Course 12.1Lesson 3 of 3

Respiratory volumes and capacities

A spirometer measures the volumes of air that move in and out of the lungs. There are four volumes (TV, IRV, ERV and RV), and the capacities are sums of them (IC, EC, FRC, VC and TLC). In an average adult the tidal volume is about 500 mL.

about 25 min1 simulation1 practice question

Lesson 3 of 3Show all lessons

Try the code and the simulations beside the lesson.

Counting the air of a breath

How much air do your lungs hold? There is no single number, because we can breathe in a little or a lot. Doctors use a spirometer to measure the air that a person breathes in and out. The graph that it draws is a spirogram. It helps in the clinical assessment of the lungs.

The four volumes

A spirogram, a graph of the volume of air in the lungs in millilitres against time. A wavy line first shows three quiet breaths between 2300 and 2800 millilitres. Then comes one deep breath in up to 5800 millilitres, a long forced breath out down to 1200 millilitres, and quiet breaths again. Brackets on the right mark the inspiratory reserve volume (2800 to 5800), the tidal volume (2300 to 2800), the expiratory reserve volume (1200 to 2300) and the residual volume (0 to 1200, shaded, not seen on a spirometer). Taller brackets mark the capacities: IC, EC, FRC, VC and TLC, each with its sum.

Volume	What it is	Average value
Tidal volume (TV)	Air breathed in or out in a normal quiet breath	about 500 mL
Inspiratory reserve volume (IRV)	Extra air that a forceful inspiration can take in, after a normal one	2500 to 3000 mL
Expiratory reserve volume (ERV)	Extra air that a forceful expiration can push out, after a normal one	1000 to 1100 mL
Residual volume (RV)	Air that stays in the lungs even after a forceful expiration	1100 to 1200 mL

Notice the words reserve and residual. Reserve air can be used when we need it. Residual air cannot be breathed out at all.

NCERT prints ranges for these volumes, as in the table. The sums in this course use one set of values, the ones that Guyton and Hall give for a young adult man: TV 500 mL, IRV 3000 mL, ERV 1100 mL and RV 1200 mL. These fall inside the NCERT ranges. Volumes depend on sex, age, height and fitness. Women have smaller volumes. OpenStax gives a total lung capacity of about 6 L for men and about 4.2 L for women.

The capacities

A capacity is a sum of two or more volumes.

Inspiratory capacity (IC) is the total air that can be breathed in after a normal expiration. IC = TV + IRV.
Expiratory capacity (EC) is the total air that can be breathed out after a normal inspiration. EC = TV + ERV.
Functional residual capacity (FRC) is the air that stays in the lungs after a normal expiration. FRC = ERV + RV.
Vital capacity (VC) is the largest volume of air that can be breathed out after the deepest breath in, or breathed in after the deepest breath out. VC = ERV + TV + IRV.
Total lung capacity (TLC) is the total air in the lungs after the deepest breath in. TLC = VC + RV, which is IRV + ERV + TV + RV.

A worked example

Take TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL and RV = 1200 mL.

IC = 500 + 3000 = 3500 mL.
EC = 500 + 1100 = 1600 mL.
FRC = 1100 + 1200 = 2300 mL.
VC = 500 + 3000 + 1100 = 4600 mL.
TLC = 4600 + 1200 = 5800 mL.

Check the other way: TLC = IC + FRC = 3500 + 2300 = 5800 mL. Both ways agree.

You should find that RV changes only FRC and TLC, and that the vital capacity does not move when RV moves.

Try it in Python

The program turns the four volumes of a person into the five capacities.

Practise with new numbers

Common mistakes

Putting the residual volume into the vital capacity. VC is what can be breathed out, and RV cannot be breathed out.
Mixing up ERV and EC. ERV is one volume. EC is TV + ERV.
Saying that FRC is IRV + RV. FRC is ERV + RV, the air left after a normal expiration.
Forgetting that the reserve volumes are extra air. TV is the normal breath, and IRV and ERV come after it.
Thinking that a spirometer can show the residual volume. That air never leaves the lungs, so the trace does not show it.

Remember these

TV about 500 mL. IRV 2500 to 3000 mL. ERV 1000 to 1100 mL. RV 1100 to 1200 mL.
IC = TV + IRV. EC = TV + ERV. FRC = ERV + RV.
VC = ERV + TV + IRV. TLC = VC + RV.
Breathing 12 to 16 times a minute with a tidal volume of 500 mL moves 6000 to 8000 mL a minute.

Check yourself

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

Define tidal volume, and give its average value.

Which volumes add up to the functional residual capacity?

Why is the residual volume not part of the vital capacity?

A person has TV = 500 mL, IRV = 2800 mL and ERV = 1000 mL. Find the vital capacity.

Write the total lung capacity in two different ways.

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.
OpenStax (2022). Anatomy and Physiology 2e, chapter 22, The Respiratory System, section 22.3, The Process of Breathing.