The document summarizes gas exchange in mammals. It describes the key structures involved in mammalian respiration including the nasal cavity, pharynx, larynx, trachea, bronchi, lungs, respiratory bronchioles, alveolar ducts and alveoli. It explains how air moves through these structures into the alveoli where gas exchange occurs via diffusion across the alveolar surface. The alveoli have a thin moist surface and extensive blood capillaries to facilitate efficient oxygen and carbon dioxide exchange.

1. Chapter 7 Gas Exchange
Ong Yee Sing
2017

2. Gas exchange
• Gas exchange is the biological
process by which gases move by
passive diffusion across a surface.
• Here, it refers to the process of
diffusion of oxygen gas into the
body through wet surface and the
diffusion of carbon dioxide gas
out of the body.

3. Simple diffusion
• Unicellular organisms (yeast, amoeba
and paramecium) only need to carry
out gaseous exchange at their body
surface through a simple mechanism
diffusion as they have a high total
surface area to volume ratio (TSA/V
ratio).
• Gas exchange by direct diffusion
across surface membranes is efficient
for organisms less than 1 mm in
diameter.
Amoeba
Flat worm

4. Specialisation of organs
• Large organisms required
specialized organs to undergo
gas exchange.
• the TSA/V ratio of large organisms
is small
• the body surface of terrestrial
animals/plants is water-proof to
reduce water loss

5. Skin Gill
Trachea Lung

6. Increasing complexity of respiratory system

7. 7.1 Respiratory
Surface

8. Respiratory surface
Respiratory surfaces Surrounding environment Examples of organisms
1. Cell membrane Water Yeast, amoeba, paramecium
2. Gills Water Bony fish
3. Trachea Land Cockroach and grasshopper
4. Skin & lungs Water & land Frog
5. Lungs Land Reptiles, birds and human
The gills of the cartilaginous fish are exposed.

10. Characteristics of respiratory surfaces
• Thin surface
• Facilitate the diffusion of gases
• Moist surface
• Oxygen and carbon dioxide must be dissolved in water speed up rate of
diffusion.
• Large surface area
• Speed up the rate of gaseous exchange.
• Network of blood capillaries which are closely packed
• Facilitate transport of gases after the exchange.

11. Size of organisms and gaseous exchange
• The increase in volume is much faster than the increase in surface area.
• This means that the surface for gaseous exchange is too small for the numerous
cells found in the body.
• To overcome this problem, organisms have to change the surface area of their
body or to have it increase the entry and exit of gases and the rate of transport
of gases.

12. Lower surface-to-volume ratio = faster diffusion

13. Quiz
• Which of the following is not a way to increase the efficiency of a
respiratory system?
A. increase the surface area available for diffusion of gases
B. decrease the distance over which the gases must diffuse
C. increase the concentration differences of gases inside and outside
the system
D. dry the system out so the gases do not have to diffuse through
water
E. all of the above will increase efficiency

14. Conclusion
• Type of respiratory surfaces includes skin, cell surface, gills, trachea
and lungs.
• Characteristics of respiratory surfaces includes thin surface, moist
surface, large surface area, and network of blood capillaries which are
closely packed.

15. 7.2 The mechanism
of gaseous
exchange

16. Mass flow
• Mass flow集体流动, also known as “mass
transfer” and “bulk flow”, is the
movement of fluids down a pressure or
temperature gradient.
• Diffusion is the movement of
substances down a concentration
gradients.
• Examples of mass flow include blood
circulation and transport of water in
vascular plant tissues.

17. 7.2.1 Gas Exchange
in Insects

18. Tracheal system
• Tracheae气管系统(air-tube) are
distribute throughout the body
of the insects.
• Air enters the tracheal system
through the spiracles气孔 located
on both sides of the thorax胸部
and abdomen腹部.

