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Chapter 48
Lecture 17
Gas exchange in animals
Dr. Chris Faulkes
Gas exchange in animals
Aims:
•  To examine the principles behind gas exchange in
animals
•  To examine how different animals maximise
respiratory gas exchange
•  To examine how human lungs work
Gas exchange in animals
Aims:
•  To examine the principles behind gas exchange in animals
•  To examine how different animals maximise respiratory gas exchange
•  To examine how human lungs work
These lecture aims form part of the knowledge
required for learning outcomes 3 and 4.
Describe mechanisms for life processes (LOC3).
Appreciate how the physiology of an organism fits
it for its environment (LOC4).
48 Gas Exchange in Animals
• 48.1 What Physical Factors Govern
Respiratory Gas Exchange?
• 48.2 What Adaptations Maximize Respiratory
Gas Exchange?
• 48.3 How Do Human Lungs Work?
Essential reading
•  Pages 1024-1035
48.1 What Physical Factors Govern Respiratory Gas Exchange?
O2 and CO2 are respiratory gases
exchanged by diffusion along their
concentration gradients.
Partial pressure is the concentration of
a gas in a mixture.
Barometric pressure: Atmospheric
pressure at sea level is 760 mm Hg.
Partial pressure of O2 (PO2) is 159
mm Hg
48.1 What Physical Factors Govern Respiratory Gas Exchange?
Fick s law of diffusion applies to all
gas exchange systems.
P1– P2
Q = DA
L
Q: the rate of diffusion
D: the diffusion coefficient—a
characteristic of the diffusing
substance, the medium, and the
temperature
48.1 What Physical Factors Govern Respiratory Gas Exchange?
P1– P2
Q = DA
L
A: the area where diffusion occurs
P1 and P2: partial pressures of the gas
at two locations
L: the path length between the
locations
(P1 – P2)/L is a partial pressure
gradient.
48.1 What Physical Factors Govern Respiratory Gas Exchange?
Oxygen is easier to obtain from air
than from water:
• O2 content of air is higher than that of
water.
• O2 diffuses much faster through air.
• Air and water must be moved by the
animal over its gas exchange
surfaces; requires more energy to
move water than air.
48.1 What Physical Factors Govern Respiratory Gas Exchange?
The slow rate of diffusion of oxygen in
water limits the size and shape of
species without internal systems for
gas exchange.
These species have evolved larger
surface areas, central cavities, or
specialized respiratory systems.
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Figure 48.1 Keeping in Touch with the Medium
48.1 What Physical Factors Govern Respiratory Gas Exchange?
An aquatic animal s body temperature
and metabolic rate rise with an
increase in water temperature.
The animal s need for oxygen is
increased while the available oxygen
decreases in the warmer water.
An increase in altitude reduces
available oxygen for air breathers
due to the lower partial pressure of
oxygen at high altitudes.
Figure 48.2 The Double Bind of Water Breathers
48.1 What Physical Factors Govern Respiratory Gas Exchange?
CO2 diffuses out of the body as O2
diffuses in.
The concentration gradient of CO2
from air-breathers to the environment
is always large.
CO2 is very soluble in water and is
easy for aquatic animals to
exchange.
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Some respiratory systems have
adaptations to maximize the
exchange of O2 and CO2.
• Increased surface area
• Maximized partial pressure gradients
• Minimized diffusion path length
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Surface area (A) is increased by:
•  External gills: also minimize the diffusion path
length (L) of O2 and CO2 in water
•  Internal gills: protected from predators and
damage
•  Lungs: internal cavities for respiratory gas
exchange with air
•  Tracheae: air-filled tubes in
insects
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Partial pressure gradients are
increased by:
• Minimization of the diffusion path
length (L) of O2 and CO2
• Ventilation: active moving of the
respiratory medium over the gas
exchange surfaces
• Perfusion: circulating blood over the
gas exchange surfaces
48.2 What Adaptations Maximize Respiratory Gas Exchange?
A gas exchange system is made up
of the gas exchange surfaces and
mechanisms for ventilation and
perfusion of those surfaces.
Examples: tracheal system in insects,
fish gills, lungs in birds and humans.
Figure 48.3 Gas Exchange Systems
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Insects have a tracheal system:
Spiracles in the abdomen open to
allow gas exchange and close to limit
water loss.
Spiracles open into tracheae, that
branch to tracheoles, that end in air
capillaries.
Figure 48.4 The Tracheal Gas Exchange System of Insects
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Fish gills use countercurrent flow to
maximize gas exchange.
Gills are supported by gill arches that
lie between the mouth and the
opercular flaps.
Water flows unidirectionally into the
mouth, over the gills, and out from
under the opercular flaps.
Figure 48.5 Fish Gills
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Constant water flow maximizes PO2
on the external gill surfaces and
blood circulation minimizes PO2 on
the internal surfaces.
Gills are made up of gill filaments that
are covered by folds, or lamellae.
Lamellae are the site of gas exchange
and minimize the diffusion path
length (L) between blood and water.
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Afferent blood vessels bring blood to
the gills and efferent vessels take
blood away.
Blood flows through the lamellae in the
direction opposite to the flow of
water.
The countercurrent flow optimizes
the PO2 gradient.
