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ACTIVITY OVERVIEW
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Gas Exchange
BO RA
T
SUMMARY
This activity explores the role of the respiratory system in the regulation of gases in
the blood. Students investigate how to quantitatively measure the amount of carbon
dioxide in their exhaled breath by using an indicator to perform a titration.
KEY CONCEPTS AND PROCESS SKILLS
1.
Creating models is one way to understand and communicate scientific
information.
2.
Each system within the human body is defined by the functions that it
performs. For example, the respiratory system is a group of organs that brings
in oxygen and releases carbon dioxide for use within the body.
3.
Chemical reactions occur at surfaces. The greater the surface area available for
contact, the faster the reaction. This is why mechanical breakdown facilitates
chemical breakdown.
4.
Lungs provide surface area for oxygen to enter and carbon dioxide wastes to
leave the blood.
5.
An indicator can help measure changes, such as shifts in the concentration of
carbon dioxide present in a solution.
KEY VOCABULARY
control
qualitative/quantitative data
indicator
range
function
respiratory system
organ
structure
Teacher’s Guide
B-75
Activity 17 • Gas Exchange
MATERIALS AND ADVANCE PREPARATION
For the teacher
1
Transparency 17.1, “Use of Dropper Bottles”
1
Transparency 17.2, “Composition of Breath”
1
large sheet of chart paper (or overhead transparency)
*
1
overhead projector
*
1
large sponge
*
1
large bottle of carbonated water (optional)
For each group of four students
1
dropper bottle of bromthymol blue indicator
5
plastic cups
*
supply of water (may require distilled water—see below)
For each pair of students
1
dropper bottle of 0.05 M sodium hydroxide
1
SEPUP tray
1
dropper
1
30-mL graduated cup
For each student
*
1
1-gallon plastic bag
1
straw
1
stir stick
access to a wall clock or watch with a second hand
*Not supplied in kit
In areas with extremely soft water (in which the addition of the bromthymol
blue results in a yellow-green color rather than a blue color), use distilled
water instead of tap water. It is important that the BTB solution that acts as
the control be blue; only then is a solution containing excess dissolved carbon dioxide obviously yellow in contrast.
Prepare a large sheet of chart paper (or an overhead transparency) for students
to post their results in Part Two. Construct two columns, as shown below.
Name
B-76
Science and Life Issues
Number of Drops of Sodium Hydroxide
Gas Exchange • Activity 17
TEACHING SUMMARY
Getting Started
1.
Discuss how energy from food is related to gas exchange.
Doing the Activity
2.
Student pairs test solutions for carbon dioxide using BTB (Part One).
3.
Students work in groups to measure the carbon dioxide in their own
exhaled breath (Part Two).
Follow-Up
4.
Review the role of the respiratory system.
Extension 1
Students repeat Part Two of the Procedure, holding their breaths before
exhaling.
Extension 2
Students design and conduct an investigation to determine how the body
gets more oxygen during exercise.
INTEGRATIONS
Mathematics
Students collect quantitative data on the amounts of carbon dioxide in their
breath. These data can be further analyzed to determine the mean, median,
and mode, as well as the significance of those values.
Teacher’s Guide
B-77
Activity 17 • Gas Exchange
BACKGROUND INFORMATION
Indicators
to-volume ratio allows for rapid diffusion of oxygen
Bromthymol blue (BTB) is a chemical indicator that
and carbon dioxide across the alveolar membranes,
is yellow in acidic solution and blue in basic solu-
in the direction of high to low concentration. Since
tion. Its equivalence point is at pH = 6.7; conse-
the blood that enters the lungs has been partially
quently, it is blue in a neutral solution (pH = 7). As a
depleted of its oxygen supply and is laden with
solution of BTB gradually changes color, it appears to
waste carbon dioxide, oxygen flows into the capil-
go through an intermediate green stage. The green
laries while carbon dioxide flows out. As a result,
color results from some of the BTB molecules being
over a quarter of the oxygen inhaled is transferred
in the blue state and some in the yellow state. Since
to the bloodstream, while the exhaled air acquires a
carbon dioxide in solution produces carbonic acid,
concentration of carbon dioxide 100 times that of
BTB is yellow in the presence of sufficient carbon
the inhaled air (see Transparency 17.2, “Composi-
dioxide. Addition of a base (such as sodium hydrox-
tion of Breath”).
ide) to a yellow BTB solution eventually turns the
solution blue, as the sodium hydroxide neutralizes
the acidity and then turns the overall solution basic.
