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CHAPTER 49
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Gas Exchange
• One of the major physiological challenges facing
all multicellular animals is obtaining sufficient
oxygen and disposing of excess carbon dioxide
• In vertebrates, the gases diffuse into the
aqueous layer covering the epithelial cells that
line the respiratory organs
• Diffusion is passive, driven only by the difference
in O2 and CO2 concentrations on the two sides
of the membranes and their relative solubilities
in the plasma membrane
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Gas Exchange
• Gases diffuse directly into unicellular organisms
• However, most multicellular animals require
system adaptations to enhance gas exchange
• Amphibians respire across their skin
• Insects have an extensive tracheal system
• Fish use gills
• Mammals have a large network of alveoli
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Gills
• Specialized extensions of tissue that
project into water
• Increase surface area for diffusion
• External gills are not enclosed within body
structures
– Found in fish and amphibians
– Two main disadvantages
• Must be constantly moved to ensure contact with
oxygen-rich fresh water
• Are easily damaged
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Gills
• Gills of bony fishes are located between
the oral (buccal or mouth) cavity and the
opercular cavities
• These two sets of cavities function as
pumps that alternately expand
• Move water into the mouth, through the
gills, and out of the fish through the open
operculum or gill cover
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Gills
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Gills
• Some bony fish have immobile opercula
– Swim constantly to force water over gills
– Ram ventilation
• Most bony fish have flexible gill covers
• Remora switch between ram ventilation
and pumping action
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Gills
• 3–7 gill arches on each side of a fish’s
head
• Each is composed of two rows of gill
filaments
• Each gill filament consist of lamellae
– Thin membranous plates that project into
water flow
– Water flows past lamellae in 1 direction only
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Gills
• Within each lamella, blood flows opposite
to direction of water movement
– Countercurrent flow
– Maximizes oxygenation of blood
– Increases Dp
• Fish gills are the most efficient of all
respiratory organs
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Gills
• Many amphibians use cutaneous
respiration for gas exchange
• In terrestrial arthropods, the respiratory
system consists of air ducts called
trachea, which branch into very small
tracheoles
– Tracheoles are in direct contact with individual
cells
– Spiracles (openings in the exoskeleton) can
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be opened or closed by valves
Lungs
• Gills were replaced in terrestrial animals
because
– Air is less supportive than water
– Water evaporates
• The lung minimizes evaporation by moving
air through a branched tubular passage
• A two-way flow system
– Except birds
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Lungs
• Air exerts a pressure downward, due to
gravity
• A pressure of 760 mm Hg is defined as
one atmosphere (1.0 atm) of pressure
• Partial pressure is the pressure
contributed by a gas to the total
atmospheric pressure
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Lungs
• Lungs of amphibians are formed as
saclike outpouchings of the gut
• Frogs have positive pressure breathing
– Force air into their lungs by creating a positive
pressure in the buccal cavity
• Reptiles have negative pressure breathing
– Expand rib cages by muscular contractions,
creating lower pressure inside the lungs
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Lungs
• Lungs of mammals are packed with
millions of alveoli (sites of gas exchange)
• Inhaled air passes through the larynx,
glottis, and trachea
• Bifurcates into the right and left bronchi,
which enter each lung and further
subdivide into bronchioles
• Alveoli are surrounded by an extensive
capillary network
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Lungs
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Lungs
• Respiration in birds occurs in two cycles
– Cycle 1 = Inhaled air is drawn from the
trachea into posterior air sacs, and exhaled
into the lungs
– Cycle 2 = Air is drawn from the lungs into
anterior air sacs, and exhaled through the
trachea
• Blood flow runs 90o to the air flow
– Crosscurrent flow
– Not as efficient as countercurrent flow
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Lungs
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Gas Exchange
• Gas exchange is driven by differences in partial
pressures
• Blood returning from the systemic circulation,
depleted in oxygen, has a partial oxygen
pressure (PO2) of about 40 mm Hg
• By contrast, the PO2 in the alveoli is about 105
mm Hg