19. • The tracheal system contains a
few main tracheal trunks气管干
branch which run through the
body from the head to the tail.
• The tracheal trunks branch
into numerous tracheae气管
(singular: trachea).
• The tracheae branches into
tracheoles微气管 which directly
connects to the muscles or
tissue cells.
Branching of the tracheal system

20. Walls of tracheae and tracheoles
• The walls of the tracheae have spiral bands of chitin几丁质which thicken the walls.
• These walls can prevent the tracheae from collapsing when the pressure in the
tracheae fall.
• The walls of the tracheoles are not thickened by chitin.
• Walls of tracheoles are thin and moist.
• Gas exchange occurs at the surface of tracheoles through mechanical diffusion.

21. Control of airflow
• The opening and closure of the
spiracles of the grasshopper are
controlled by muscular valves气孔瓣膜.
• Muscles contract to close the spiracle,
or relax to open it.
SEM of a crickets spiracle valve. 342x

22. Breathing in mantis

23. Air sac气囊
• Through the expansion and contraction of
the abdomen, the air sacs help to drive air
in and out of the tracheae.
• Air sacs also provide temporary air supply
so that an insect to conserve water by
closing its spiracles during periods of high
evaporative stress.
Unidirectional airflow during abdominal
pumping in a grasshopper. During
inspiration, air flows in through open
thoracic spiracles (sp), along the
longitudinal trachea, and into the air
sacs. At low metabolic rates, air flows
out only through the tenth abdominal
spiracles; in more active animals, air
flows out all abdominal spiracles.

24. Blood of insects
• Insects do not need to use blood for
the transport of gases.
• The blood of insects does not contain
haemoglobin.
• This colourless fluid is also known as
haemolymph血淋巴.
Hemolymph of cockroach under
light microscopy. It is not very
cellular though it contains
hemocytes (white blood cells).

25. Conclusion

26. Quiz
• Insect respiratory systems contain all of these structures EXCEPT:
A. Parabronchi
B. Spiracles
C. Tracheoles
D. Tracheae

27. Quiz
• When a grasshopper’s head is submerged into the water, it will
A. be drown
B. die
C. still be breathing

28. Write your name,
class number,
class and date.
Total mark: 2
• A
• B
• C
• D
• E
• F
• G
• H
• I
A
B
C D
E
F
G
H
I

29. 7.2.2 Gas
exchange in fish

30. Fish gills鳃
• The gills of fish are the
organs for gaseous exchange.
• In bony fish, four gills are
presence in each opercular
cavity.
• In the textbook it is refers as
“four pairs of gills” inside each
opercular cavity.
• The opercular cavity鳃腔 is
covered by the operculum鳃
盖.
Operculum
Oncorhynchus mykiss
Pontinus nematophthalmus
Aracana aurita
Peristedion gracile

31. Structure of a gill
• Each gill is made up with
• gill arch鳃弧
• gill filaments鳃丝
• gill rakers鳃耙

32. Gill arches鳃弧and gill rakers鳃耙
• Gill arches hold the gill filaments.
• Gill rakers are bony or cartilaginous processes
that project from the gill arch.
• Functions of gill rakers:
• To prevent the potentially damaging passage of
solid material through the gill slits and over the
gill filaments.
• To divert food particles into the esophagus

33. Gill filaments鳃丝
• Gill filaments are fleshy processes that project from the gill arch.
• They are red in colour due to the extensive blood capillary
systems in the lamella鳃板.
F: filament; L: lamella

34. Opercular movement鳃盖运动
• The opercular movement
is responsible for fish
breathing.
• Fish take in water
through the mouth. The
opeculum is close.
• The mouth closes, forcing
the water back over the
gill filaments and out
through the gill slits.

35. Gas exchange at the gill filament
• The structure of gill filaments with numerous blood capillaries in the
lamella provide a large and effective respiratory surface for gaseous
exchange.
• Blood carries oxygen from the gills to other body tissues and carbon
dioxide from deeply seated tissues to the gill filaments.

