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