investigating breathing

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Edexcel practical materials created by Salters-Nueld Advanced Biology, ©University o York Science Education Group.

Practical 7.2 Investigating breathing

Using a spirometerThe apparatus shown below is a spirometer. Spirometers allow us to study both breathing

and respiration. In this activity you will learn how a spirometer works and how to interpret

the spirometer trace that is produced.

A spirometer

The general principle behind a spirometer is simple. It is eectively a tank o water with

an air-lled chamber suspended in the water. It is set up so that adding air to the chamber

makes the lid o the chamber rise in the water, and removing air makes it all. Movements

o the chamber are recorded using either a kymograph (pen writing on a rotating drum), a

chart recorder, computer or datalogger.

• Toinvestigatelungvolumes

andrateofbreathing.

Purpose

Use eye protection when handling soda lime.

Soda lime is corrosive. Do not handle directly: use a spatula.

A spirometer should only be used with supervision. I you have

breathing or circulation (heart) problems or suer rom epilepsy

you should not use the spirometer. Read the manuacturer’s

instructions and saety notes beore using the equipment.

Stop using the spirometer at once i you experience any unusual

breathing problems or eel dizzy or uncomortable.

(Asthmatics may use a spirometer i they are otherwise in goodhealth.)

A trained member o sta should use an oxygen cylinder to fll

the spirometer.

Safety

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Practical 7.2 (cont.) Investigating breathing

Tubes run rom the chamber to a mouthpiece and back again. Breathing in and out throughthe tubes makes the lid o the chamber all and rise. The volume o air the person inhales and

exhales can be calculated rom the distance the lid moves.

The apparatus can be calibrated so that the movement o the lid corresponds to a given

volume. A canister containing soda lime is inserted between the mouthpiece and the foating

chamber. This absorbs the CO2 that the subject exhales. In which direction will the pen move

when the subject inhales?

Procedure

Calibration

In order to interpret the spirometer trace you need to know what both the vertical and the

horizontal scales represent.

Finding the vertical scale

The vertical scale measures the volume o air in the chamber. The spirometer’s foating-chamber lid has markings on it showing how much gas it contains.

1 First, empty the chamber completely and, i using a kymograph, make a mark on the

paper while it is stationary, to show where the pen lies when there is no gas in the tank.

Then orce a known volume o air into the tank (e.g. 500 cm3) and make a second mark

on the kymograph trace.

2 Repeat this procedure until the chamber has been completely lled with air. I using a

kymograph, i the trace is too large or too small, the length o the arm supporting the pen

can be adjusted so that the trace ts onto the paper.

3 Write the values next to your calibrating marks – they will help with interpretation o the

trace later. Once the marks have been made on the paper it should be possible to count

how many squares on the trace represent 1 dm3.

Finding the horizontal scale

4 On most kymographs there is a switch allowing you to set the speed at which the drum turns.

Choose a speed close to 1 mm per second. This is your horizontal scale. Make a note o the

speed on your trace, so that the trace can be interpreted once the experiment is complete.

Collecting data on breathing5 Ater calibration, the spirometer is lled with oxygen. A disinected mouthpiece

is attached to the tube, with the tap positioned so that the mouthpiece is connected to

the outside air. The subject to be tested puts a nose clip on, places the mouthpiece in

their mouth and breathes the outside air until they are comortable with breathing

through the tube.

• Disinfectantsolution

• Eyeprotection

Youwillneed:• Spirometer

• Kymograph,chartrecorderorcomputer

• Sodalime(forthespirometercanister)

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6Switch on the recording apparatus and at the end o an exhaled breath turn the tap sothat the mouthpiece is connected to the spirometer chamber. The trace will move down as

the person breathes in. Ater breathing normally the subject should take as deep a breath

as possible and then exhale as much air as possible beore returning to normal breathing.

A sketch of a trace showing normal breathing and one forced breath in and out

A diagram o a spirometer trace is shown above. In this example the subject has breathed inand out normally three times, then taken as deep a breath in as possible, then orced the air

rom their lungs. Several pieces o inormation about the subject’s breathing can be read o

this kind o trace, or worked out rom it.

• Thetidalvolumeisthevolumeofairbreathedinandoutinonebreathatrest.Thetidal

volume or most adults is only about 0.5 dm3.

• Vitalcapacityisthemaximumvolumeofairthatcanbebreathedinoroutofthelungsin

one orced breath.

• Breathingrateisthenumberofbreathstakenperminute.

• Minuteventilationisthevolumeofairbreathedinto(andoutof)thelungsinoneminute.

Minute ventilation 5 tidal volume 3 rate o breathing (measured in number o breaths

per minute).

Some air (about 1 dm3) always remains in the lungs as residual air and cannot be breathed

out. Residual air prevents the walls o the bronchioles and alveoli rom sticking together. Any

air breathed in mixes with this residual air.

Collecting data on rate of respirationEach time we take a breath, some oxygen is absorbed rom the air in the lungs into our

blood. An equal volume o carbon dioxide is released back into the lungs rom the blood.

When we use the spirometer, each returning breath passes through soda lime, which absorbs

the carbon dioxide so, with the canister in place, less gas is breathed back into the spirometer

vitalcapacity

tidalvolume

V o l u m e 0 2 / d m 3

Time/minutes

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chamber than was breathed in. I we breathe into and out o the spirometer or (say) 1minute, a steady all in the spirometer trace can be seen. The gradient o the all is a measure

o the rate o oxygen absorption by the blood, and so is a measure o the rate o respiration

by the body.

1 Using the trace produced in class, or one provided by your teacher/lecturer, nd the

ollowing values:

a tidal volume

b vital capacity

c breathing rate

d minute ventilation.

2 Use the trace produced in class, or one provided by your teacher/lecturer, to work out the

rate o oxygen consumption in someone at rest.

3 What dierences would you expect i the subject had been exercising beore a trace was

taken?

4 Describe how you could use the apparatus to measure changes in breathing and

respiration rates due to exercise. (Note that the apparatus you have used may not be

suitable or use during exercise, and that measurements need to be taken immediately

ater exercise has stopped. Discuss with your teacher which is the best method or

use with your apparatus.) State what exercise would be appropriate, and any hazards

involved. Sketch the shape o the trace you would expect beore and ater exercise.

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investigating breathing

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