The mechanism of breathing

Why does air move?

Pull the handle of a bicycle pump out. The space inside grows, the pressure falls, and air rushes in through the nozzle. Push it in and the air goes out again.

Air always flows from the place of higher pressure to the place of lower pressure. At a fixed temperature, a gas that is given more space has a lower pressure. Your chest uses this idea. It changes its volume, the lungs change with it, and the pressure inside the lungs changes. Then the air flows.

The pressure inside the lungs is the intrapulmonary pressure. It is compared all the time with the pressure of the air outside, the atmospheric pressure.

Inspiration, breathing in

1. The diaphragm contracts and flattens. This increases the volume of the thoracic chamber. NCERT says that it is in the antero-posterior axis. In plain words, the floor of the chamber moves down.
2. The external intercostal muscles contract. They lift the ribs and the sternum, and this increases the volume of the chamber in the dorso-ventral axis, from the back to the front.
3. The volume of the lungs rises with the volume of the chest.
4. The intrapulmonary pressure falls below the atmospheric pressure. Air flows from outside into the lungs.

Expiration, breathing out

1. The diaphragm and the external intercostal muscles relax.
2. The diaphragm goes back to its dome shape. The ribs and the sternum go back to their normal places.
3. The volume of the chest and the lungs falls.
4. The intrapulmonary pressure rises slightly above the atmospheric pressure. Air flows out of the lungs.

The pressure differences are small. The change is only a few millimetres of mercury, but it is enough to move the air.

Quiet and forceful breathing

Quiet expiration needs no muscle to contract. The muscles only relax. We can also make the breaths stronger. We do this with the help of additional muscles in the abdomen.

How fast do we breathe?

A healthy human breathes 12 to 16 times a minute. Each breath moves about 500 mL of air, so in a minute the lungs move about 6000 to 8000 mL. A spirometer measures these volumes. We study it in the next lesson.

NCERT prints 12 to 16 breaths a minute. Other books give a slightly wider range for a resting adult: 12 to 18 in OpenStax and 12 to 20 in clinical books. The rate is not one fixed number, and it rises with exercise, fever and height.

You should find that the air moved in a minute grows when the rate grows, and also when each breath grows. The pressure words change at the top and the bottom of the curve.

Try it in Python

The program reads a set of volumes taken every half second and tells you the phase of the breath, the tidal volume and the rate.

Practise with new numbers

Common mistakes

• Saying that the lungs suck the air in. The lungs have no muscle. They follow the chest, and the pressure difference moves the air.
• Saying that the diaphragm relaxes in inspiration. It contracts in inspiration and relaxes in expiration.
• Saying that the pressure in the lungs rises in inspiration. It falls below atmospheric pressure.
• Thinking that quiet expiration needs muscle work. It is the relaxation of the muscles.
• Forgetting the abdominal muscles. They are the extra muscles for stronger breathing.

Remember these

• Inspiration: diaphragm contracts, external intercostals contract, chest volume rises, lung pressure falls below atmospheric.
• Expiration: both relax, chest volume falls, lung pressure rises above atmospheric.
• Air flows from higher pressure to lower pressure.
• A healthy human breathes 12 to 16 times a minute and moves 6000 to 8000 mL of air a minute.

Check yourself

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

1. Which muscles contract in a quiet inspiration?

2. What happens to the pressure in the lungs in inspiration, and why does that make the air move?

3. Why is quiet expiration called a passive process?

4. What helps us to breathe more strongly than usual?

5. A woman breathes 15 times a minute and each breath moves 500 mL. How much air is moved in a minute?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• OpenStax (2022). Anatomy and Physiology 2e, chapter 22, The Respiratory System, section 22.3, The Process of Breathing.