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AP Biology
March 2008
Respiration
Chapter 42
Gas exchange occurs across specialized respiratory surfaces.
1) Gas exchange: the uptake of molecular oxygen (O2) from
the environment and the discharge of carbon dioxide
(CO2) to the environment.
a) Relies on the diffusion of gases down pressure
gradients.
i) At sea level, atmosphere exerts a total pressure of
760 mm Hg.
ii) This is the downward force equal to that exerted by
a column of mercury 750 mm high.
iii) Since the atmosphere is 21% O2 (by volume)
Then the partial pressure of oxygen is
0.21 x 760
=
160 mm Hg
b) Gases will diffuse down a pressure gradient across a
respiratory surface if it is:
i) permeable
ii) moist
2) The amount of diffusion depends on the surface area of
the membrane and the differences in partial pressure.
Factors Influencing Gas Exchange
1) Surface-to-volume ratio: animals must have a surface
area large enough to allow the exchange of gases in
large enough quantities to sustain life.
2) Ventilation
a) Many animals have developed adaptations to move
air, or water, over respiratory surfaces.
3) Transportation pigments
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a) Aids in the movement of gases throughout the body,
increasing the quantity of molecules the blood can
carry.
b) Hemoglobin is the respiratory pigment found in almost
all vertebrates.
i) It consists of four subunits, each of which is a heme
group with an embedded iron atom.
ii) The iron atom binds O2, thus one hemoglobin can
carry 4 oxygen molecules.
c) Hemocyanin is a transport pigment found in many
invertebrates.
i) Contains copper, the oxygen-binding component.
ii) Common in arthropods and many mollusks.
iii) Dissolved in hemolymph rather than being confined
to cells.
Invertebrate Respiration
1) Respiratory medium: the source off the oxygen.
a) Air for terrestrial animals
b) Water for aquatic animals
2) Respiratory surface: the part of an animal where oxygen
from the environment diffuses into living cells and carbon
dioxide diffuses out.
a) Always moist
b) O2 and CO2 will first dissolve in the moist environment
surrounding the cells and then diffuse through the
membrane.
3) Integumentary Exchange
a) Used by small invertebrates, such as flatworms and
earthworms.
b) Epidermis at the body surface is used for integumentary
exchange.
c) Earthworms have a mucus coating on their surface that
helps keep the surface moist to allow the O2 to diffuse
inward through the epidermis.
4) Gills
a) Used by invertebrates in aquatic environments.
b) A gill has a thin, moist, vascularized epidermis.
c) Gill walls are highly folded to increase surface area.
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5) Tracheal respiration
a) Used by terrestrial arthropods.
b) Consists of internal tubes that branch repeatedly and
deliver air directly to body cells.
c) Air enters the tracheae through openings called
spiracles on the body surface and passes into smaller
tubes called tracheoles.
i) Tracheoles terminate on the plasma membranes of
individual cells.
d) In most cases no participation by the circulatory system
is needed, neither is any respiratory pigments needed.
Vertebrate Exchange
1) Gills of Fishes and Amphibians
a) Gills: variously shaped outfoldings of the body surface
specialized for gas exchange.
i) The total surface are of the gills is often much
greater than that of the rest of the body.
b) The internal gills of adult fishes are positioned where
water can enter the mouth and then flow over them as
it exits just behind the head.
c) Water flows over the gills and blood circulates through
them in OPPOSITE DIRECTIONS.
i) Countercurrent Exchange: the opposite flow of
adjacent fluids that maximizes transfer rates.
d) This mechanism is highly efficient in extracting O2 from
water whose oxygen content is lower than air.
2) Lungs
a) Lungs contain internal respiratory surfaces shaped as a
cavity or sac.
b) Lungs provide a membrane for gaseous exchange with
blood.
i) Air moves by bulk flow into and out of the lungs.
ii) Gases diffuse across the inner respiratory surfaces of
the lungs.
iii) Pulmonary circulation enhances the diffusion of
dissolved gases into and out to lung capillaries.
iv) In body tissues,
O2 diffuses from
blood ------> interstitial fluid ------> cells
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CO2 travels the route in reverse.
c) Lungs also participate in sound production by forcing
air to pass through the glottis opening causing the
vocal cords on either side to vibrate.
d) Amphibians have small lungs or no lungs, and they rely
heavily on the diffusion of gases across other body
surfaces.
i) Frogs: the skin supplements gas exchange.
e) Turtles are another vertebrate exception.
i) Rigid shell restricts breathing movements.
ii) Supplement with gas exchange across moist
epithelial surfaces in their mouth and anus.
