Physiology
Respiratory
Regarding carbon dioxide transport in the blood, which of the following statements is CORRECT:
Answer:
CO2 generated in the tissues and water combine to form carbonic acid which readily dissociates to form HCO3- and H+. The first part of this reaction is very slow in plasma, but is accelerated dramatically by the enzyme carbonic anhydrase present in red blood cells. Bicarbonate is therefore formed preferentially in red cells, from which it freely diffuses down its concentration gradient into plasma where it is transported to the lungs. The red cell membrane is impermeable to H+ ions which remain in the cell. To maintain electroneutrality, Cl- ions diffuse into the cell to replace HCO3-, an effect known as the chloride shift. Deoxygenated haemoglobin acts as a buffer for H+, allowing the reaction to continue.Carbon Dioxide Transport
Physiology / Respiratory / Gas Transport in Circulation
Last Updated: 26th July 2024
Carbon dioxide is transported in the blood from tissues to the lungs in three ways as bicarbonate ions, as carbamino compounds with proteins or simply dissolved in plasma.
Bicarbonate Ions
About 60% of CO2 is transported in the form of bicarbonate ions.
CO2 generated in the tissues and water combine to form carbonic acid which readily dissociates to form HCO3- and H+. The first part of this reaction is very slow in plasma, but is accelerated dramatically by the enzyme carbonic anhydrase present in red blood cells. Bicarbonate is therefore formed preferentially in red cells, from which it freely diffuses down its concentration gradient into plasma where it is transported to the lungs. The red cell membrane is impermeable to H+ ions which remain in the cell. To maintain electroneutrality, Cl- ions diffuse into the cell to replace HCO3-, an effect known as the chloride shift. Deoxygenated haemoglobin acts as a buffer for H+, allowing the reaction to continue.
In the lungs, all of the above reactions occur in reverse. Oxygenated haemoglobin does not bind H+ as well as it is more acidic and so in the lungs H+ dissociates from haemoglobin and shifts the CO2-HCO3- equation to the left, assisting CO2 unloading from the blood. This contributes to the Haldane effect which states that for any given PCO2, the CO2 content of deoxygenated blood is greater than that of oxygenated blood.
Carbamino Compounds
About 30% of CO2 is transported as carbamino compounds. CO2 combines rapidly with terminal amine groups on proteins to form carbamino compounds, primarily with haemoglobin forming carbaminohaemoglobin. Reduced haemoglobin forms carbamino compounds more readily than oxygenated haemoglobin and this also contributes to the Haldane effect.
Dissolved Carbon Dioxide
CO2 is 20 times more soluble than O2 in plasma and about 10% of CO2 is transported dissolved in solution.
Transport of Carbon Dioxide. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
