CO₂ transport in blood ◦ Bicarbonate and blood pH ◦ CO₂ transport by haemoglobin ◦ CO₂ content and PaCO₂ ◦ CO₂ uptake from tissues and release in the lungs

You might want to read the Gas basics


Overview
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 uptake from tissues and release in the lungs