Carbon Dioxide Transport Quiz

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Which form accounts for most of the carbon dioxide carried in venous blood?

A Carbon dioxide stored within red blood cells as carbonic acid
B Carbon dioxide dissolved in the fluid inside red blood cells
C Carbon dioxide transported as hydrogen carbonate ions in plasma ✓
D Carbon dioxide dissolved directly in plasma
E Carbon dioxide bound to haemoglobin as carbaminohaemoglobin

The answer is C: Carbon dioxide transported as hydrogen carbonate ions in plasma

Explanation

Why C is correct

See Carbon Dioxide in the Venous Blood. Option C is correct. Tissues make carbon dioxide all the time, and venous blood carries it back to the lungs in three ways. About 70 percent travels as hydrogen carbonate (bicarbonate) ions, about 20 percent is attached to haemoglobin, and about 7 to 10 percent is simply dissolved. The bicarbonate route works like this. Carbon dioxide diffuses into a red blood cell, where the enzyme carbonic anhydrase (which speeds up the reaction) combines it with water to make carbonic acid. The carbonic acid splits into hydrogen ions and hydrogen carbonate ions. Haemoglobin takes up the hydrogen ions, which stops the red cell becoming acidic, and the hydrogen carbonate leaves the cell in exchange for chloride, a swap called the chloride shift that keeps the electrical charge balanced. Most of the hydrogen carbonate is therefore carried in the plasma. In the lungs the reactions run backwards and carbon dioxide is released to be breathed out.

Bicarbonate: about 70 percent of the carbon dioxide.
Haemoglobin: about 20 percent attached.
Dissolved: about 7 to 10 percent.
Chloride shift: hydrogen carbonate leaves the red cell in exchange for chloride.

Why the other options are incorrect

A. Carbon dioxide stored within red blood cells as carbonic acid: Carbonic acid is only a brief intermediate in red cells: it splits almost at once into hydrogen ions and hydrogen carbonate. It is not a store that carries a large share of the carbon dioxide. The hydrogen carbonate it forms is what accounts for most of the transport.
B. Carbon dioxide dissolved in the fluid inside red blood cells: Dissolved carbon dioxide, whether in plasma or inside red cells, makes up only a small fraction of the total. Most of the carbon dioxide that enters a red cell is quickly converted to hydrogen carbonate. Dissolved gas would not be able to carry the volume of carbon dioxide made by the tissues.
D. Carbon dioxide dissolved directly in plasma: Only a small fraction of carbon dioxide, around 7 to 10 percent, travels simply dissolved in plasma. Although carbon dioxide is more soluble than oxygen, this route cannot carry the large amounts the tissues produce. It would matter if the question asked about the smallest transport route.
E. Carbon dioxide bound to haemoglobin as carbaminohaemoglobin: About 20 percent of carbon dioxide is carried bound to the globin part of haemoglobin as carbaminohaemoglobin. This is an important second route but not the main one. It would be the answer to a question asking for the second most common form.

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1
Which statement best describes how bicarbonate helps to buffer acids in the plasma?
2
Which process allows carbon dioxide to continue being converted to hydrogen carbonate ions within red blood cells?

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