Regulation of respiration

Who sets the pace?

Run up a flight of stairs. Your breathing gets faster and deeper, and you did not decide it. When you are asleep, you still breathe. Something in the body sets the pace of breathing and keeps it matched to what the body needs.

The centres of the brain

• Respiratory rhythm centre. It lies in the medulla region of the brain. It is primarily responsible for the regulation of respiration.
• Pneumotaxic centre. It lies in the pons region of the brain. It can moderate the work of the rhythm centre. A signal from it can reduce the duration of inspiration, and this changes the rate of breathing.
• Chemosensitive area. It lies next to the rhythm centre. It is highly sensitive to carbon dioxide and hydrogen ions. When their level rises, this area is activated. It signals the rhythm centre to make changes in the breathing, so that these substances are got rid of.

Receptors outside the brain

Receptors in the aortic arch and the carotid artery also recognise changes in the carbon dioxide and hydrogen ion concentrations of the blood. They send signals to the rhythm centre, and the rhythm centre acts to correct the change.

The role of oxygen

The role of oxygen in the regulation of the respiratory rhythm is quite insignificant. The main signal is carbon dioxide, with hydrogen ions. This is why a person who holds the breath feels the need to breathe because of the carbon dioxide that builds up, and not mainly because of a lack of oxygen. NCERT is describing the usual conditions. At great heights the pO$_2$ of the blood can fall below about 60 mm Hg. Then the receptors of the carotid and aortic bodies do respond to oxygen, and they raise the breathing.

A feedback loop

1. The body makes more carbon dioxide, for example in exercise.
2. The carbon dioxide and hydrogen ion level of the blood rises.
3. The chemosensitive area and the receptors of the aortic arch and carotid artery sense it.
4. The rhythm centre sends signals to the diaphragm and the intercostal muscles. Breathing becomes faster and deeper.
5. More carbon dioxide leaves the body, and the level falls back.

This is a feedback loop. The result of the action switches off the cause.

You should find that a small rise in the pCO$_2$ raises the air moved in a minute a great deal, because both the rate and the size of each breath grow.

Try it in Python

The program follows the feedback loop in a model. The carbon dioxide starts high, the breathing speeds up, and the carbon dioxide falls back.

Practise with new numbers

Common mistakes

• Putting the respiratory rhythm centre in the pons. It is in the medulla. The pneumotaxic centre is in the pons.
• Saying that the pneumotaxic centre lengthens inspiration. It can shorten it.
• Saying that oxygen is the main signal. Its role is insignificant. Carbon dioxide and hydrogen ions are the main signal.
• Saying that the chemosensitive area is far from the rhythm centre. It lies next to it.
• Forgetting the receptors outside the brain. The aortic arch and the carotid artery have them.

Remember these

• Rhythm centre: medulla, the main control. Pneumotaxic centre: pons, shortens inspiration and changes the rate.
• Chemosensitive area: next to the rhythm centre, sensitive to carbon dioxide and hydrogen ions.
• Receptors in the aortic arch and the carotid artery also sense carbon dioxide and hydrogen ions.
• The role of oxygen is insignificant.

Check yourself

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

1. Where is the respiratory rhythm centre?

2. What does the pneumotaxic centre do?

3. To which two substances is the chemosensitive area highly sensitive?

4. Where, outside the brain, are there receptors that sense these changes?

5. Why do you breathe faster after a run?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• West J. B. and Luks A. M. (2021). West's Respiratory Physiology: The Essentials, 11th edition, Wolters Kluwer, chapter Control of Ventilation.