36. Counter-current exchange system逆流交换机制
• The blood flows through the
blood vessels in the opposite
direction to the water flowing
through the lamellae.

37. Advantages of the counter-current exchange
system
• This system maximises the amount of oxygen diffused into the blood
by having the most oxygenated blood meet the most oxygenated
water, and the least oxygenated blood meet the least oxygenated
water to maintain the concentration gradient the whole way
through.

38. Quiz
• A countercurrent flow system between substance A and substance B
A. maximizes the exchange by having A and B flow in the same
direction
B. minimizes the exchange by having A and B flow in the same
direction
C. maximizes the exchange by having A and B flow in opposite
directions
D. minimizes the exchange by having A and B flow in opposite
directions

39. Limitations of the counter-current exchange
system
• A limitation of this gas exchange system is that fish can only live in water.
• They need water to support the filaments and hold the lamellae apart to
maintain the large surface area.
• In air, the filaments and lamellae would stick together, greatly reducing
the surface area: volume ratio, and therefore decreasing the efficiency of
diffusion of gases.
• This may permanently destroy the structure of the filaments as well.
• The gills would also dry out without water keeping them moist, so gases
would no longer be able to dissolve in order to diffuse into the blood.

40. Conclusion

41. Quiz
• The efficiency of gills in fish is derived from
A. the countercurrent flow of water over the gills
B. the increasing temperature of blood within the gills
C. continuous diffusion of oxygen into the blood
D. a and b
E. a and c

43. Quiz
• A countercurrent flow system between substance A and substance B
A. maximizes the exchange by having A and B flow in the same
direction
B. minimizes the exchange by having A and B flow in the same
direction
C. maximizes the exchange by having A and B flow in opposite
directions
D. minimizes the exchange by having A and B flow in opposite
directions

44. Quiz
• The efficiency of gills in fish is derived from
A. the countercurrent flow of water over the gills
B. the increasing temperature of blood within the gills
C. continuous diffusion of oxygen into the blood
D. a and b
E. a and c

45. 7.3 Gaseous
exchange in
mammals

46. Respiratory system of
mammals
• The respiratory system of
mammals includes lungs and other
structures which help to drive gas
in and out of the lung.

47. Nasal cavity鼻腔
• Nasal/nose hair鼻毛
• Filtering foreign particles e.g. dust from entering the
nasal cavity
• Collecting moisture
• Mucous membrane鼻腔黏膜
• Contain microvasculatures/microvessels微血管 to
warm the absorbed air
• Secrete slime to moisture the air, and trap dust and
bacteria
• Olfactory cells嗅细胞
• Latin olfacere ‘to smell’ + adjective –ory
• Contain nerve endings
• Reception of sensory stimuli caused by odours

48. Quiz
• The nasal hairs and mucus
A) filter impurities from the inspired air.
B) reduce transpulmonary pressure.
C) reduce the surface tension in the alveoli.
D) keep the lungs moist so gas diffusion can occur.

49. Pharynx咽
• Greek phárynx ‘throat’
• connects nasal cavity, mouth
cavity, middle ear中耳, larynx
and oesophagus
• acts as the common passage
for food and gases.

50. Larynx喉
• from Greek larynx "the upper windpipe"
• Epiglottis会厌软骨 is a flap-like structure
that covers the opening of larynx when
swallowing to prevent food or liquids
from entering the trachea气管.

51. Larynx
• Vocal cords声带 are a pair of fibrous sheets of tissue with gaps that
produce sounds.

52. Trachea气管and bronchus支气管
• Trachea (sg.), tracheae/tracheas (pl.),
from Greek τραχεῖα “windpipe”
• Bronchus (sg.), bronchi (sg.), from
Latin bronchus, from Greek βρόγχος
(brónkhos) "wind pipe“.
• No gas exchange occurring here.
• Part of the conducting zone.

