Updated: 7/21/2019

Carbon Dioxide Transport

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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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          • aldane 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.

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