounds (~5% in arterial blood) - formed by the combination of CO₂ with terminal amine groups in blood proteins

• Most common protein is Hb eg. Hb.NH₂ + CO₂ <-> Hb.NH.COOH
• This occurs rapidly without an enzyme, and reduced Hb can bind more CO₂ as carbaminohaemoglobin than HbO2, promoting CO₂ uptake in peripheries

CO2 Dissociation Curve

• Much more linear than the O₂ dissociation curve
• Curve moves rightwards with deoxygenation - Haldane Effect - due to:

• Improved ability of reduced Hb to mop up H⁺ ions when carbonic acid dissociates
• Improved ability of reduced Hb to form carbaminohaemoglobin

• This steep curve is why there is only a small PCO₂ difference between arterial and venous blood, whereas PO₂ has a big one