8.2.3—Plasma role in CO2 transport

Carbon dioxide from respiring tissues reaches the blood in three forms. Most is converted inside red blood cells and then carried in plasma as hydrogencarbonate ions; smaller amounts remain dissolved in plasma or bind to haemoglobin as carbaminohaemoglobin.

• Dissolved CO2 — a small fraction (about 5% in the SME comparison): CO2 remains in solution in plasma → it can be carried directly in the blood, but this is not the main transport form.
• Carbaminohaemoglobin — a smaller bound fraction (about 10%): CO2 binds to haemoglobin in red blood cells → this carries CO2 in a reversible haemoglobin-bound form.
• Hydrogencarbonate — the major fraction (about 85%): tissue CO2 diffuses into a red blood cell → carbonic anhydrase catalyses reaction with water and carbonic acid dissociates → HCO3− leaves the red cell and enters plasma → plasma carries it in solution towards the lungs.

Division of labour: red blood cells provide the rapid conversion site and haemoglobin binding; plasma provides the main fluid route for hydrogencarbonate. At the lungs, the transport pathway is reversed so carbon dioxide can be regenerated and removed.

Boundary: the detailed HCO3−/Cl− membrane exchange belongs to the chloride-shift card, and CO2/H+ effects on haemoglobin oxygen affinity belong to the Bohr-shift card.

Do not say that all carbon dioxide is dissolved molecular CO2 in plasma or that plasma formed the hydrogencarbonate without red-cell involvement. The percentages are approximate comparison values from the SME note; no image generated or bound.