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13 Gas exchange in humans
13.1 What does the human breathing system consist of?
(Book 2, p.126)

The human breathing system is specially developed for gas exchange. It consists of the
respiratory tract and structures that help in breathing: rib cage (made up of ribs, sternum and
vertebral column), external and internal intercostal muscles, and diaphragm.
A
B
J
K
L
C
M
D
E
N
F
G
O
H
I
P
 The human breathing system
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Structure
(1)
A
Nasal
) Date:
Description

Lined with
a
mucus-secreting cells
b ciliated epithelial cells
c
blood capillaries
d olfactory cells (嗅覺細胞)

Guarded by hairs

Makes up the rib cage to protect the lungs and
the heart
cavity
(鼻腔)
B
Nostril (鼻孔)
C
(2)
D
Intercostal muscle (肋間肌)

Controls the movement of the rib cage
(3)


Does not have any cartilage
Lined with mucus-secreting cells and
E
Rib
Bronchiole
(肋骨)
(小支氣管)
ciliated epithelial cells
F
Inner pleural membrane (內胸膜)

In contact with the lung
G
Outer pleural membrane (外胸膜)

In contact with the internal intercostal muscles
(4)

Acts as a lubricant to help the lungs move easily
inside the thoracic cavity during breathing

Forms the respiratory surface (呼吸表面) where
gas exchange takes place

Where the respiratory tract and the alimentary
canal meet

Covers the entrance of the larynx to prevent
choking (窒息)


Made of cartilage to prevent it from collapsing
Contains the vocal cords to produce sounds

Supported by C-shaped cartilage
(氣管)

Lined with mucus-secreting cells and
ciliated epithelial cells
(支氣管)


Supported by circular rings of cartilage
Lined with mucus-secreting cells and
H
Pleural
fluid
(胸膜液)
I
(5)
J
Pharynx (咽)
K
(6)
Epiglottis
(會厭)
L
(7)
Larynx
(喉)
M
N
(8)
(9)
Alveolus
Trachea
Bronchus
(肺泡)
ciliated epithelial cells
O
Lung (肺)
P
(10)
Diaphragm
(橫膈膜)
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
Made up of alveoli, bronchioles and a large
network of blood capillaries

Separates the thoracic cavity from the abdomen
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 Rib cage
 Transverse section through the trachea
 The structure of a lung

Air flow along the respiratory tract: nostrils  nasal cavity  pharynx  larynx 
(1)
(4)
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trachea
air
 (2)
sacs
bronchi
 (3)
(in lungs)  (5)
bronchioles
alveoli
(in lungs) 
(in lungs)
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Go To …
Practical 13.1 Examination of the mammalian breathing system
(Student’s Book 2 p.127; Activity Book 2 p.87)
Practical 13.2 Examination of the pig lung
(Student’s Book 2 p.129; Activity Book 2 p.89)
Filtering, moistening and warming of the incoming air (Book 2, p.130)

Air contains dust particles, which would block the air passage, and (1)
bacteria
that
may cause infection.

The incoming air is filtered by the (2)
(3)

cilia
hairs
in the nostrils, and mucus and
in the nasal cavity, trachea and bronchi.
It is moistened by the (4)
mucus
and warmed by the (5)
blood
capillaries
in the respiratory tract.

Consequently, the air reaching the lungs is clean, moist and warm.
 Dust particles are removed from the respiratory tract
Go To …
Quick check (Student’s Book 2 p.130)
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13.2 How does gas exchange take place in the alveoli?
(Book 2, p.131)
direction of
blood flow
blood capillary from
pulmonary artery
red blood cell
alveolus
O2
CO2
film of moisture
blood capillary to
pulmonary vein
 Gas exchange between an alveolus and a blood capillary

The transport of oxygen from the atmosphere into the blood:
1
Oxygen is inhaled into the alveoli of the lungs.
2
Oxygen then (1)
3
Oxygen concentration is (2)
dissolves
in the film of moisture lining the alveoli.
higher
in the alveoli than in the blood capillaries.
Oxygen diffuses from the alveoli into the (3)