Human Respiratory System
1) Anatomy of respiratory system
a) Located in the thoracic (chest) cavity.
b) Air enters through the nostrils.
c) Then filtered by hairs, warmed, humidified, and
sampled for odors as it flows through a maze of spaces
in the nasal cavity.
d) Nasal cavity leads to the pharynx.
i) An intersection where the paths for air and food
cross.
ii) When food is swallowed, the larynx (the upper part
of the respiratory tract) moves upward and tips the
epiglottis over the glottis (the opening of the wind
pipe).
iii) This allows food to go down the esophagus to the
stomach.
iv) The rest of the time the glottis is open and we can
breath.
e) From the larynx, air passes into the trachea, or
windpipe.
f) The trachea forks into two bronchi.
i) One leading to each lung.
g) Within the lung, the bronchus branches repeatedly into
finer and finer tubes called bronchioles.
h) At their tips, the tiniest bronchioles dead-end as a
cluster of air sacs called alveoli.
i) The thin epithelium of the millions of alveoli in the lung
serves as the respiratory surface.
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i) O2 in the air conveyed to he alveoli dissolves in the
moist film and diffuses across the epithelium and into
a web of capillaries that surrounds the alveolus.
ii) CO2 diffuses from the capillaries, across the
epithelium of the alveolus, and into the air space.
Breathing – Cyclic Reversals in Air Pressure Gradients
1) The Respiratory Cycle
a) In inhalation, the diaphragm contracts and flattens,
muscles lift the rib cage upward and outward, the
chest cavity volume increases, internal pressure
decreases, air rushes in.
i) Negative pressure breathing: works like a suction
pump, pulling air, instead of pushing it, down into
the lungs.
ii) Results from changes in the volume of the lungs.
iii) Diaphragm: a sheet of skeletal muscle that forms
the bottom wall of the chest cavity.
b) In exhalation, the actions listed above are reversed; the
elastic lung tissue recoils passively.
i) Rib muscles and diaphragm relax.
ii) The lung volume is reduced.
iii) The increase in air pressure within the alveoli forces
air up the breathing tubes and out through the
nostrils.
2) Lung Volumes
a) Vital capacity: the maximum volume that can be
moved in or out.
b) Lungs cannot be completely emptied.
c) Tidal volume: the amount of air that enters and leaves
with each breath -- about 500 ml.
Gas Exchange and Transport
1) Gas Exchange
a) Each alveolus consists of a single layer of epithelial cells
through which gases can readily diffuse to and from
interstitial fluid and blood capillaries.
b) The partial pressure gradients are sufficient to move O2
in and CO2 out of the blood passively.
2) Oxygen transport
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a) Blood cannot carry sufficient O2 and CO2 in dissolved
form to satisfy the body’s demands.
b) Hemoglobin is a protein with four heme groups that
bind oxygen.
c) O2 diffuses down a pressure gradient into the blood
plasma, then into the red blood cells, where it binds
reversibly to hemoglobin
i) Each hemoglobin molecule can bind 4 O2
molecules.
ii) Called oxyhemoglobin.
iii) Consists of four subunits, each with a cofactor called
a heme group that has an iron atom at its center.
iv) Iron actually binds the oxygen.
d) Hemoglobin gives up its O2 in tissues where:
i) partial pressure of O2 is low.
ii) blood is warmer
iii) partial pressure of CO2 is higher
iv) pH is lower
3) Carbon Dioxide Transport
a) Because the concentration of CO2 is higher in body
tissues, it diffuses into the blood.
b) Ten percent is dissolved in plasma.
c) 30 percent binds with hemoglobin to form
carbaminohemoglobin.
d) 60 percent is in bicarbonate form.
4) Matching Air Flow with Blood Flow
a) Gas exchange in the alveoli is most efficient when
airflow equals the rate of blood flow.
b) The nervous system controls O2 and CO2 levels for the
entire body by adjusting contraction rates of the
diaphragm and chest wall muscles.
c) The brain monitors input from CO2 sensors in the
bloodstream and from receptors sensitive to decreases
in O2 partial pressures (carotid bodies and aortic
bodies)
i) Breathing is under automatic control from the brain.
ii) Breathing control center located in the medulla
oblongata and the pons.
iii) Medulla oblongata sets the basic rhythm
iv) Pons moderates it.