37
Gas Exchange in Animals
Chapter 37 Gas Exchange in Animals
Key Concepts
• 37.1 Fick’s Law of Diffusion Governs
Respiratory Gas Exchange
• 37.2 Respiratory Systems Have Evolved to
Maximize Partial Pressure Gradients
• 37.3 The Mammalian Lung Is Ventilated by
Pressure Changes
Chapter 37 Gas Exchange in Animals
Key Concepts
• 37.4 Respiration Is under Negative
Feedback Control by the Nervous System
• 37.5 Respiratory Gases Are Transported
in the Blood
Chapter 37 Opening Question
How are bar-headed geese able to
sustain the high metabolic cost of flight
at altitudes higher than Mount Everest?
Concept 37.1 Fick’s Law of Diffusion Governs Respiratory
Gas Exchange
Organisms must exchange O2 and CO2—
respiratory gases exchanged only 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.
In-Text Art, Ch. 37, p. 730
Concept 37.1 Fick’s Law of Diffusion Governs 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
Concept 37.1 Fick’s Law of Diffusion Governs 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.
Concept 37.1 Fick’s Law of Diffusion Governs 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
Concept 37.1 Fick’s Law of Diffusion Governs 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, or central cavities, or specialized
respiratory systems.
In-Text Art, Ch. 37, p. 731 (1)
In-Text Art, Ch. 37, p. 731 (2)
Concept 37.1 Fick’s Law of Diffusion Governs Respiratory
Gas Exchange
O2 availability is limited in some
environments due to temperature.
For water-breathers, body temperature and
metabolic rate rise with an increase in
water temperature—need for oxygen
increases while the available oxygen
decreases.
For air-breathers, increase in altitude
reduces available oxygen due to lower
partial pressure of oxygen at high
altitudes.
Concept 37.1 Fick’s Law of Diffusion Governs Respiratory
Gas Exchange
Respiratory gas exchange is a two-way
process: CO2 diffuses out of the body as
O2 diffuses in.
The concentration gradient of CO2 from airbreathers to the environment is always
large.
CO2 is very soluble in water and is easy for
aquatic animals to exchange.
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
Gas exchange systems are made up of
surfaces and the mechanisms that
ventilate and perfuse those surfaces.
Adaptations to maximize the exchange of
O2 and CO2:
• Increase surface area
• Maximize partial pressure difference
• Minimize diffusion path length
• Minimize the diffusion that takes place in
an aqueous medium
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
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
Figure 37.1 Gas Exchange Systems
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
Transporting gases optimizes partial
pressure gradients—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
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
Insects have a tracheal system throughout
their bodies.
Spiracles in the abdomen open to allow gas
exchange and close to limit water loss.
Spiracles open into tracheae that branch to
tracheoles, which end in air capillaries.
Figure 37.2 The Tracheal Gas Exchange System of Insects (Part 1)
Figure 37.2 The Tracheal Gas Exchange System of Insects (Part 2)
Figure 37.2 The Tracheal Gas Exchange System of Insects (Part 3)
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
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 37.3 Countercurrent Exchange Is More Efficient
Figure 37.4 Fish Gills (Part 1)
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
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.
Figure 37.4 Fish Gills (Part 2)
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
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.
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
Most terrestrial vertebrates use tidal
ventilation in lungs—air flows in and out by
the same path.
Lungs and airways are never completely
empty—contain some dead space
The residual volume (RV) is the air that
cannot be expelled from the lungs and
contains “stale” (low O2) air.
Each inhalation brings a mixture of outside
air and stale air to the exchange area.
Concept 37.2 Respiratory Systems Have Evolved to Maximize
Partial Pressure Gradients
Bird lungs use unidirectional air flow to
maintain a high PO2 gradient.
Air enters through the posterior end of the
lung and flows through parabronchi, and
then into air capillaries—the sites of gas
exchange.
Birds have air sacs that receive inhaled air
but are not sites of gas exchange.
Posterior air sacs store fresh air and release
it to lungs during exhalation—anterior sacs
receive air from lungs.
Figure 37.5 The Respiratory System of a Bird (Part 1)
Figure 37.5 The Respiratory System of a Bird (Part 2)
Concept 37.3 The Mammalian Lung Is Ventilated by
Pressure Changes
In mammals, air enters the lung through the
oral cavity and nasal passage, which join
in the pharynx.
Below the pharynx, the esophagus directs
food to the stomach, and the trachea leads
to the lungs—at the beginning is the
larynx, or voice box.
The trachea branches into two bronchi, then
into bronchioles, and then into alveoli—
the sites of gas exchange.
Figure 37.6 The Human Respiratory System (Part 1)
Figure 37.6 The Human Respiratory System (Part 2)
Concept 37.3 The Mammalian Lung Is Ventilated by
Pressure Changes
Capillaries surround and lie between the
alveoli—diffusion path between blood and
air is less than two micrometers.
Mammalian lungs produce two secretions
that affect ventilation—mucus and
surfactant.
Mucus lines the airways and captures dirt
and microorganisms.
A surfactant reduces the surface tension
of liquid lining the alveoli.
Figure 37.6 The Human Respiratory System (Part 3)
Figure 37.6 The Human Respiratory System (Part 4)
Figure 37.6 The Human Respiratory System (Part 5)
Concept 37.3 The Mammalian Lung Is Ventilated by
Pressure Changes
Tidal volume (TV)—the amount of air that
moves in and out per breath, at rest.
Inspiratory (IRV) and expiratory (ERV)
reserve volumes are the additional
amounts of air that we can forcefully inhale
or exhale.
The vital capacity (VC) is the sum of TV +
IRV + ERV.