Figure 48.6 Countercurrent Exchange Is More Efficient
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Bird lungs use unidirectional air flow to
maintain a high PO2 gradient.
Birds also have air sacs that receive
inhaled air but are not sites of gas
exchange.
Air enters through the trachea, which
divides into bronchi, then into
parabronchi, and then into air
capillaries.
Figure 48.7 The Respiratory System of a Bird
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Air sacs keep air moving through the
lungs in a continuous and
unidirectional flow:
• Air flows unidirectionally through the
parabronchi.
• Inhalation expands the air sacs and
exhalation compresses them: fresh
air is forced out and passes over the
lungs.
Figure 48.8 The Path of Air Flow through Bird Lungs
Figure 48.8 The Path of Air Flow through Bird Lungs (Part 1)
Figure 48.8 The Path of Air Flow through Bird Lungs (Part 2)
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Ventilation in lungs is tidal: air flows in
and out by the same path.
Tidal volume: the amount of air that
moves in and out per breath, at rest
is measured by a spirometer.
Inspiratory and expiratory reserve
volumes are the additional amounts
of air that we can inhale or exhale.
Figure 48.9 Measuring Lung Ventilation
48.2 What Adaptations Maximize Respiratory Gas Exchange?
The vital capacity is the sum of the
tidal volume, the inspiratory reserve
volume, and the expiratory reserve
volume.
The residual volume is the air that
cannot be expelled from the lungs.
The total lung capacity is the sum of
the vital capacity and the residual
volume.
48.2 What Adaptations Maximize Respiratory Gas Exchange?
Tidal breathing reduces PO2 and does
not permit countercurrent gas
exchange.
Two features offset the inefficiency of
tidal breathing in mammals:
• An enormous surface area
• A very short path length for diffusion
48.3 How Do Human Lungs Work?
Air enters the human lung through the
oral cavity or nasal passage via the
trachea.
The trachea branches into two
bronchi, then into bronchioles, and
then into alveoli—the sites of gas
exchange.
Figure 48.10 The Human Respiratory System
48.3 How Do Human Lungs Work?
Mammalian lungs produce two
secretions that affect ventilation:
mucus and surfactant.
• Mucus: lines the airways and
captures dirt and microorganisms.
The mucus escalator is a group of
cells with cilia that sweep the mucus
and particles out of the airways.
48.3 How Do Human Lungs Work?
A surfactant reduces the surface
tension of a liquid.
The fluid covering the alveoli has
surface tension that makes the lungs
elastic.
Lung surfactant is released by cells in
the alveoli when they are stretched—
less force is needed to inflate the
lungs.
48.3 How Do Human Lungs Work?
Premature babies may not have
developed the ability to make lung
surfactant.
Without it, they have great difficulty
breathing, known as respiratory
distress syndrome.
48.3 How Do Human Lungs Work?
Human lungs are inside a right and left
thoracic cavity.
The diaphragm is a sheet of muscle
at the bottom of the cavities.
The pleural membrane lines each
cavity and covers each lung, and
encloses the pleural space.
48.3 How Do Human Lungs Work?
The pleural space contains fluid to
help the membranes slide past each
other during breathing.
A negative pressure is created in the
pleural space when the volume
increases in the thoracic cavity.
The slight negative pressure present in
between breaths keeps the alveoli
inflated.
48.3 How Do Human Lungs Work?
Inhalation begins when the diaphragm
contracts.
The diaphragm pulls down on the
thoracic cavity and on the pleural
membranes.
The pleural membranes pull on the
lungs, air enters through the trachea,
and the lungs expand.
48.3 How Do Human Lungs Work?
Exhalation begins when the diaphragm
stops contracting and relaxes.
The elastic lung tissues pull the
diaphragm back up and push air out
of the airways.
48.3 How Do Human Lungs Work?
The intercostal muscles, located
between the ribs, can also change
the volume of the thoracic cavity.
The external intercostal muscles lift
the ribs up and outward, expanding
the cavity.
The internal intercostal muscles
decrease the volume by pulling the
ribs down and inward.
Figure 48.11 Into the Lungs and Out Again
Into the Lungs and Out Again
Endoscopic view of trachea, bronchi, and bronchioles
Gas exchange in animals
Check out
•  48.1 Recap, page 1027
•  48.1 CHAPTER SUMMARY, page 1041
•  48.2 Recap, page 1033
•  48.2 CHAPTER SUMMARY, page 1041, See WEB/CD Activity 48.1
•  48.3 Recap, page 1035
•  48.3 CHAPTER SUMMARY, page 1041, See WEB/CD Activity 48.2
Self Quiz
page 1041-1042: Chapter 48, questions 1-5
For Discussion
•  page 1042: Chapter 48, question 3
Gas exchange in animals
Key terms:
afferent, alveoli, bronchiole, bronchus (pl. bronchi),
countercurrent, diaphragm, efferent, expiratory
reserve volume, Fick’s law, gills, inspiratory reserve
volume, intercostal, lamellae, larynx (voice box),
lungs, mucus, mucus escalator, opercular flaps,
parabronchi, perfusion, pharynx, plearual space,
spiracles, spirometer, surfactant, thoracic cavity,
tidal ventilation, trachea, ventilation, vital capacity