53. • C-shape cartilage
rings C形软骨环
• Supporting structures
which open up the
lumen of the trachea
to conduct air
• Mucous membrane黏膜
• Secrete mucus to trap
dust and germs
• Possesses cilia纤毛that
lash towards the
larynx, driving the
mucus粘液out of the
body to become
phlegm痰

54. Lungs
• two lobes for the left lungs, three
lobes for the right lungs

55. Respiratory
bronchioles
and alveolar
ducts 肺泡管
• Bronchus divides into
bronchioles.
• Alveolar ducts
connect bronchioles
to the alveoli.
• Bronchioles and
alveolar ducts have no
cartilage at all.
• Gaseous exchange
also takes place here.
呼吸性细支气管

56. Alveoli肺泡
• Alveolus (sg.), alveoli (pl.)
• from Latin, ‘small cavity,’ diminutive
of alveus
• Made up of a single layer of epithelial
cells
• Surrounded by a network of blood
capillaries and elastic fibre.
• The main site of gaseous exchange

57. Alveoli
SEM
TEMLight microscopy

58. Quiz
• Which of these is an adaptation for efficient gas exchange in the air
sacs (alveoli)?
A. Thick walls
B. Few blood capillaries
C. Moist surface

59. Movement of air
• Movement of air into the lungs
• Nostril  Nasal cavity  pharynx  larynx  trachea  bronchus
 lung  bronchiole  respiratory bronchiole  alveolar duct 
alveolus

60. Lungs in motion
• An experimental device keeps a lung warm, breathing and nourished
while outside the body.
• Allow the donor lung to extending the life of an organ outside the
body.

61. Quiz
• Gas exchange in the lungs occurs in the
A. Nasal cavity
B. Larynx
C. Bronchi
D. Respiratory bronchiole

62. Quiz
• Alveoli are microscopic air sacs branching off the
A) tertiary bronchi.
B) bronchioles.
C) terminal bronchioles.
D) respiratory bronchioles.

63. Quiz
• Name the following parts.
A
B
C
D
E
F

64. Write your name,
class number,
class and date.
Total mark: 2
• A
• B
• C
• D
• E
• F
• G
• H
A
B
C
D
E
F
G
H

65. Asthma
• Asthma is a long-term inflammatory
disease of the airways.
• Symptoms include episodes of
wheezing气管响声, coughing, chest
tightness, and shortness of breath.
• Symptoms caused by hyperactive
bronchial tube with swelling airway
wall, contracting muscle (reduce
diameter of airway), and increase
mucosal secretion.
• Symptoms can be prevented by
avoiding triggers, such as allergens敏
感物质 and irritants刺激物, and by the
use of inhaled corticosteroids using
an inhaler吸入器.
epithelium (Ep)
basement membrane (Bm)
smooth muscle (Sm)
blood vessel (Bv).

67. 7.3.1 Gaseous
exchange at the
lung alveoli and
tissues

68. Partial pressure气体分压
• In a mixture of gases, each gas has a partial pressure.
• Partial pressure is the hypothetical pressure of that gas if it alone occupied
the entire volume of the original mixture at the same temperature.
• Or the partial pressure value of a gas is the ratio of the gas occupies in a mixture of
gases and the total pressure of the mixture of gases.
• The partial pressure of oxygen or carbon dioxide varies in the blood
capillaries found in alveoli, the blood in vein, the blood in artery and the
body tissue.
Ptotal = Pn+Pn+1

69. Partial pressure of gasses in difference tissues
• The partial pressure of oxygen or carbon dioxide varies in the blood
capillaries found in alveoli, the blood in vein, the blood in artery and
the body tissue.
Gasses Alveoli
Deoxygenated
blood
Oxygenated
blood
Body tissues
Oxygen 13.3 kPa 5.3 kPa 13.3 kPa 5.3 kPa
Carbon
dioxide
5.3 kPa 6.0 kPa 5.3 kPa 6.0 kPa

70. Partial pressure of gasses in difference tissues

71. Partial pressure gradient
• The driving force of gaseous
exchange is the difference in
partial pressure (partial
pressure gradient/difference)
分压差of the gasses in the
alveoli and the blood
capillaries.
• Gasses will move from a
region with higher partial
pressure to a region of lower
partial pressure.

