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Chapter 22
Gas Exchange
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Surviving in Thin Air
• The high mountains of the Himalayas
o Have claimed the lives of even the world’s
top mountain climbers
• The air at the height of the world’s highest peak,
Mt. Everest is so low in oxygen that most
people would pass out instantly if exposed to it
• Your body will not increase in the affinity of hemoglobin for
oxygen.
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• Twice a year, flocks of geese migrate over the Himalayas
They are able to fly at such a high altitude because of the efficiency
of their lungs
o These birds have blood with hemoglobin with a very high affinity for
oxygen
o This adaptation allows them to carry large amounts of oxygen to their
tissues to exchange with carbon dioxide
o Birds do not have mitochondria that are more efficient than those of
other vertebrates.
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• The process of gas exchange, often called
respiration
o Is the interchange of O2 and CO2 between an
organism and its environment
o Animals need oxygen because without it,
animals cannot obtain enough energy from their
food.
MECHANISMS OF GAS EXCHANGE
22.1 Overview: Gas exchange involves breathing, transport
of gases, and exchange of gases with tissue cells
• The three phases of gas exchange
O2
1 Breathing
CO2
Lung
Circulatory
system
2 Transport
of gases by
the circulatory
system
Mitochondria
O2
3Exchange
Figure 22.1
of gases
with
body
cells
CO2
Capillary
Cell
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• Gas exchange provides O2 for cellular respiration
and removes its waste product, CO2
• The process of moving air in and out of the lungs is
called breathing.
• When blood passes by body cells, the body cells
take up oxygen and release carbon dioxide to the
blood.
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22.2 Animals exchange O2 and CO2 across moist
body surfaces
• Respiratory surfaces must be thin and moist for
diffusion of O2 and CO2 to occur
• The part of an animal where gas exchange occurs
is called the respiratory surface.
• Animals that use their body surface for gas
exchange must have a high ratio of body surface
area to volume.
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• Some animals, like the earthworm
o Use their entire skin as a gas-exchange organ
Cut
Cross section
of respiratory
surface (the
skin covering
the body)
CO2
O2
Figure 22.2A
Capillaries
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• In most animals
o Specialized body parts provide large respiratory
surfaces for gas exchange
Body surface
Body surface
Respiratory
surface
(gill)
CO2
Body cells
(no capillaries)
O2
CO2
Figure 22.2C
Capillary
O2
Body surface
Figure 22.2B
CO2
CO2
Figure 22.2D
Respiratory
surface
(air tubes)
O2
Respiratory
surface
(within lung)
O2
Capillary
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22.3 Gills are adapted for gas exchange in aquatic
environments
• Gills are extensions of the body that absorb O2
dissolved in water
• Gills are generally unsuitable for animals living on
land because the animals would lose too much
water.
• The chief advantage of exchanging gases in water
is that no energy need be expended to keep the
exchange surface wet.
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• In a fish, gas exchange is enhanced by ventilation and the
countercurrent flow of water and blood
• In the countercurrent exchange systems of fish gills the blood and
water flow in opposite directions.
Gill arch
Oxygen-poor
blood
Direction
of water flow
Lamella
Oxygen-rich
blood
Gill arch
15%
40%
70%
30% 5%
60%
100%
80%
Blood
vessels
% O2 in water
flowing over
lamellae % O2 in blood
Gill
filaments
Figure 22.3
flowing through capillaries
in lamellae
Countercurrent exchange
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22.4 The tracheal system of insects provides
direct exchange between the air and body cells
• Land animals
o Exchange gases by breathing air
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• Tracheal systems in insects
o Transport O2 directly to body cells through a
network of finely branched tubes
Air sacs
Tracheae
Opening
for air
Body
cell
Tracheole
Air
sac
LM 250
Trachea
O2
Figure 22.4A, B
CO2 Body wall
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22.5 Terrestrial vertebrates have lungs
• In mammals, air inhaled through the nostrils
o Passes through the pharynx and larynx into the
trachea, bronchi, and bronchioles
Nasal
cavity
Pharynx
(Esophagus)
Larynx
Left lung
Trachea
Right lung
Bronchus
Bronchiole
Diaphragm
Figure 22.5A
(Heart)
• The nasal cavities in humans is used for:
o
o
o
o
humidification of inhaled air
warming of inhaled air
filtering of inhaled air
sampling of inhaled air for odors
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• The bronchioles end in clusters of tiny sacs called
alveoli
o Where gas exchange occurs
Oxygen-rich
blood
Oxygen-poor
blood
Bronchiole
Alveoli
Blood
capillaries
Figure 22.5B, C
Colorized SEM 6,200
CONNECTION
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22.6 Smoking is a deadly assaults on our
respiratory system
• Mucus and cilia in the respiratory passages
o Protect the lungs
o Can be destroyed by smoking causing coughing.