Respiration
Cellular respiration is the complex series of chemical reactions by which cells use oxygen to burn glucose for energy, producing waste carbon dioxide.
(Respiration at the cellular level is addressed in the
Micro-Life unit of Science and Life Issues.) Respiration by the respiratory system is better described as
“breathing and gas exchange.”
During exercise, the primary way in which the body
meets the need for increased oxygen is by increasing
the number of breaths per minute. This increases
the average oxygen concentration in the air inside
the lungs, so there is increased oxygen diffusion
across the lung surfaces. Meanwhile, oxygen delivery increases due to an increased rate of blood flow
to the exercising muscles. Oxygen extraction by the
muscles also increases, since they are using more
oxygen in cellular respiration. This results in the
venous blood carrying less oxygen than at rest,
which in turn increases the rate of oxygen diffusion
The internal structure of each lung shows several
across the lung surfaces. However, the concentra-
stages of branching, from the bronchi (tubes from
tion of oxygen in the arterial blood shows no
the trachea into each lung) to the 300 million or so
change during exercise.
tiny alveoli, or air sacs. This incredibly high surface-
B-78
Science and Life Issues
Gas Exchange • Activity 17
TEACHING SUGGESTIONS
n
1.
GETTING STARTED
Discuss how energy from food is related to
gas exchange.
n
2.
DOING THE ACTIVIT Y
Student pairs test solutions for carbon dioxide using BTB (Part One).
Ask students to recall Activity 16, “Balancing Act,”
Discuss the important Safety Note on page B-38 in
and ask such questions as, How does the body obtain
the Student Book before students begin. In Part One
energy? Students should know that food provides
of the Procedure, students observe that the presence
energy. Ask, What happens to the food as it goes
of dissolved carbon dioxide causes a change in the
through the body? Reinforce the idea that food breaks
color of bromthymol blue (BTB). Note that they are
down into nutrients. Inform students that the human
required to create their own data table in Step 3. You
body uses a gas dissolved in the blood to help break
may wish to use the “Organizing Data” ele-
down the food chemically. Encourage them to con-
ment of the DESIGNING
sider a burning candle as an analogy. The human
INVESTIGATION (DCI) variable to score the organiza-
body burns food just as a candle flame burns wax.
tion and completeness of students’ data tables.
People even give off heat like a candle does! What gas
Their tables may look something like Table 1, in
is required to keep a candle burning? Many students
which the expected results have been entered.
will know that it is oxygen; fewer students may be
aware that, just like a candle, humans give off waste
carbon dioxide.
AND
CONDUCTING
DCI
od
As student pairs complete Part One, they can go on
to work on Analysis Questions 1 and 2. Depending
on your student population, it may be necessary to
Have students read the introduction to the activity,
discuss the responses to these questions before
either individually or as a class. Students will be col-
moving on to Part Two.
lecting evidence about this gas exchange by using
an indicator to test for the presence of carbon diox-
Table 1: Testing for Carbon Dioxide (Part One)
ide in their exhaled breath. Be sure to discuss the
second paragraph, which introduces indicators. In
Table 1: Testing for Carbon Dioxide
Initial
BTB Color
Final
BTB Color
A (control)
blue
blue
B (air)
blue
blue
C (sodium hydroxide)
blue
blue
D (exhaled breath)
blue
yellow
E (partner’s
exhaled breath)
blue
yellow
this activity, students will be using bromthymol
blue (BTB) as an indicator for carbon dioxide.
Use Transparency 17.1, “Use of Dropper Bottles,” to
review how to hold SEPUP dropper bottles correctly. When used this way, the bottles produce consistent drops, allowing for quantification.
