Canonical Question
Gas Transport – CO2
Master answer
CO2
- End-product of aerobic metabolism
- Produced almost entirely in mitochondria
- Normal production approx. 200ml/min
- Total CO2 in blood
- Venous 52mL/dL
- Arterial 48mL/dL
CO2 Carriage in 4 forms:
| Arterial % | % of A-V diff | Details | |
|---|---|---|---|
| 1. Bicarbonate | 90 | 60 | Main form of CO2 delivery in blood Via Hendersen-hasselbach: CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3– Process 1. CO2 diffuses into RBCs 2. Carbonic anhydrase converts it to Bicarb 3. Hydrogen ions are buffered by binding to Hb molecules inside RBCs (30% Haldane.) Higher percentage in venous blood as: • Deoxygenated pKa 8.2 • Oxygenated pKa 6.6 • decreased pH = greater increase in [A-]/[HA] of DeoxyHb rel to OxyHb thus greater ability to accept protons 4. Bicarb is transferred out of cell in exchange for Cl ion by an exchange transporter called Band 3. (Hamburger Shift) |
| 2. Carbamino compounds | 5 | 30 | CO2 able to bind amino end of proteins Hb largely responsible as in much greater concentrations than any other protein (15 g/dL vs 7 g/dL) in serum CO2 combines with terminal amine to form carbamic acid dissosciates → at physiological pH to carbamate Affinity of Hb for CO2 increases in venous blood via Conformational change (70% Haldane): • Unbinding of O2 causes a conformational change in the beta chain • CO2 binding site at the N-terminus moves 1nm • Affinity for CO2 is increased. • Deoxy-Hb is 2-3x better at forming carbamino compounds than oxy-Hb |
| 3. Dissolved | 5 | 10 | CO2 ~20 x more soluble than O2 thus via Henry’s Law more CO2 is dissolved PvCO2 ~46mmHg (PaCO2 40mmHg) |
| 4. Carbonic Acid | <1% | <1% | Carried as dissolved H2CO3 Conversion to CO2 via Hendersen-Hasselbach equation above has pKa 6.1 At physiological pH Carbonic acid is ~96% dissociated Reaction is catalysed by Carbonic Anhydrase Not present in plasma, present in high concentrations in erythrocytes and pulmonary capilaries |
Gladwin / Sakurai 2016
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2015A Q13 | Describe how carbon dioxide (CO2) is carried in the blood? | historical_member | — |
| 2020A Q01 | Describe the carriage of carbon dioxide in blood. | historical_member | — |
| 2012A Q06 | Describe the carriage of carbon dioxide (CO2) in blood | historical_member | — |
| 2018B Q05 | Describe the carriage of carbon dioxide (CO2) in the blood. | historical_member | — |