Course 12.3Lesson 1 of 3

Transport of carbon dioxide

Carbon dioxide travels in the blood in three forms: about 70 per cent as bicarbonate, 20 to 25 per cent bound to haemoglobin as carbamino-haemoglobin, and about 7 per cent dissolved in plasma. The enzyme carbonic anhydrase of the red blood cells works in both directions, forward in the tissues and backward in the alveoli. Each 100 mL of deoxygenated blood delivers about 4 mL of carbon dioxide to the alveoli.

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A fizzy drink

A bottle of a fizzy drink holds carbon dioxide dissolved in water. Open it and the gas escapes. Blood is mostly water too, but it cannot carry all of the carbon dioxide of the body in that way. The cells make carbon dioxide all the time, and it must be carried to the lungs. The blood has three ways to do it.

The three forms

Two drawings side by side, each showing a large red blood cell inside a capillary. On the left, a tissue capillary. An arrow brings carbon dioxide from the tissues into the cell. Inside, a column of steps runs downward: carbon dioxide plus water, then the enzyme carbonic anhydrase, then carbonic acid, then a hydrogen ion plus a bicarbonate ion. Below that, carbon dioxide plus haemoglobin gives carbamino-haemoglobin. A blue arrow takes bicarbonate out of the cell to the plasma and a green arrow brings a chloride ion in. A note says that about 70 per cent travels as bicarbonate, 20 to 25 per cent on haemoglobin and 7 per cent dissolved. On the right, an alveolar capillary: the same steps run upward in reverse, bicarbonate comes back in, chloride goes out, and carbon dioxide is released to the alveoli.

Form	Share	Where
Dissolved in plasma	about 7 per cent	plasma
Carbamino-haemoglobin	20 to 25 per cent in NCERT, 23 per cent in the sums	red blood cells
Bicarbonate, HCO $_3^-$	about 70 per cent	red blood cells and plasma

The three shares make up all of the carbon dioxide that the blood carries. Most of it travels as bicarbonate.

These shares are teaching values, and books do not agree on them. The carbamino share in particular runs from about 5 to about 35 per cent, depending on the book and the method. This is the share of the carbon dioxide that the blood takes up in the tissues. NCERT prints 20 to 25 per cent. The sums in this course use 23 per cent, which is inside the NCERT range, so that 70, 23 and 7 add up to 100. If you count the whole carbon dioxide content of blood and not just the part taken up in the tissues, bicarbonate is nearer 90 per cent. For an exam, learn the NCERT shares.

Carbamino-haemoglobin

Carbon dioxide can bind to haemoglobin. The compound is carbamino-haemoglobin. About 20 to 25 per cent of the carbon dioxide travels in this way.

The binding depends on the partial pressure of carbon dioxide. The partial pressure of oxygen is also a major factor.

In the tissues the pCO $_2$ is high and the pO $_2$ is low. More carbon dioxide binds to haemoglobin.
In the alveoli the pCO $_2$ is low and the pO $_2$ is high. Carbon dioxide comes off haemoglobin and is delivered to the alveoli.

Bicarbonate and carbonic anhydrase

The red blood cells hold a very high concentration of an enzyme, carbonic anhydrase. Plasma has only minute quantities of it. The enzyme helps this reaction go in both directions.

\displaystyle \text{CO}_2 + \text{H}_2\text{O} \;\underset{\text{carbonic anhydrase}}{\rightleftharpoons}\; \text{H}_2\text{CO}_3 \;\rightleftharpoons\; \text{HCO}_3^- + \text{H}^+

At the tissues the pCO $_2$ is high, because the cells make carbon dioxide. The carbon dioxide enters the blood and goes forward. It forms carbonic acid, and then bicarbonate and hydrogen ions.
At the alveoli the pCO $_2$ is low. The reaction goes backward. Carbon dioxide and water are formed again, and the carbon dioxide is released.

So the carbon dioxide that was trapped as bicarbonate in the tissues is carried to the alveoli and let out as carbon dioxide.

The chloride shift

Bicarbonate ions are made inside the red blood cell. Most of them move out into the plasma. To keep the electric charges balanced, chloride ions move into the cell in exchange. This movement of chloride ions is called the chloride shift. In the alveoli the movement goes the other way. The hydrogen ions are taken up by haemoglobin in the tissues, so the blood does not become too acid. These details are beyond the textbook, but they are asked in tests.

How much does the blood deliver?

Every 100 mL of deoxygenated blood delivers about 4 mL of carbon dioxide to the alveoli. Compare this with the 5 mL of oxygen that each 100 mL of oxygenated blood gives to the tissues.

You should find that a person at rest, with 5000 mL of blood a minute passing the tissues, releases about 200 mL of carbon dioxide a minute. About 140 mL of it goes as bicarbonate.

Try it in Python

The program splits the carbon dioxide released in a minute into its three forms.

Practise with new numbers

Common mistakes

Saying that most carbon dioxide travels on haemoglobin. Most of it, about 70 per cent, travels as bicarbonate.
Saying that carbonic anhydrase works only in one direction. It helps the reaction go both ways.
Putting carbonic anhydrase mainly in the plasma. The red blood cells hold a very high concentration of it.
Forgetting that low pO $_2$ and high pCO $_2$ favour carbamino-haemoglobin. That is the condition of the tissues.
Mixing up the carbon dioxide and oxygen figures. The blood gives up 5 mL of oxygen and takes up 4 mL of carbon dioxide for each 100 mL.

Remember these

Carbon dioxide: about 70 per cent as bicarbonate, 20 to 25 per cent as carbamino-haemoglobin, about 7 per cent dissolved in plasma.
Carbonic anhydrase is in the red blood cells. It makes the reaction $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+$ go in both directions.
Tissues: the reaction goes forward and more carbon dioxide binds to haemoglobin. Alveoli: it goes backward, and carbon dioxide is released.
Chloride shift: chloride moves into the red blood cell as bicarbonate moves out.
Every 100 mL of deoxygenated blood delivers about 4 mL of carbon dioxide to the alveoli.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

Name the three forms in which the blood carries carbon dioxide, with their shares.

Which enzyme helps the reaction of carbon dioxide and water, and where is it found in a high concentration?

What happens to the reaction of carbonic anhydrase in the alveoli?

What conditions favour the binding of carbon dioxide to haemoglobin?

How much carbon dioxide does 100 mL of deoxygenated blood deliver to the alveoli?

References

NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
Geers C. and Gros G. (2000). Carbon dioxide transport and carbonic anhydrase in blood and muscle. Physiological Reviews, 80(2), 681-715. DOI 10.1152/physrev.2000.80.2.681.
Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids.