Notes to Instructors 277
Notes to Instructors
Chapter 42 Circulation and Gas Exchange
What is the focus of these activities?
All the cells of the body need to be continuously supplied with oxygen and food to
support cellular respiration (Chapter 9). Wastes need to be removed. Oxygen and carbon
dioxide are transported by diffusion into and out of cells. Food and wastes are transported
What are the particular activities designed to do?
Activity 42.1 How is mammalian heart structure related to function?
This activity is designed to give students an understanding of the normal function of the
Activity 42.2 How do we breathe, and why do we breathe?
This activity is designed to give students an understanding of how and why we breathe
Activity 42.3 How are heart and lung structure and function related to
metabolic rate?
This activity asks students to integrate their understanding of the structure and function
What misconceptions or difficulties can these activities reveal?
Activity 42.1
Question 1: Many students have difficulty understanding the overall flow pattern from the
left side of the heart to the body and back to the right side of the heart, to the lungs, and
278 Notes to Instructors
then back to the left side of the body. They may be confused by realistic diagrams and
pictures of the heart that show all the major vessels “coming out the top of the heart.”
You can simplify the system for students by drawing a schematic heart that has a vessel
Question 5: If the students have worked out and understand how to track blood flow in
the body, they will have little difficulty with this question compared to those who
memorized the pattern of flow.
Activity 42.2
Question 1: Most students understand how we breathe. Many fewer make the connection
between why we breathe and the requirements of cellular respiration. If you ask students
why we breathe, most will automatically reply, “Because we need oxygen to live.” If you
Questions 2 and 3: Students may have difficulty reading oxygen dissociation graphs because
they try to read them from left to right (the way most graphs are read). It is useful to point out
Activity 42.1 279
Answers
Activity 42.1 How is mammalian heart structure related to
function?
1. Diagram and describe the path a red blood cell takes from a capillary in your big toe
to your heart and back to your big toe. At each point in the pathway, indicate
whether the red blood cell is most likely picking up or losing oxygen. Indicate also
the relative blood pressure that part of the path is likely to have. Include all of the
following terms in your diagram.
aorta
venules
right ventricle
left ventricle
280 Activity 42.1
a. Draw the heart and its
chambers, including
differences in musculature.
b. Explain why the
differences in musculature
might exist by explaining
the normal functions of
each chamber.
c. Include below the
functions of the SA node
(pacemaker), the AV
node, and the AV and
semilunar valves.
the ventricular contraction.
The right ventricle pumps
its contents to the lungs.
valves lie between the atria
and the ventricles. These
are one-way valves that
2. The degree of musculature differs in these chambers of the heart: atria, right
ventricle, left ventricle.
Activity 42.1 281
42.1 Test Your Understanding
1. While in utero, the wall between the two atria of the human fetal heart is not
complete. An opening, the foramen ovale, allows blood from the two atria to mix.
Normally, at birth, this hole seals over and the two atria are separated from each other.
What would be the consequences to the infant if this hole did not seal over at birth?
If the foramen ovale did not seal at birth, oxygenated blood from the lungs
2. One of the most common congenital defects of the cardiovascular system is called
“transposition of the great arteries.” In infants who have this defect, the pulmonary
artery exits from the heart where the aorta should and the aorta exits where the
pulmonary artery should. All other circulatory connections are normal.
a. Diagram and describe the circulation of blood in an infant who has this genetic
defect.
When the positions of the aorta and pulmonary artery are reversed, two separate
b. What type of treatment would such an infant need?
Unless something is done immediately, the infant will die. An operation to
282 Activity 42.2
Activity 42.2 How do we breathe, and why do we breathe?
This activity is designed to give you an understanding of how and why we breathe and
some of the mechanisms that control the rates of breathing.
How do we breathe? Explain the mechanics of breathing. To do this, diagram (or model)
and explain the general movement of oxygen and carbon dioxide into and out of the
lungs, lung alveoli, blood, and tissues. Be sure to include all of the following terms in
your diagram.
diaphragm
diffusion
CO2removal
O2demand
lungs
alveoli
expired air
inspired air
oxygen
carbon dioxide
brain
PCO2sensors
medulla oblongata
lung tissue cells
skeletal muscle cells
carbohydrate + O2→CO2+ H2O + energy
CO2+ H2O ↔H2CO3↔HCO3
⫺+ H+
Use your diagram or model to answer the questions.
1. Explain briefly why we breathe. Or, more specifically, where is what we breathe in
used in the body and where is what we breathe out produced in the body?
We breathe in to supply our cells with oxygen for use in cellular respiration—
Activity 42.2 283
2. Oxygen is transported in the blood in reversible combination with hemoglobin.
a. How does the level of carbon dioxide in the blood contribute to blood pH?
(Write the chemical reaction.)
CO2+ H2O →H2CO3(carbonic acid) →H++ HCO3⫺
b. How does the level of CO2in the blood affect the affinity of hemoglobin for
oxygen? That is, how does the concentration of CO2in the blood change its
ability to carry oxygen (the effect of the Bohr shift)? What is the Bohr shift?