blood
capillaries
.
The transport of carbon dioxide from the blood into the atmosphere:
1
Carbon dioxide concentration is (4)
higher
in the blood capillaries than in the
alveoli. Carbon dioxide then diffuses across the capillary walls and the alveolar
epithelium into the (5)
2
alveoli
.
Carbon dioxide is exhaled out of the lungs.
Note:
Not all of the oxygen in the alveoli diffuses into the blood capillaries.
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The adaptations of the alveoli (Book 2, p.132)
1
The surface area of the alveoli is (1)
large
. There are about 700 millions of
alveoli in the lungs, giving a total surface area of over 70 m2.
2
The alveoli are richly supplied with (2)
and the blood capillaries is also (3)
blood
small
. The distance between the alveoli
. Therefore, a (4)
steep
concentration gradient is maintained to allow rapid diffusion of gases.
alveolus
network of
blood capillaries
 Lung model
3
The layer of (5)
epithelium
of the alveoli is very thin. It is only one-cell thick.
4
The inner surface of the alveoli is kept (6)
moist
. This allows the gases to
dissolve in the moisture before diffusing across the alveoli membranes.
Go To …
Practical 13.3 Examination of the mammalian air sacs (Student’s Book 2 p.132; Activity Book 2 p.91)
Transport of respiratory gases (Book 2, p.133)
1
Transport of oxygen (Book 2, p.133)
Red blood cells in the blood are specially adapted for carrying oxygen.

Their (7)
biconcave
(雙凹)
whole
shape provides a large surface area
for oxygen to diffuse in and out more efficiently.

Their (8)
disc
shape shortens the distance so that oxygen
reaches the inside of the red blood cells and combines with haemoglobin
section
(血紅蛋白) more quickly.

The (9)
haemoglobin
molecules in the cytoplasm of the
red blood cells are oxygen carriers. 98% of oxygen transported in blood
is carried by haemoglobin.
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 Red blood cell
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In the alveoli (Book 2, p.133)
In the alveoli, the oxygen concentration is (1)
high
due to breathing in fresh air.
Oxygen dissolves and diffuses into red blood cells and combines with haemoglobin to form
(2)
oxyhaemoglobin
(氧合血紅蛋白), which
is then carried in the blood to the body cells.
in alveoli
(high oxygen concentration)
haemoglobin + oxygen
b
oxyhaemoglobin
In the body cells (Book 2, p.133)
In the body cells, the oxygen concentration is (3)
oxyhaemoglobin breaks down into (4)
low
owing to respiration. The
haemoglobin
and (5)
oxygen
.
The oxygen then diffuses into the body cells.
in body cells
(low oxygen concentration)
oxyhaemoglobin
haemoglobin + oxygen
 How oxygen is transported in the blood
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2
Transport of carbon dioxide (Book 2, p.135)
a
In the body cells (Book 2, p.135)
) Date:
Carbon dioxide produced in respiration by body cells reacts with water inside red blood cells to
form (1)
hydrogencarbonate
cells and are carried in the (2)
ions
plasma
. These ions diffuse out of the red blood
to the alveoli.
in body cells
(high CO2 concentration)
H+ + HCO3
CO2 + H2O
b
In the alveoli (Book 2, p.135)
When blood reaches the alveoli, the hydrogencarbonate ions break down to (3)
dioxide
and (4)
water
carbon
again. Carbon dioxide then diffuses into the alveoli
and is exhaled.
in alveoli
(low CO2 concentration)
H+ + HCO3
CO2 + H2O
 How carbon dioxide is transported in the blood
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Go To …
Practical 13.4 Study of the effects of oxygen and carbon dioxide on citrated chicken blood
(Student’s Book 2 p.136; Activity Book 2 p.92)
Section concept diagram (Student’s Book 2 p.137)
Quick check (Student’s Book 2 p.137)
13.3

How does breathing take place? (Book 2, p.138)
(1)
Ventilation
(換氣)
is the movement of air over the respiratory surface to facilitate
gas exchange across the surface. It is brought about by the breathing action in humans, which
includes (2)

inhalation
(吸氣)
and (3)
exhalation
(呼氣).
Breathing is brought about by the movements of the ribs and the diaphragm. Their
movements can be shown by the (4)
(5)
bell-jar
rib-cage
model and the
model respectively.
1 Inhalation (Book 2, p.138)
air
5
1a
1b
3a
3b
1a
External intercostal muscles (6)
contract
1b
Ribs move upwards and (7)
2a
Diaphragm muscles contract.
2b
Diaphragm (8)
3a
Volume of the thoracic cavity (9)
3b
Pressure of the thoracic cavity (10)
4a
Lungs expand.
4b
Air pressure in the lungs is (11)
outwards
flattens
.
.
.
increases
.
4a
4b
2a
decreases
lower
than the atmospheric pressure.
2b
5
Air rushes into the lungs.
 The movements of the ribs and the diaphragm during inhalation
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2 Exhalation (Book 2, p.138)
air
5
1a
1b
1a
External intercostal muscles (1)
relax
.
1b
Ribs move downwards and (2)
inwards
2a
Diaphragm muscles relax.
2b
Diaphragm returns to (3)
3a
Volume of the thoracic cavity returns to original
.
3a
dome
shape.
3b
4a
volume.
4b
2a
2b
3b
Pressure of the thoracic cavity (4)
4a
Lungs contract.
4b
Air pressure in the lungs is (5)
increases
.
higher
than the atmospheric pressure.
5
Air is forced out of the lungs.
 The movements of the ribs and the diaphragm during exhalation
Exhalation is also contributed by the (6)
recoiling
(反彈)
of the elastic lung tissue.
Differences in composition between inhaled and exhaled air (Book 2, p.140)

(7)
Exhaled
air can be collected by using the following set-up:
exhaled air
blow slowly into
gas jar
rubber tubing
water
water trough
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Exhaled air contains (1)
less
)
Date:
oxygen because some oxygen in the inhaled air
has dissolved and has diffused from the alveoli into the blood.