Cup
Teacher’s Guide
B-79
Activity 17 • Gas Exchange
If students are having difficulty understanding the use
Before students begin Part Two, be sure to review Step
of BTB as an indicator for carbon dioxide, discuss the
17, which explains how to perform the titration. After
presence of dissolved carbon dioxide in carbonated
adding each drop of sodium hydroxide solution, it is
beverages, such as soft drinks. Have students read the
important that students wait at least 10 seconds to
label from a bottle of carbonated water (such as seltzer
compare the resulting color to the color of the control
water) and test the water for the presence of carbon
(the unchanged cup of BTB solution shared within
dioxide. Use the results of the test to reinforce the idea
each student group). If students are not sure whether
that BTB turns yellow in a solution containing large
another drop is needed, advise them to add the drop
amounts of carbon dioxide.
and simply not include it in the final count if it is clear
n Teacher’s Note: After completing Part One, some
that the drop was unnecessary. Students may be inter-
students may ask why the sodium hydroxide made
the yellow BTB solution turn back to blue. This is
due to the fact that BTB is an acid-base indicator (as
described in the Background Information) and not
simply an indicator for the presence of carbon diox-
ested in knowing that using an indicator and another
chemical (such as sodium hydroxide) to determine the
amount of another substance (like carbon dioxide) is
known as titration, a procedure used frequently by
chemists.
ide. Decide whether you want to introduce the role
Students may notice that yellow BTB solution left
of acid-base chemistry in these reactions.
open to the air gradually returns to the color of the
control. This is not a neutralization effect: the car-
3.
Students work in groups to measure the car-
bon dioxide dissolved in the solution gradually
bon dioxide in their own exhaled breath
returns to the surrounding air. (A more dramatic
(Part Two).
example of this occurs when carbonated beverages
In Part Two of the activity, each student exhales into a
are allowed to go “flat.”)
plastic bag containing BTB solution. They then titrate
with sodium hydroxide solution to find the relative
n
amount of carbon dioxide in their exhaled breath.
FOLLOW–UP
Review the role of the respiratory system.
When the color of the BTB returns to that of the con-
4.
trol solution shared within each group, enough sodi-
After students have completed the activity, discuss
um hydroxide has been added. The amount of sodium
and review their results. In Analysis Question 3, you
hydroxide needed is proportional to the amount of
may want to calculate the class average and com-
carbon dioxide in the exhaled breath.
pare it to the range of results illustrated by the bar
n Teacher’s Note: You may wish to set up a control
graphs students constructed in Step 19.
with a bag containing BTB solution left open to the
Analysis Question 4, which elicits students’ thoughts
air and then shaken. If so, use Procedure Steps 12
about the relationship between structure and func-
and 15 as a guide to preparing this control, but omit
tion, is more challenging, and is intended to be dis-
steps 13 and 14. This will provide additional evi-
cussed as a class. Discuss Figure 1 on page B-36 in the
dence that the yellow color obtained by the stu-
Student Book, which shows the organs of the respi-
dents is due to their exhaled breath.
ratory system and the internal structure of the lungs.
B-80
Science and Life Issues
Gas Exchange • Activity 17
Point out the other organs of the respiratory system
Two hypotheses are offered in the Student Book:
and then focus attention on the lungs. Students often
that one breathes faster, or that more oxygen is
have the mistaken idea that the lungs are like two
absorbed from the air with each breath. Note that
empty balloons in the chest. Hold up a large sponge.
the regulation of the breathing rate in response to
Point out that the sponge is filled with numerous
physiological needs relates to the body’s balance of
spaces for holding water, yet the sponge is a solid
food input/energy use, explored in Activity 16, “Bal-
object. Human lungs are solid organs, containing
ancing Act.”
millions of air spaces. The tissue of the lungs forms
the walls of these spaces. These air spaces branch
In fact, if plastic resin is injected into the spaces of a
SUGGESTED ANSWERS
TO ANALYSIS QUESTIONS
human lung and the tissue removed from around it,
Part One: Using BTB to Test for Carbon Dioxide
through the lungs, similar to the branches of a tree.
the plastic mold looks like a small, upside-down tree,
as shown on page B-37 of the Student Book. When a
1.
Cup A was the control for BTB color.
person inhales, air flows into these smallest spaces,
known as alveoli (or air sacs). This is where the
What was the purpose of the solution in Cup A?