72. Gas exchange in the lungs
• Inhaled air contains a high
concentration of oxygen
molecules.
• The oxygen partial pressure in
the alveoli (13.3 kPa) is much
higher than the oxygen partial
pressure in the alveolar blood
capillaries (5.3 kpa) (Table 7.4).
• The oxygen molecules diffuse
into the blood capillaries.

73. Transportation of oxygen in
the blood
• Oxygen combine with haemoglobin血红蛋
白in the red blood cells to form oxy-
haemoglobin氧合血红素.
Hb + O2  HbO2
• Oxyhaemoglobin is transport to other
parts of the body.

74. Structure of haemoglobin血红蛋白
• Form by four polypeptides
of two different type.
• Each polypeptide contain a
Heme group血红素with a
Fe2+.
• The Heme group can
combine with oxygen.
• Each haemoglobin can
carry 4 oxygen.

75. Quiz
• Oxygen binds to the ____ of deoxyhemoglobin.
A) alpha chains
B) beta chains
C) iron atom in the heme groups
D) organic portion of the heme group

76. Releasing of oxygen into the tissues
• Oxyhaemoglobin is a unstable
compound.
• The partial pressure of oxygen in
the tissues (5.3 kPa) is lower than
the partial pressure of
oxygenated blood (13.3 kPa).
• Oxyhaemoglobin releases the
oxygen.
HbO2  Hb + O2

77. Removal of carbon dioxide
• Tissue cells produce large amount
of CO2 in the metabolism processes.
• The partial pressure of CO2 in the
tissues is higher (6.0 kPa) than that
of the blood plasma (5.3 kPa).
• Carbon dioxide diffuses into the
plasma.
• Most carbon dioxide is transported
as bicarbonate / hydrogen
carbonate ion (HCO3
- )碳酸氢离子 to
the aveoli.
CO2 + H2O  HCO3
- + H+

78. Expel of carbon dioxide
• The carbon dioxide enters the
lung with the deoxygenated
pulmonary artery肺动脉.
• The carbon dioxide partial
pressure in the alveoli is lower
(5.3 kPa) than that of the blood
capillaries (6.0 kPa).
• The HCO3
- ions in the blood
capillaries are rapidly converted
into carbon dioxide and water
HCO3
- + H+  CO2 + H2O
• Carbon dioxide diffuses into the
alveoli and is removed from the
body through the respiratory
tract.

79. Quiz
• Bicarbonate ion (HCO3
-) and hydrogen (H+) ions result from a reaction
of ____ with water.
A) oxygen
B) hydrogen
C) carbon dioxide
D) carbon monoxide

81. Summary
• Gasses diffuse from a region of high partial pressure to a region of
low partial pressure.
• Aka gasses move down the partial pressure gradient.
• Oxygen is transported as oxyhaemoglobin.
• Carbon dioxide is transported as bicarbonate (HCO3
-).
Gases Alveoli ⇄ deoxygenated blood Oxygenated blood ⇄ tissue
Oxygen 13.3 KPa  5.3 KPa 13.3 KPa  5.3 KPa
Carbon dioxide 5.3 KPa  6.0 KPa 5.3 KPa  6.0 KPa

82. 7.3.2 Adaptations of the alveoli to
the exchange of gases

83. Large surface area
• Alveoli provide an
extremely large surface
area for the exchange of
gases.

84. Extensive blood capillary network
• The alveoli are
surrounded by a
network of blood
capillary.
• Oxygen is brought away
rapidly through
diffusion and blood
flow, thus promoting
the exchange of gases.

