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Updated: Jul 21 2019

Carbon Dioxide Transport

  • Overview
      • Forms of Carbon Dioxide (CO2) in the Blood
      • Dissolved CO2
      • Bicarbonate (HCO3-)
      • Carbaminohemoglobin
      • ~5-10% of total CO2 content
      • 70% of total CO2 content
      • 20-25% of total CO2 content
      • CO2 bound to N-terminus amino group of hemoglobin (Hb)
      • Stabilizes taut form of Hb → right-shift of O2-Hb dissociation curve (Bohr effect) → favors O2  release
    • Transport of CO2 in blood (peripheral tissues → RBC → lungs)
      • peripheral tissues
        • metabolically active tissues produce CO2, which diffuses across the cell membrane and eventually into the RBC at the level of the capllaries
      • RBC
        • carbaminohemoglobin
          • ↑ pCO2 is produced in peripheral tissues
            • this facilitates CO2 binding to Hb, forming carbaminohemoglobin (CO2-Hb)
          • in turn, the Taut (T) state of Hb is stabilized
            • Bohr effect
              • enhanced release of O2 in the presence of low pH or increased pCO2
                • CO2-Hb → ↓ Hb affinity for O2 → ↑ O2 unloading into tissues
                  • right shift in the hemoglobin-oxygen (Hb-O2) dissociation curve
                • H+ produced by converting CO2 + H2O to HCO3- + H+ via carbonic anhydrase (CA) which contributes in lowering the pH
                • protonated Hb stabilizes the T state, facilitating O2 release into tissues
                  • (oxyhemoglobin) HbO2 + H+ ↔ HbH + O2 (deoxyhemoglobin)
            • Haldane effect
              • at low pO2 levels, the carrying capacity of CO2 increases
                • in other words, there is an increase affinity for CO2 when there is less O2 bound to hemoglobin
        • bicarbonate (HCO3-)
          • CA reversibly catalyzes the formation of HCO3- from the hydration of CO2
          • this reaction is reversible, thus CA is also involved in the dehydration of H2CO3
            • CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
              • the produced H+ remains in the RBC, where it is buffered by deoxyhemoglobin
              • deoxyhemoglobin is a better H+ buffer than oxyhemoglobin
          • the produced HCO3- is transported into the plasma via a chloride-bicarbonate (Cl-HCO3) exchanger (AE1 or band three protein)
            • this describes the chloride (or Hamburger) shift
      • lung
        • the reactions mentioned above occur in reverse inside the lung
          • carbaminohemoglobin
            • Bohr effect
              • in lungs (low PCO2), CO2 dissociates from hemoglobin and stabilizes high O2 affinity Relaxed (R) state
                • hemoglobin-oxygen dissociation curve left shifts → ↑ hemoglobin affinity for O2 → ↑ O2 loading
            • Haldane effect
              • O2 loading → ↓ hemoglobin affinity for CO2 → ↑ CO2 unloading
          • bicarbonate
            • HCO3- is exchanged for Cl- (chloride shift) across RBC membranes
              • HCO3- enters RBCs
            • Haldane effect
              • O2 loading → ↓ hemoglobin affinity for H+→ ↑ H+ unloading
              • H+ + HCO3- → H2CO3 → CO2 + H2O
              • ↑ H+ drives equilibrium reactions to right (CO2 formation).
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