CO2 transport in blood ◦ Bicarbonate and blood pH ◦ CO2 transport by haemoglobin ◦ CO2 content and PaCO2CO2 uptake from tissues and release in the lungs

You might want to read the Gas basics

Most (70%) of the CO2 in blood is transported as bicarbonate ions (HCO3-), while 23% is carried in the form of carbamino-haemoglobin.  A small fraction (7%) is actually dissolved in blood.  CO2 transport is not carrier-mediated and does not saturate.

Carbon dioxide transport in blood
Unlike O2, CO2 is readily soluble in water (5.2 ml/kPa/per litre), but not soluble enough match the body’s CO2 production (200 ml/min).  Less than 10% of CO2 carried by blood is dissolved in plasma*. To keep up with CO2 production, most CO2 (70%) is transported in blood as bicarbonate ions (HCO3-) and the remainder is bound to haemoglobin.  This is not to say that the dissolved fraction of CO2 doesn’t matter: it is crucially important in determining blood pH.

Bicarbonate and blood pH
CO2 reacts with water to produce H+ and HCO3- in a two-step process:

Ordinarily reaction ❶ proceeds very slowly, but an enzyme in red blood cells (carbonic anhydrase) catalyses the reaction. ❷ does not require a catalyst and occurs rapidly. We can combine the two reactions into one to produce an equation that you should commit to memory:

Deoxygenated haemoglobin (deoxy-haemoglobin) has a high affinity for H+ and by mopping protons up is able to drive the reaction to the right. This allows deoxygenated blood to carry more CO2 than arterial blood. Any H+ not bound to deoxy-haemoglobin will influence the pH of blood, which should remain at about 7.4 at all times. Any change to blood pH profoundly affects every process in the body.

CO2 transport by haemoglobin
CO2 reacts with the amino (-NH2) terminal regions of proteins to form carbamino compounds. The most abundant protein in blood (at about 14 g/100 ml – another number worth committing to memory) is haemoglobin and about 20% of CO2 in blood is effectively carried by carbamino haemoglobin. The combination of the higher affinity of deoxy-haemoglobin for H+ and its increased proneness to carbamination together produce the Haldane Effect: deoxygenated blood can carry more CO2.

CO2 content
CO2 isn’t carrier mediated like O2, so blood doesn’t saturate with CO2. The relationship between PaCO2 (kPa) and CO2 content (ml/l) is hyperbolic (Figure 1). Within the usual range of PaCO2, the curve is roughly linear, rather than changing sharply as the O2-haemoglobin dissociation curve does.
CO2 carbon dioxide transport

Figure 1: Because it’s not carrier mediated, CO2 transport doesn’t saturate. It’s also fairly linear in the physiological range (dotted line), unlike the very steep sigmoid O2 dissociation curve. Venous blood can carry more CO2 than arterial blood (Haldane Effect).

CO2 uptake from tissues and release in the lungs

CO2 carbon dioxide transport

Figure 2: Transport of CO2 from tissue to the lungs. ❶ About 7% of CO2 is dissolved in plasma; this is what measures. ❷ Most CO2 (70%) is transported in the form of biocarbonate ion (HCO3-) after the reaction with water molecules to form H2CO3 is accelerated by carbonic anhydrase (CA). ❸ The reaction to form HCO3- is further accelerated by the removal of H+ by deoxy-haemoglobin. ❹ The HC03- produced by this reaction cannot pass through the cell membrane and is transported by the Cl-/HCO3- exchanger. ❺ The remaining 23% of CO2 is transported as carbamino-haemoglobin. In the lungs, each process works in reverse leaving deoxy-haemoglobin free to bind to O2.

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