In the alveoli gas exchange context (alveolar $p_{a}O_2 = 13.0\,\text{kPa}$ and pulmonary capillary points A $11.7\,\text{kPa}$, B $11.4\,\text{kPa}$, C $5.7\,\text{kPa}$), what are two forms in which carbon dioxide is transported to the lungs for excretion?
Diagram showing alveolus and pulmonary capillary with indicators:
- alveolus: paO2 = 13.0 kPa
- pulmonary capillary:
- A: paO2 = 11.7 kPa
- B: paO2 = 11.4 kPa
- C: paO2 = 5.7 kPa
Carbon dioxide is transported to the lungs mainly as hydrogen carbonate (bicarbonate) ions, $HCO_3^-$, in the plasma. It is also carried bound to haemoglobin as carbaminohaemoglobin (carbamino compounds).
What the question is really asking
Even though the diagram focuses on oxygen partial pressures, this part switches to carbon dioxide removal. You just need to name two chemical or physical forms in which $CO_2$ is carried in the blood on the way to the lungs.
Two correct transport forms of $CO_2$
Common accepted forms include:
-
As bicarbonate (hydrogen carbonate) ions in plasma: Most $CO_2$ reacts in red blood cells and ends up transported in the blood as $HCO_3^-$.
-
Bound to haemoglobin (carbaminohaemoglobin): Some $CO_2$ attaches to haemoglobin (not the same binding site as oxygen), forming carbamino compounds.
A third form you might also see
- Dissolved $CO_2$ in plasma: a small fraction travels simply dissolved in the blood.
Any two of these three typically earn full credit, but the first two are the most often expected.
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