KS3 · Biology

Estimating lung volume

You cannot see the air in your lungs, so how do you measure it? Make your breath push water out of a bottle, then count how far the water drops.

Biology · Measuring lung volume

Air in, water out

View: Start. Showing 4 layers: Tank of water, Starting line, Upturned bottle, Tube

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Explore

Tap a stage to move the water level, or tap a part to see what it does.

Before breathing out: the water is at the starting line, so 0 lines have fallen. Note this line first.

Predict first: as someone breathes out into the tube, which way will the water level in the bottle move? Then step through the breath and check.

Biology · Measuring accurately

Getting 200 cm³ right

View: 1 · Pour. Showing 2 layers: Measuring cylinder, Pouring from a beaker

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Layers

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Switch view to follow the technique, or tap a part to see why it matters.

Pour from a beaker until the water almost reaches the mark.

Every line on the bottle comes from a measured 200 cm³. Tap a view, or tap a part of the picture.

Biology · Working it out

From lines to lung volume

In a practical like this one, the water level falls by 12 lines and each line is 200 cm³. Estimate the vital capacity, then use it to estimate the lung volume.

The water level fell by 12 lines, and each line is 200 cm³. Two jobs, one after the other: lines into vital capacity first, then vital capacity into lung volume.
missing step

Which line is step 2?

Vital capacity = 12 lines × 200 cm³ = 2,400 cm³Vital capacity = 12 lines + 200 cm³ = 212 cm³Vital capacity = 200 cm³ ÷ 12 lines = 16.7 cm³

Biology · What changes vital capacity?

Bigger breath or smaller breath?

Vital capacity is not the same for everyone. Decide who is likely to have a larger vital capacity and who a smaller one, then read the reason.

Still to sort

Likely larger vital capacity (0)

Something that pushes vital capacity up.

Where the line is: Only being physically fit pushes it up in this list.

Likely smaller vital capacity (0)

Something that pushes vital capacity down.

Where the line is: Age at either end of life, smoking and asthma all push it down.

5 of 5 still to sort.

Watch out: A smaller vital capacity on its own does not show that someone smokes. Age, fitness and asthma can all change it too.

Biology · How sure is the number?

What does “estimate” really mean?

A class counts how many lines the water fell, works out the vital capacity, and then uses it to work out lung volume. Their teacher says: “Both of those numbers are estimates.”

Which idea is closest to what you think “estimate” means here?

“An estimate is just a guess, so the number isn’t worth much.”“We counted the lines, so the vital capacity is exact.”“It’s a rough value from a real measurement that can be slightly off.”“Once we have the vital capacity, the lung volume worked from it is exact.”

How sure are you?

I'm sureI think soI'm guessing

“An estimate is just a guess, so the number isn’t worth much.”

This is a common misconception

Not quite. An estimate is an approximate (rough) value worked out without taking accurate measurements. You did measure something real, the lines the water fell, so it is not just a guess. It is a rough value, not a wild guess.

“We counted the lines, so the vital capacity is exact.”

This is a common misconception

Counting the lines is exact, but the reading it gives is not. The person may not have breathed in or out fully, the lines may not have been marked accurately, and the water level may end up between two lines. Any of these means the vital capacity can only be an estimate.

“It’s a rough value from a real measurement that can be slightly off.”

This matches the evidence

Yes. That is what an estimate is: approximate, not a wild guess, and not exact. The breath may not have been full, the lines may be slightly out, or the level may sit between two lines.

“Once we have the vital capacity, the lung volume worked from it is exact.”

This is a common misconception

No. The vital capacity was already an estimate, and vital capacity is only usually around 80% of lung volume: it differs from person to person. So the lung volume is an estimate too, on top of an estimate.

An estimate is an approximate value found without accurate measuring. It is not just a guess. The vital capacity is an estimate, and so is the lung volume worked out from it.

Predict, then check

You have seen what can go wrong. Now: can the method be improved?

A student marks the bottle in steps smaller than 200 cm³, so the lines are closer together. They also repeat the whole measurement several times and work out a mean. Would the estimate of vital capacity be better or worse?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Turn a breath into a number with a bottle, a tank and a tube, then see why the number is an estimate.

What you need to know

Vital capacity is the most air you can breathe out after breathing in as fully as you can. Some air always stays behind in your lungs.
In the bottle-and-tank apparatus, the air you breathe out takes the place of water in the upturned bottle, so the water level falls.
Vital capacity = number of lines the water falls × 200 cm³.
Vital capacity is usually around 80% of lung volume, so lung volume ≈ (vital capacity ÷ 80) × 100.
Both values are estimates: approximate, not guesses and not exact.

The big picture

You can measure the air you breathe out by letting it push water out of an upturned bottle. Count the lines the water falls and multiply by 200 cm³ to estimate your vital capacity. Then work out an estimate of lung volume. Both are estimates, and there are good reasons why, and good ways to improve them.

Key points

1Breathing out into the tube moves air into the bottle, which displaces water into the tank.

2Every line the water falls is 200 cm³ of air breathed out.

3Measure the 200 cm³ at eye level, and read from the bottom of the meniscus.

4Age, activity level, smoking and asthma all affect vital capacity, so a low reading alone does not show that someone smokes.

5Closer lines and a mean of repeats give a better estimate.

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Worked example

Problem

At the end of a breath out, the water level has fallen past 8 lines and sits exactly halfway between the 8th and 9th lines. Estimate the vital capacity.

⚠ Watch out

Counting the line the water ends at instead of how many lines it has fallen. That is why you note the starting line first: it is the fall that you multiply by 200 cm³.

🧠

Memory hook

Air in, water out. Lines fallen × 200 gives the breath. For the whole lung: divide by 80 to get 1%, then times 100 for the lot.

✓

Check yourself

In your own words: what does the number of lines the water falls tell you, and what do you do with it to estimate vital capacity?

Flashcards

(14)

What is vital capacity?

The maximum volume of air that can be breathed out after breathing in fully.

Why does some air stay in your lungs even after a full breath out?

Most of the air moves out, but some stays behind to stop the lungs from collapsing.

In the bottle-and-tank apparatus, why does the water level fall?

Air breathed out moves into the bottle and displaces some of the water into the tank.

How do you work out vital capacity from the apparatus?

Count the lines the water level has fallen by and multiply by 200 cm³.

What does “accurate” mean for a measurement?

The closer a measurement is to the true value, the more accurate it is.

How do you measure 200 cm³ of water accurately?

Eye level with the mark, looking from the side. Pour from a beaker until it almost reaches the mark, then trickle the rest in from a pipette down the inside wall.

What is a meniscus, and where do you read it?

The curved surface of a liquid. Read the volume from the bottom of the meniscus.

Which two factors reduce vital capacity?

Smoking and asthma.

Who has a smaller vital capacity than teenagers and adults, and who has a larger one?

Babies and elderly people have a smaller one. Physically fit people have a larger one.

A person has a low vital capacity. Does that show they smoke?

No. Age, activity level and asthma also change vital capacity.

What is an estimate?

An approximate (rough) value obtained without accurate measurements. It is not just a guess.

Give the three reasons a vital capacity reading may not be accurate.

The person may not have breathed in or out fully, the lines may not be marked accurately, and the water level may end between two lines.

How can you get a better estimate of vital capacity?

Mark the bottle in volumes smaller than 200 cm³ so the lines are closer, and repeat several times and calculate a mean.

How do you estimate lung volume from vital capacity?

Vital capacity is usually around 80% of lung volume: divide by 80 to find 1%, then multiply by 100.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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From lines to lung volume

Bigger breath or smaller breath?

What does “estimate” really mean?

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