In contrast, (2)
more
carbon dioxide is present in exhaled air than in inhaled air,
as carbon dioxide produced in respiration has diffused from the blood into the alveoli.

Exhaled air is saturated with (3)
water
and warmer.
Inhaled air (%)
Exhaled air (%)
21
16
0.03
4
78
78
Variable
Saturated
1
1
Oxygen
Carbon dioxide
Nitrogen
Water vapour
vapour
Other gases
Go To …
Practical 13.5 Investigation of the differences in composition between inhaled and exhaled air
(Student’s Book 2 p.140; Activity Book 2 p.94)
Section concept diagram (Student’s Book 2 p.142)
Quick check (Student’s Book 2 p.142)
E 13.4 What is the relation of exercise to energy
requirement, rates of respiration and ventilation?
(Book 2, p.143)

The (4)
rate
of
breathing
(呼吸率)
is the number of
breaths per minute. It measures how fast we breathe.

The (5)
depth
of
breathing
(呼吸深度)
is the volume of
air breathed in after an exhalation. It can be measured by a data logger, a spirometer (肺量計)
and a breath volume kit.
 Breath volume kit
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Exercise (1)
(
increases
) Date:
both the rate and the depth of breathing.
Exercise
↓
Energy requirement of muscle cells increases
↓
Rate of respiration in muscle cells increases
↓
Rate of ventilation increases
(to supply more oxygen to and remove
carbon dioxide from muscle cells)
↓
Both rate of breathing and depth of breathing increase
 Graphs showing the rate and depth of breathing (a) at rest and (b) during exercise

Since an extra amount of oxygen is required to oxidize the (2)
lactic
acid
produced during anaerobic respiration of muscles, it takes some time for the rate and the depth
of breathing to return to normal after exercise.
Go To …
Practical 13.6 Study of the effect of exercise on breathing using a breath volume kit
(Student’s Book 2 p.143; Activity Book 2 p.97)
Practical 13.7 Study of the effect of exercise on breathing using a data logger
(Student’s Book 2 p.144; Activity Book 2 p.99)
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13.5 What are the health hazards caused by smoking?
(Book 2, p.146)

(1)
Tars
(焦油)
cause extra mucus secretion in
the respiratory tract and prevent the cilia from removing
dust particles in the respiratory tract.

The smoke irritates (刺激) the (2)
bronchi
and
causes them to contract. This makes breathing more
difficult and reduces the amounts of gases for exchange.

(3)
Nicotine
(尼古丁)
in cigarette is an addictive drug
which increases the heart rate and the blood pressure.

Smoking increases the natural (4)
abortion
rate (流產率) in pregnant women.

The physical and mental development of babies born
to women who smoke during pregnancy is affected.

Smoking may lead to many diseases,
e.g. lung cancer, (5)
emphysema
(肺氣腫)
and coronary heart disease (冠心病).
Go To …
STS connection 13.1 Relationship between smoking and the incidence of emphysema and lung cancer
(Student’s Book 2 p.146; Activity Book 2 p.102)
Review (Student’s Book 2 p.147)
Summary concept diagram (Student’s Book 2 p.148)
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 Practice question
The diagram below shows an alveolus and its associated blood capillary of a heavy smoker.
X
a
According to the diagram, state two adaptations of the alveoli for gas exchange. (2 marks)
Alveoli have a large surface area to volume ratio. / The thin epithelium provides a short
distance for rapid diffusion of gases. / Blood capillaries are in close contact with the
alveoli. (1m x 2)
b
Name the gas that moves from the alveolar air to the blood. Explain how this happens.
(4 marks)
Oxygen (1m).The oxygen concentration is higher in the alveoli than in the blood
capillaries (1m). Oxygen then dissolves in the film of moisture lining the alveolis (1m)
and diffuses from the alveoli into the blood capillaries (1m).
c
Substance X deposited on the inner surface of the alveolus comes from cigarette smoke.
What is substance X?
(1 mark)
Tar (1m).
d
According to the diagram, explain how gas exchange is affected by substance X. (2 marks)
Substance X deposits on the inner surface of the alveolus reduces the surface area for gas
exchange (1m) and makes the diffusion of gases more difficult (1m).
Total: 9 marks
－ END －
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57