2.
a.
exchange of oxygen and carbon dioxide between the
Which of the solutions in Part One contained carbon dioxide? Support your answer
air and the bloodstream occurs.
with evidence from your experimental results.
Use Transparency 17.2, “Composition of Breath,” to
The solutions in Cups D and E contained car-
review with the class that neither inhaled breath nor
bon dioxide. These were the solutions into
exhaled breath is composed entirely of a single gas,
which the students exhaled. BTB is an indicator
and that exhaled breath (as corroborated by this
that turns yellow in solutions containing car-
activity) contains about 100 times as much carbon
bon dioxide. After blowing into the solutions,
dioxide as inhaled air. Emphasize the interaction
both turned yellow. This provides evidence that
between the human body and the environment. The
the solutions contained carbon dioxide.
body uses oxygen, so oxygen flows in through the
lungs; it produces carbon dioxide as waste, so carbon
dioxide flows out.
b.
What does this tell you about the exhaled
breath of human beings?
Testing the air (Cup B) for carbon dioxide is
Extension 1
equivalent to testing a person’s inhaled breath.
Students repeat Part Two of the Procedure, holding
Comparing the results in Cup B with those in
their breaths before exhaling.
Cups D and E provides evidence for a significant
increase in the carbon dioxide composition of
Extension 2
exhaled vs. inhaled breath.
Students design and conduct an investigation to
determine how the body gets more oxygen during
exercise.
Teacher’s Guide
B-81
Activity 17 • Gas Exchange
Part Two: Using BTB to Measure Carbon
5.
a.
Dioxide in Exhaled Breath
Were the data collected in Part One qualitative
or quantitative? Explain.
Qualitative. Students described color changes.
3.
Review the class data table. What was the
b.
range of carbon dioxide in exhaled breath
Were the data collected in Part Two qualitative
or quantitative? Explain.
(as measured by drops of sodium hydroxide)? Hint:
You looked at the range of class data in Activity 9,
Quantitative. Students measured (counted) the
“Data Toss.”
number of drops of sodium hydroxide, provid-
The range is likely to be rather narrow, espe-
ing data that can easily be compared among
cially if students have taken comparably deep
students.
breaths, exhaled completely, and kept their
bags air-tight. Emphasize the reproducible
6.
a.
Look carefully at Figure 1, “Human Respiratory System.” What are some of the important
nature of the procedure and the fact that the
structures in the respiratory system?
data from different students can be compared.
The nose, mouth, trachea (windpipe), lungs,
4.
Look again at Figure 1, “Human Respira-
and alveoli (air sacs) are important structures of
tory System.” Considering all the oxygen
the respiratory system. The blood also plays an
that has to get into your blood and all the carbon
important role in this system.
dioxide that has to escape from your blood why do
b.
you think the inside of the lung is structured the way
Explain how gases are exchanged within the
respiratory system.
it is?
One function of the respiratory system is the
The tree-like structure of the lungs, with many
exchange of gases between the human body and
levels of branching from the trachea to tiny
the environment. Oxygen in the environment
alveoli, give the lungs far more surface area
comes in through the nose or mouth, travels
than they would have if structured like empty
through the trachea, and enters the lungs. In the
balloons. This enables the lungs to accomplish
alveoli of the lungs, oxygen is absorbed into the
gas exchange efficiently enough to supply oxy-
blood and carried to different parts of the body.
gen to and remove carbon dioxide from the
Carbon dioxide, which has been produced by
entire bloodstream. In a similar way, the innu-
the body as waste, leaves the blood and is
merable villi in the small intestine enable that
exhaled back into the environment.
organ to absorb and digest nutrients rapidly
enough that they are not allowed to pass
through the digestive tract unused.
B-82
Science and Life Issues
©2006 The Regents of the University of California
Use of Dropper Bottles
Science and Life Issues Transparency 17.1
B-83
Composition of Breath
Components of
Earth’s Atmosphere
Composition of Air
Breathed In (%)
Nitrogen
78
75
Oxygen
21
16
Argon
Carbon dioxide
0.9
0.9
0.035
4.0
0.4
4.0
©2006 The Regents of the University of California
Water vapor
Composition of Air
Breathed Out (%)
Science and Life Issues Transparency 17.2
B-85