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• Smoking causes lung cancer, heart disease, and
emphysema & reduces the lungs’ capacity for gas
exchange.
Lung
Heart
Figure 22.6
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22.7 Breathing ventilates the lungs
• Breathing is the alternation of inhalation and
exhalation
• Inhalation in humans is achieved by contraction of
the diaphragm and chest muscles.
• Exhalation results mainly from the relaxation of the
chest muscles and diaphragm.
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• The contraction of rib muscles and the diaphragm
o Expands the chest cavity and reduces air pressure in the
alveoli (negative pressure breathing)
Rib cage
expands as
rib muscles
contract
Rib cage gets
smaller as
rib muscles
relax
Air
inhaled
Air
exhaled
Lung
Diaphragm
Figure 22.7A
Diaphragm contracts
(moves down)
Diaphragm relaxes
(moves up)
Inhalation
Exhalation
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• Vital capacity is the maximum volume of air we can
inhale and exhale
o But our lungs still hold a residual volume
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• Air flows in one direction through the more efficient lungs
of birds
• The function of air sacs in birds is to permit one-way
ventilation of the lungs.
Anterior
air sacs
Air
Posterior
air sacs
Air
Trachea
Lungs
Air
tubes
in lung
Figure 22.7B
Inhalation:
Exhalation:
Air sacs fill
Air sacs empty; lungs fill
1 mm
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22.8 Breathing is automatically controlled
• Breathing control centers in the brain
o Keep breathing in tune with body needs, sensing and
responding to the CO2 level in the blood
Brain
Cerebrospinal
fluid
Pons
Medulla
Breathing
control
centers
stimulated by:
Nerve signals
trigger
contraction
of muscles
CO2 increase /
pH decrease
in blood
CO2 and O2
sensors in
aorta
Diaphragm
Figure 22.8
Rib muscles
Nerve signals
indicating CO2
and O2 levels
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• A drop in blood pH
o Triggers an increase in the rate and depth of
breathing
TRANSPORT OF GASES IN THE BODY
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22.9 Blood transports respiratory gases
• The heart pumps oxygen-poor blood to the lungs
o Where it picks up O2 and drops off CO2
• Then the heart pumps the oxygen-rich blood to
body cells
o Where it drops off O2 and picks up CO2
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• Gas transport and exchange in the body
Inhaled air
Exhaled air
Alveolar
epithelial
cells
Air spaces
CO2
O2
CO2
O2
Alveolar
capillaries of lung
CO2-rich,
O2-poor
blood
O2-rich,
CO2-poor
blood
Heart
Tissue
capillaries
CO2
O2
CO2 Interstitial O2
fluid
Figure 22.9
Tissue cells
throughout body
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• Gases diffuse down partial-pressure gradients
o In the lungs and the tissues
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22.10 Hemoglobin carries O2 and helps transport
CO2 and buffer the blood
• Hemoglobin in red blood cells
o Transports oxygen, helps buffer the blood, and
carries some CO2
Iron atom
O2 loaded
in lungs
O2 unloaded
in tissues
Heme group
Figure 22.10
Polypeptide chain
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• Most CO2 in the blood
o Is transported as bicarbonate ions in the plasma
CO2
Carbon
dioxide
+
H2O
Water
H2CO3
Carbonic
acid
H+
+
Hydrogen
ions
HCO3–
Bicarbonate
CONNECTION
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22.11 The human fetus exchanges gases with the
mother’s bloodstream
• A human fetus
o Exchanges gases with maternal blood in the placenta
Placenta, containing
maternal blood vessels
and fetal capillaries
Umbilical cord,
containing fetal
blood vessels
Amniotic
fluid
Uterus
Figure 22.11
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• Fetal hemoglobin
o Enhances oxygen transfer from maternal blood
• At birth, increasing CO2 in the fetal blood causes
the baby to start breathing.