What are the major transport mechanisms for CO2? Explain.

Carbon dioxide travels in three principal forms; on average 60–70 % moves as bicarbonate ions, 20–30 % as carbamino‑compounds, and 5–10 % simply dissolves in plasma.

Bicarbonate route

• • CO₂ diffuses from tissues into red blood cells and, under carbonic anhydrase, hydrates to carbonic acid; this instantly dissociates into HCO₃⁻ and H⁺.

  • HCO₃⁻ exits the erythrocyte via the Band 3 Cl⁻/HCO₃⁻ exchanger, while Cl⁻ enters (chloride shift), maintaining electroneutrality; the reverse shift in pulmonary capillaries reforms CO₂ for exhalation

  • Proton buffering by deoxy‑hemoglobin prevents large pH swings and drives further CO₂ uptake; oxygenation in lungs releases H⁺, facilitating CO₂ release (Haldane effect).

Carbamino‑compound route

• • CO₂ binds reversibly to terminal α‑amino groups of globin chains, forming carbaminohemoglobin and minor carbamino‑proteins in plasma.

  • Affinity rises when O₂ tension falls, so venous blood carries more carbamino‑CO₂ than arterial—another facet of the Haldane effect that augments total CO₂ carriage from active tissues.

  • Unloading in the lungs is rapid because O₂ binding shifts hemoglobin to its R‑state, displacing CO₂ back to plasma and finally to alveolar air.

Dissolved CO₂ route

• • Henry’s law allows a small but physiologically important fraction of CO₂ to remain in true solution; its partial pressure gradient (≈45 mm Hg in tissues to ≈40 mm Hg in alveoli) drives bulk diffusion.

  • Although only 5–10 % of total load, this fraction is crucial for minute‑to‑minute regulation because chemoreceptors monitor arterial pCO₂, not total CO₂ content.

Integrated significance

• • Rapid enzymatic conversion, chloride exchange, hemoglobin buffering, and the Haldane‑Bohr interplay cooperatively raise the CO₂‑carrying capacity of blood roughly twentyfold above simple dissolution alone—ensuring efficient clearance of ~200 mL CO₂ produced per minute at rest.