CO2 dissociation curve

Physiology - Respiratory

Describes the relationship between PCO2 and total CO2 concentration in blood.

Three Forms of Carbon Dioxide in the Blood

1.       Physically dissolved CO2 (~5%)

            Henry’s law : dissolved CO2 increases linearly with increases in PCO2

CO2 solubility = 0.06 mL CO2/dL blood/mm Hg (20 times higher than O2 solubility)

2.         Carbamino compounds (small)

CO2 joins reversibly with non-ionized terminal amino groups (-NH2) of blood borne proteins

3.        Lost CO2 in the blood is transported in the bicarbonate ion form via carbonic anhydrase:

CO2 + H2O ⇔ H2CO3⇔ H+ + HCO3-

RBCs are source of carbonic anhydrase, and histidine residue of Hb carries the H+

The CO2 dissassociation curve is linear in shape and steep compared to O2 dissociation curve.  Thus, when increasing ventilation, the CO2 excretion is increased in lung regions of high and low V/Q ratios. In contrast, increasing ventilation preferentially increases O2 content of blood in low V/Q ratio areas of lung.

Haldane effect:  Deoxygenation of the blood increases its ability to carry carbon dioxide, and vice versa.


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