• The blood leaving the lungs, as a result of this
gas exchange, normally contains a PO2 of about
100 mm
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Lung Structure and Function
• Outside of each lung is covered by the
visceral pleural membrane
• Inner wall of the thoracic cavity is lined by
the parietal pleural membrane
• Space between the two membranes is
called the pleural cavity
– Normally very small and filled with fluid
– Causes 2 membranes to adhere
– Lungs move with thoracic cavity
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Lung Structure and Function
• During inhalation, thoracic volume
increases through contraction of two
muscle sets
– Contraction of the external intercostal
muscles expands the rib cage
– Contraction of the diaphragm expands the
volume of thorax and lungs
• Produces negative pressure which draws
air into the lungs
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Lung Structure and Function
• Tidal volume
– Volume of air moving in and out of lungs in a person
at rest
• Vital capacity
– Maximum amount of air that can be expired after a
forceful inspiration
• Hypoventilation
– Insufficient breathing
– Blood has abnormally high PCO2
• Hyperventilation
– Excessive breathing
– Blood has abnormally low PCO2
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Lung Structure and Function
• Each breath is initiated by neurons in a
respiratory control center in the medulla
oblongata
• Stimulate external intercostal muscles and
diaphragm to contract, causing inhalation
• When neurons stop producing impulses,
respiratory muscles relax, and exhalation occurs
• Muscles of breathing usually controlled
automatically
– Can be voluntarily overridden – hold your breath
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Lung Structure and Function
• Neurons are sensitive to blood PCO2 changes
• A rise in PCO2 causes increased production of
carbonic acid (H2CO3), lowering the blood pH
• Stimulates chemosensitive neurons in the aortic
and carotid bodies
• Send impulses to respiratory control center to
increase rate of breathing
• Brain also contains central chemoreceptors that
are sensitive to changes in the pH of
cerebrospinal fluid (CSF)
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Respiratory Diseases
• Chronic obstructive pulmonary disease
(COPD)
– Refers to any disorder that obstructs airflow
on a long-term basis
– Asthma
• Allergen triggers the release of histamine, causing
intense constriction of the bronchi and sometimes
suffocation
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Respiratory Diseases
• Chronic obstructive pulmonary disease
(COPD) (cont.)
– Emphysema
• Alveolar walls break down and the lung exhibits
larger but fewer alveoli
• Lungs become less elastic
• People with emphysema become exhausted
because they expend three to four times the
normal amount of energy just to breathe
• Eighty to 90% of emphysema deaths are caused
by cigarette smoking
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Respiratory Diseases
• Lung cancer accounts for more deaths than any
other form of cancer
• Caused mainly by cigarette smoking
• Follows or accompanies COPD
• Lung cancer metastasizes (spreads) so rapidly
that it has usually invaded other organs by the
time it is diagnosed
• Chance of recovery from metastasized lung
cancer is poor, with only 3% of patients surviving
for 5 years after diagnosis
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Hemoglobin
• Consists of four polypeptide chains: two a and
two b
• Each chain is associated with a heme group
• Each heme group has a central iron atom that
can bind a molecule of O2
• Hemoglobin loads up with oxygen in the lungs,
forming oxyhemoglobin
• Some molecules lose O2 as blood passes
through capillaries, forming deoxyhemoglobin
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Hemoglobin
• Hemoglobin’s affinity for O2 is affected by
pH and temperature
• The pH effect is known as the Bohr shift
– Increased CO2 in blood increases H+
– Lower pH reduces hemoglobin’s affinity for
O2
– Results in a shift of oxyhemoglobin
dissociation curve to the right
– Facilitates oxygen unloading
• Increasing temperature has a similar effect38
Hemoglobin
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Transportation of Carbon Dioxide
• About 8% of the CO2 in blood is dissolved in
plasma
• 20% of the CO2 in blood is bound to hemoglobin
• Remaining 72% diffuses into red blood cells
– Enzyme carbonic anhydrase combines CO2 with H2O
to form H2CO3
– H2CO3 dissociates into H+ and HCO3–
– H+ binds to deoxyhemoglobin
– HCO3– moves out of the blood and into plasma
– One Cl– exchanged for one HCO3– – “chloride shift”
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Transportation of Carbon Dioxide
• When the blood passes through
pulmonary capillaries, these reactions are
reversed
• The result is the production of CO2 gas,
which is exhaled
• Other dissolved gases are also
transported by hemoglobin
– Nitric oxide (NO) and carbon monoxide (CO)
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