Measure lung volumes (spirometry)

Read a spirometer's volumes and add the right ones together to find a person's vital capacity.

Builds on (2 levels back)inferred · high confidence

Reading volumes in liters: Spirometry numbers are volumes in liters; you must read and compare them before combining them.
Adding measured quantities: Vital capacity is a sum of three separate volumes, so you must add measured amounts correctly.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

Vital capacity is the largest amount of air you can move in one breath. You find it by adding inspiratory reserve volume + tidal volume + expiratory reserve volume.

Step 1: Define the volumes

Tidal volume (TV) is one normal breath. Inspiratory reserve volume (IRV) is the extra air you can still breathe in after a normal breath in. Expiratory reserve volume (ERV) is the extra air you can still push out after a normal breath out.

Step 2: State the rule

Vital capacity (VC) = IRV + TV + ERV: the most air you can move in one breath. It does NOT include residual volume, the air that always stays in the lungs.

Step 3: Do the addition

Read the three volumes off the spirometer, line up the liters, and add them. The sum is the vital capacity you report.

Practice

A spirometry report lists these volumes. Using the rule Vital Capacity = IRV + TV + ERV, what is this person's vital capacity?

Reviewed

Volume	Value
Tidal volume (TV)	0.5 L
Inspiratory reserve volume (IRV)	3.0 L
Expiratory reserve volume (ERV)	1.1 L

A.3.5 L
B.4.1 L
C.4.6 L
D.5.1 L

Show the worked solution ▾

Answer: C. 4.6 L

Step 1: List the three volumes: From the table: TV = 0.5 L, IRV = 3.0 L, ERV = 1.1 L.
Step 2: Add them: VC = IRV + TV + ERV = 3.0 + 0.5 + 1.1 = 4.6 L.

Why it's right: Adding the three breathing volumes (3.0 + 0.5 + 1.1) gives 4.6 L, which is the vital capacity.

Why the others miss:

A: 3.5 L only adds TV and IRV and leaves out the expiratory reserve volume.
B: 4.1 L only adds IRV and ERV and leaves out the tidal volume.
D: 5.1 L adds an extra liter that is not in the table (a residual volume is not part of vital capacity).

Aligned to Human Body Systems: vital capacity calculation · reading level ~grade 9

Where you'd see this

A clinic chart shows TV, IRV, and ERV separately; the therapist sums them to report one vital capacity number for the doctor.

Careers that use this

Pulmonologist: Compares a patient's measured vital capacity to expected values to detect restricted breathing.

Video library

Watch: Measure lung volumes (spirometry)

Respiratory | Spirometry: Lung Volumes & Capacities

Ninja Nerd · ~22 min

Guided notes

Fill these in as you work through the lesson.

Big idea: A spirometer measures how much air you move; vital capacity is the largest amount you can move in one breath, found by adding three smaller volumes.

Key terms: write the meaning

Tidal volume (the amount in one normal, relaxed breath):
Vital capacity (the most air you can move in one breath):
Inspiratory reserve volume (extra air you can pull in after a normal breath in):
Expiratory reserve volume (extra air you can push out after a normal breath out):

The rule

Vital capacity = + + (add the three breathing volumes a spirometer measures).

Check yourself

Which single volume is the air in one calm, normal breath?
Vital capacity is the sum of which three volumes?
If you only knew tidal volume, why could you not yet report vital capacity?

Work one example

A spirometer reads tidal volume = 0.5 L, inspiratory reserve volume = 3.0 L, and expiratory reserve volume = 1.1 L. Add the three volumes to find this person's vital capacity, and show your addition.