Most carbon dioxide (CO₂) in the blood is transported in two main forms: dissolved in plasma and converted into bicarbonate ions (HCO₃⁻) within red blood cells (RBCs).
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Dissolved in Plasma: A small portion of carbon dioxide is directly dissolved in the plasma, which is the liquid component of blood.
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Bicarbonate Formation: The majority of carbon dioxide is transported in the form of bicarbonate ions. When carbon dioxide enters the bloodstream from tissues, it primarily reacts with water (H₂O) to form carbonic acid (H₂CO₃). This reaction is catalyzed by the enzyme carbonic anhydrase, which is found in red blood cells.
The chemical equation that represents this reaction is as follows:
\[ \text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \]
Carbonic acid is unstable and quickly dissociates into bicarbonate ions and hydrogen ions:
\[ \text{H}_2\text{CO}_3 \leftrightarrow \text{HCO}_3^- + \text{H}^+ \]
In this process:
- Carbon dioxide combines with water to form carbonic acid.
- Carbonic acid then dissociates into bicarbonate ions and hydrogen ions.
After this transformation, bicarbonate ions enter the plasma, where they can be transported to the lungs. In the lungs, the process is reversed: bicarbonate ions re-enter the RBCs, combining with hydrogen ions to form carbonic acid, which then converts back to carbon dioxide and water. The carbon dioxide is then expelled from the body during exhalation.
In summary, the transportation of carbon dioxide in the blood involves a combination of direct dissolution in plasma and conversion into bicarbonate by the enzyme carbonic anhydrase, facilitating the efficient transport of CO₂ from tissues to the lungs.
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