The conformation or shape of the hemoglobin molecule changes in response to
c. Is the Bohr shift more likely to occur after exercise or after hyperventilation
(rapid breathing with no exercise involved)? Explain.
Exercise increases the amount of ATP required per unit time (compared to no
284 Activity 42.2
3. Compare these oxygen dissociation curves for hemoglobin.
O
2
saturation of hemoglobin (%)
P
O
2
(mm Hg)
P
O
2
(mm Hg)
0 20406080100
100
80
60
40
20
0
O
2
saturation of hemoglobin (%)
0 20406080100
100
80
60
40
20
0
O
2
saturation of hemoglobin (%)
0 20406080100
100
80
60
40
20
0
P
O
2
(mm Hg)
Fetus
Fetal vs. Adult
Reduced pH conditions
Note: The solid line indicates the curve
for reduced pH conditions.
Normal adult
Mother
a. How do the curves differ?
i. Normal versus reduced pH?
Adult hemoglobin becomes completely saturated with oxygen at 80 mm Hg
Activity 42.2 285
ii. Fetus versus adult?
Adult hemoglobin becomes completely saturated with oxygen (carries as
b. What would happen if fetal and adult hemoglobin had the same affinities for
oxygen?
Oxygenated blood from the mother’s left ventricle passes through other tissues
before it reaches the placenta. As a result, the partial pressure of oxygen in the
4. If they don’t get what they want, some small children threaten to “hold their breath
until they die.” Children who are very strong-willed may turn blue, but they will not
die because they will pass out and the autonomic breathing response will take over.
What types of control are involved in holding your breath? Why (for what
physiological reason) will you pass out if you hold your breath too long?
When you hold your breath the carbon dioxide level in your blood increases and pH
decreases. Changes in blood pH (carbon dioxide levels) are sensed in the medulla of
Activity 42.3 How are heart and lung structure
and function related to metabolic rate?
1. Describe the simplest type of closed circulatory system. Include the function of each
part of the system and how the structure of each part is related to its function.
The simplest closed circulatory system would include a heart, a muscular organ that
2. If you want to increase the rate of metabolism in an organism, you need to increase:
a) the rate of food and oxygen delivery to the cells (or body) and b) the rate of
removal of waste products from the cells (or body). In the evolution of the
vertebrates, what modifications of the circulatory system and/or circulatory
function have occurred to increase metabolic rate?
• Fish have two-chambered hearts (one atrium and one ventricle). Deoxygenated
blood is pumped from the heart to the gills and oxygenated blood then travels
286 Activity 42.3
3. Both reptiles and mammals obtain their energy via aerobic respiration:
C6H12O6+ 6O2→6CO2+ 6H2O + 36ATP
Both also need to be able to maintain internal homeostasis in order to survive.
a. Given this, would you expect the lungs of the lizard (an ectotherm) to have more
or fewer alveoli per unit area than the lungs of the rat (an endotherm)? Assume
the overall SA/V ratios of the rat and lizard are about the same. Explain the
physiological reasoning behind your answer.
The lizard will have fewer alveoli per unit area of lung tissue than the rat. This is
reasonable because, on average, the ectothermic lizard requires less energy per
b. On average, would you expect the heart rate of the lizard to be higher or lower than
the heart rate of the rat? Explain the physiological reasoning behind your answer.
The argument here is similar to that above. To maintain a high metabolic rate
4. Based on your knowledge of SA/V ratios, metabolic rates and cardiopulmonary
physiology, would you expect the following to be higher or lower in a human infant
as compared to an adult? Explain the physiological reasoning behind your answer(s).
a. body temperature
Body temperature is lost across surface area. Since an infant has a much higher
b and c. heart rate and oxygen consumption rate/unit mass of body
To maintain a constant body temperature, the infant will require a higher metabolic
Activity 42.3 287
288 Activity 42.3
42.3 Test Your Understanding
1 to 3. At rest, an average human’s mean arterial blood pressure (MABP) is 120/80. As
a person increases activity however a variety of modifications must be made to
support the increased activity.
For example, assume you are late for an exam and run four blocks to get there on time.
Which of the following would occur during your four-block run? Explain your answers.
Refer to the graph below of dissociation curves for two mammalian hemoglobins to
answer questions 4 to 7.
% Saturation of
Hb with O2
Partial pressure of O2
or oxygen tension
t0
B
A
t1 t2
Activity 42.3 289
7. In addition to carbon dioxide, contracting muscles cells produce a large amount of
heat energy. It has been demonstrated that an increase in temperature, independent
of carbon dioxide levels, encourages the blood to give up its oxygen to exercising
tissues. Given this information would you expect: a or b below to be true? Explain
your answer.
a. Curve A (of the graph) would be the lower temperature curve and B would be the
higher temperature curve.
b. Curve B (of the graph) would be the lower temperature curve and A would be the
higher temperature curve.