85. Thin walls
• The alveolar wall and the
capillary wall are only
made up of a single layer
of cells.
• They share a basal
lamina.
• Their total thickness of
about 0.0005 mm,
therefore gases can
diffuse through easily.

86. Lengthened time for exchange of gases
• The diameter of the
blood capillaries
covering the alveoli
is slightly smaller
than the diameter
of the red blood
cells.
• Hence the red
blood cell found in
the capillaries
become oval in
shape and they flow
slowly through the
capillaries
• increase in the
surface area
• Increase in time
When the lungs inflated, the RBC are parallel to the
alveoli wall, maximizing their contact with the epithelial
cells.

87. Moisture
• A thin film of moisture is found at the inner surface of the alveolar wall.
• It can dissolve O2 and CO2 thereby facilitating the diffusion of gases

88. Summary
• Adaptations of the alveoli to the exchange of gases are:
• Moist surface,
• Large surface area,
• Thin alveolar walls,
• Extensive narrow blood capillary network.

89. 7.4 Gaseous exchange in plant

90. Gas exchange by diffusion through stomata气孔
and lenticels皮孔.

91. Stomata
• from Greek στόμα "mouth"
• A stoma is a pore, found in the
epidermis of leaves, stems, and other
organs, that facilitates gas exchange.
• Stomata are bordered by specialized
parenchyma cells called guard cells that
regulate the size of the stomatal
opening.
• Most of the vascular plants, including
angiosperms, gymnosperms, ferns etc.
possess stomata on the epidermis of
the leaves or green young stem.
Stomata of extant ferns and gymnosperms (a,
psilophyte松叶蕨类; b, fern; c, cycad; d, ginkgophyte
苏铁; e, f, conifers; all from Kew microscope slide
collection, except b and f, which are differential-
interference contrast images of cleared leaves)

92. Function of stomata
• The exchange of carbon dioxide
and oxygen gas, and the loss of
water vapour occur at stomata.
• It is estimated that about 90% of
the exchange of gases between the
plant body and the external
environment occurs at the
stomata.

93. Stomata on the leaves
• Most dicotyledons双子叶
plants, such as sunflower,
usually have more stomata
on the lower epidermis
than the upper epidermis.
• Most monocotyledons单子叶
plants, such as wheat, the
numbers of stomata on the
upper and lower epidermis
are about the same.

94. Gas exchange in the leaf
• The stomata are connected or linked
with the spaces formed between the
loosely arranged mesophyll cells.
• Air moves into the leaf through the
stomata, then filled the air spaces and
come into contact with the surfaces of
the mesophyll cells and other cells.
• The surfaces of these cells are always
kept moist to facilitate the exchange of
gases.

95. Gaseous exchange in the stem
• Stomata can also be found on young and
tender stem.
• The stem of woody plants has no stoma but
they have lenticels.
• From Latin lens ‘lentil’ 扁豆
Corn stem.
stoma

96. Lenticels
• A lenticel is a porous tissue
consisting of cells with large
intercellular spaces in the
periderm of woody stems and
roots of dicotyledonous
flowering plants.

97. Ventilation in the stem of perennial woody
plants
• In perennial woody plants, most of the stem are lignified dead cells,
hence oxygen is not required.
• Therefore, aeration or ventilation between the lenticels and the spaces
between the cells is enough.
Drawing of a sector of
a cross section
through a 5-year old
twig from a basswood
tree (Tilia).

98. Gas exchange in the roots
• The root hairs of roots and the epidermis of
young roots can carry out exchange of
gases with the air in the soil.
• Root hair increases the surface area.
• Gas exchange in roots requires the soil to be
moist.

99. Diffusion of air within the plant body
• The spaces between the
cells in a plant are filled
with gases.
• These spaces are
interconnected and gases
are able to diffuse to every
part of the plant body
through these air spaces.

100. Summary
• Gas exchange in plants occurs by diffusion.
• The openings on plants are stomata and lenticels.