Reading and using spirograms

Why this course goes beyond the textbook

The textbook gives the volumes and the capacities. Tests go one step further. They show a graph and ask you to read it, they change a number and ask what follows, and they ask you to reason about a disease. This lesson teaches that reading. Where a fact is beyond the textbook, the lesson says so.

Reading a spirogram

A spirogram has levels. Every volume is a difference between two levels, so subtract one level from another.

1. Find the four levels: the lowest after a forced expiration, the two ends of a quiet breath, and the highest after the deepest breath in.
2. The tidal volume is the height of one quiet breath.
3. The expiratory reserve volume is the drop from the end of a quiet expiration to the lowest level.
4. The inspiratory reserve volume is the rise from the end of a quiet inspiration to the highest level.
5. The vital capacity is the highest level minus the lowest level.

If the levels are measured from an empty lung, then the lowest level is the residual volume, and the highest level is the total lung capacity.

A simple spirometer cannot show the residual volume, because that air never leaves the lungs. So it cannot show the functional residual capacity or the total lung capacity either. Doctors use other methods for these.

What a disease does to the picture

Look at the two subjects in the figure. Subject A has healthy lungs. Subject B has emphysema.

• The residual volume of B is much larger. In emphysema the walls of the alveoli are damaged, and the lungs lose their spring. Air is trapped.
• The vital capacity of B is smaller, because the air that is trapped cannot be used.
• The total lung capacity is about the same in this figure. In advanced emphysema it is often larger, because the lungs are over-inflated.
• The quiet breathing of B takes place at a higher level.

A useful number is the residual volume divided by the total lung capacity. In a healthy young adult it is about a fifth. In emphysema it is much larger. In fibrosis, where dust has stiffened the lungs, all the volumes and the total lung capacity are smaller, and the lungs cannot be filled to the usual level. These patterns are beyond the textbook, but they follow from the damage that the textbook describes.

Air moved and air that reaches the alveoli

The conducting part ends at the terminal bronchioles. At the end of an inspiration, the last air to enter stays in the conducting part and goes out again unchanged. This is the dead space, because no gas is exchanged in it. For an adult it is about 150 mL.

• Minute volume = rate $\times$ volume of each breath.
• Alveolar ventilation = rate $\times$ (volume of each breath $-$ dead space).

Two people can move the same air in a minute and still differ greatly in the air that reaches the alveoli. A fast and shallow breather wastes most of each breath on the dead space.

You should find that the pattern with the larger breaths has the larger alveolar ventilation, even when the minute volume is the same.

Try it in Python

The program compares three patterns of breathing that move the same air in a minute.

Practise with new numbers

Common mistakes

• Reading a level as a volume. A volume is a difference between two levels.
• Measuring the residual volume with a simple spirometer. The air stays in the lungs.
• Expecting the total lung capacity to rise in fibrosis. The stiff lungs hold less.
• Forgetting the dead space. The minute volume is not the air that reaches the alveoli.
• Thinking that a faster rate always gives more alveolar air. If the breaths become small, it can give less.

Remember these

• A volume is the difference between two levels. VC = highest level minus lowest level.
• Emphysema: larger residual volume, smaller vital capacity, trapped air.
• Fibrosis: stiff lungs, a smaller total lung capacity.
• Dead space is about 150 mL. Alveolar ventilation = rate $\times$ (volume of a breath $-$ dead space).

Check yourself

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

1. How do you find the vital capacity from the levels of a spirogram?

2. Why can a simple spirometer not show the residual volume?

3. How do the residual volume and the vital capacity change in emphysema?

4. What is the dead space?

5. A person breathes 20 times a minute, 400 mL each time. What is the alveolar ventilation?

References

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