Carbon Dioxide Transport, Control of Breathing and Respiratory Disorders: Quick Revision

A brief note on Carbon Dioxide Transport, Control of Breathing and Respiratory Disorders. Read it before a test. Try each question at the end before you open its answer.

Transport of carbon dioxide

• Bicarbonate: about 70 per cent. Carbamino-haemoglobin: 20 to 25 per cent. Dissolved in plasma: about 7 per cent.
• Carbonic anhydrase, in a high concentration in the red blood cells: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+$.
• Tissues, with a high pCO$_2$: the reaction goes forward and carbon dioxide binds to haemoglobin. Alveoli, with a low pCO$_2$: 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.

Regulation of respiration

• Respiratory rhythm centre: medulla, the main control.
• Pneumotaxic centre: pons, can shorten inspiration and change the rate.
• Chemosensitive area: next to the rhythm centre, highly sensitive to carbon dioxide and hydrogen ions.
• Receptors: aortic arch and carotid artery sense carbon dioxide and hydrogen ions.
• The role of oxygen is insignificant.

Disorders

• Asthma: wheezing, difficulty in breathing, inflammation of the bronchi and bronchioles.
• Emphysema: chronic, damaged alveolar walls, smaller respiratory surface. Cigarette smoking is a major cause.
• Occupational disorders: dust from grinding or stone-breaking, then inflammation and fibrosis. Protective masks.

A worked example

The tissues release 200 mL of carbon dioxide in a minute. How much travels as bicarbonate?

1. About 70 per cent travels as bicarbonate.
2. $200 \times 70 / 100 = 140$ mL.

Practice with new numbers

A program to try

The program finds the bicarbonate share for two totals.

Mistakes to avoid

• Putting the rhythm centre in the pons. It is in the medulla.
• Saying that oxygen is the main signal for breathing. Carbon dioxide and hydrogen ions are.
• Saying that emphysema inflames the bronchi. It damages the alveolar walls.
• Saying that most carbon dioxide travels on haemoglobin. Most travels as bicarbonate.

Check yourself

1. Which centre is the main control of the rhythm of breathing?

2. To which two substances is the chemosensitive area sensitive?

3. What is damaged in emphysema?

4. What share of carbon dioxide travels as bicarbonate?

Sources

• 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.