Figure 37.7 Measuring Lung Ventilation
Concept 37.3 The Mammalian Lung Is Ventilated by
Pressure Changes
Mammalian lungs are suspended inside a
thoracic cavity.
The diaphragm is a sheet of muscle at the
bottom of the cavity.
The pleural membrane covers each lung
and lines the thoracic cavity.
The space between the membranes
contains fluid to help them slide past each
other during breathing.
Concept 37.3 The Mammalian Lung Is Ventilated by
Pressure Changes
Inhalation begins when the diaphragm
contracts—it pulls down, expanding the
thoracic cavity and pulling on on the
pleural membranes.
The pleural membranes pull on the lungs,
which expand and draw air in from outside.
Exhalation begins when the diaphragm
relaxes.
The elastic lung tissues pull the diaphragm
up and push air out of the airways.
Figure 37.8 Into the Lungs and Out Again (Part 1)
Figure 37.8 Into the Lungs and Out Again (Part 2)
Concept 37.3 The Mammalian Lung Is Ventilated by
Pressure Changes
Additional muscles are used during
exercise.
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.
Concept 37.4 Respiration Is under Negative Feedback Control by
the Nervous System
Breathing is controlled in the medulla
oblongata, in the brain stem.
Groups of respiratory motor neurons
increase their firing rate just before an
inhalation.
The breathing rate is modulated to meet
demands for O2 supply and CO2
elimination.
Concept 37.4 Respiration Is under Negative Feedback Control by
the Nervous System
In humans and mammals, the breathing rate
is more sensitive to increases in CO2 than
to falling levels of O2.
The PCO2 of blood is the primary metabolic
feedback for breathing. When breathing or
metabolism changes, it alters PO2 and PCO2
in the blood.
Ventilation increases rapidly with exercise,
in anticipation of a rise in PCO2.
Figure 37.9 Sensitivity of Respiratory Control System Changes With Exercise (Part 1)
Figure 37.9 Sensitivity of Respiratory Control System Changes With Exercise (Part 2)
Concept 37.4 Respiration Is under Negative Feedback Control by
the Nervous System
The major site of sensitivity to PCO2 is on the
ventral surface of the medulla.
Cells respond to H+ ions produced when
CO2 diffuses from blood into extracellular
fluid.
CO2 + H2O H2CO3 H+ + HCO3–
H+ ions stimulate cells to increase
respiratory gas exchange—respiration is
controlled by pH.
Concept 37.4 Respiration Is under Negative Feedback Control by
the Nervous System
Sensitivity to PO2 is monitored by the
carotid and aortic bodies in the blood
vessels leaving the heart.
If PO2 falls, chemoreceptors in these bodies
send nerve impulses to the brain stem to
stimulate breathing.
Figure 37.10 Feedback Information Controls Breathing
Concept 37.5 Respiratory Gases Are Transported in the Blood
Respiratory gases are transported in the
blood.
O2 is non-polar and does not dissolve in the
blood plasma, or liquid part of blood.
In most animals blood contains transport
molecules that bind reversibly to O2.
One such O2 transporter is hemoglobin—a
protein in red blood cells
Concept 37.5 Respiratory Gases Are Transported in the Blood
Hemoglobin is a protein with four
polypeptide subunits.
Each polypeptide surrounds a heme group
that can bind a molecule of O2.
One molecule of hemoglobin can bind up to
four molecules of O2.
O2 is picked up where PO2 is high and is
released where PO2 is lower.
Figure 37.11 Binding of O2 to Hemoglobin Depends on PO2
Concept 37.5 Respiratory Gases Are Transported in the Blood
The relationship between PO2 and the
amount of O2 that binds is S-shaped.
Hemoglobin will pick up or release O2
depending on the PO2 of the environment.
If PO2 of the plasma is high, as in the lungs,
hemoglobin will pick up its maximum of
four O2 molecules.
As blood circulates through tissues with
lower PO2, hemoglobin will release only
some O2.
Concept 37.5 Respiratory Gases Are Transported in the Blood
Myoglobin is a single polypeptide molecule
in muscles and can bind one molecule of
O2.
It has a higher affinity for O2, binds it at low
PO2 values when hemoglobin molecules
would release their O2.
It provides a reserve for high metabolic
demand for O2.
Concept 37.5 Respiratory Gases Are Transported in the Blood
The affinity of hemoglobin for O2 varies.
Three factors are:
• Hemoglobin composition
• pH—in the Bohr effect, blood circulating
through active tissues has a lower pH and
H+ ions bind to the hemoglobin molecule in
place of O2
• 2,3-bisphosphoglyceric acid (BPG)—also
lowers the affinity for O2
Figure 37.12 Oxygen-binding Adaptations
Concept 37.5 Respiratory Gases Are Transported in the Blood
Besides delivering O2, blood also transports
CO2 away from the tissues.
In the blood plasma, CO2 is slowly
converted into bicarbonate ions (HCO3–).
In endothelial cells and red blood cells,
carbonic anhydrase speeds up the
conversion.
Concept 37.5 Respiratory Gases Are Transported in the Blood
The conversion keeps PCO2 low and
facilitates diffusion away from the tissues.
Some CO2 binds to hemoglobin molecules.
In the lungs the conversion reaction is
reversed—CO2 diffuses from the blood into
the alveoli and is exhaled.
Figure 37.13 Carbon Dioxide Is Transported as Bicarbonate Ions
Answer to Opening Question
Bar-headed geese have adaptations in
respiration that give them an advantage:
• As birds, they have a continuous,
unidirectional air flow.
• Because they live at high altitude, their
respiration is driven by increases in
low O2.
• They also have a point mutation in their
hemoglobin gene that gives their
hemoglobin a higher affinity for O2.