Animal Respiratory Systems

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Animal Respiratory Systems

• The Details

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The Three Requirements for Gas Exchange
1. Concentration Gradient
Gas exchange occurs by diffusion when there is a
concentration gradient. Both CO2 and O2 will
diffuse from regions of high concentration to
regions of low concentration. The processes of
photosynthesis and respirations create these
concentration gradients.
2. A surface for exchange
The exchange surface must be large enough to
supply the metabolic needs of the entire mass (or
volume) of the organism. Small organisms use
their exterior surface for gas exchange. Larger
organisms use specialize gas exchange organs with
enlarged surfaces like gills and lungs.
3. Moisture
The exchange surface must be wet, because both
CO2 and O2 are dissolved in water. The gas
exchange surfaces of terrestrial organisms are
covered by a film of moisture.
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The Requirements of Gas Exchange are Embodied in
Ficks Law of Diffusion
Ficks Law of Diffusion
Surface Area Surface area defines the capacity
for gas exchange. The surface must be
sufficiently large to satisfy the gas exchange
needs of the volume of the entire organism
Concentration Difference The concentration
difference or gradient of the respiratory gases
between the cell and the environment occurs
because of the processes of photosynthesis and
cellular respiration.
Distance Since diffusion is a slow process, cells
must be close to the supply of respiratory gases.
The path of diffusion is short.
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The Surface Area to Volume Ratio The Problem of
Large Organisms
Large organisms do not use their exterior
surfaces for gas exchange.
1 Volume defines the need for gas exchange.
Since metabolic reactions like photosynthesis and
cellular respiration take place throughout the
entire volume (or mass) of an organism, the
volume of the organism dictates its gas exchange
requirements.

6 CO2 (g) 6 H2O ? C6H12O6 6 O2
(g)
Photosynthesis
C6H12O6 6 O2 (g) ? 6 CO2 (g) 6
H2O
Respiration
2 Surface area defines the capacity for gas
exchange.
The size of the exchange surface dermines the
quantity of CO2 and O2 that can be exchanged.
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Assume a spherical cow.
Although organisms come in a wide variety of
sizes and shapes, the argument that follows will
not change if we simplify the mathematics of the
calculation. Assume each organism listed below
has the shape of a cube.
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Adaptations Reflect the Constraints Imposed by
Ficks Law
Gas exchange requires relatively high rates of
diffusion. Ficks Law demonstrates that large
surface areas, large concentration differences
and short distances assure high rates of
diffusion. Large organisms have adaptations that
assure these conditions are satisfied.
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Planaria (Dugesia tigrina) is an aquatic flatworm
about 5 mm long and less than 1 mm thick. Gas
exchange occurs through its exterior surface.
O2
CO2
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The earthworm (Lumbricus terrestris) is a
terrestrial animal. Gas exchange occurs through
its exterior surface only if it is kept moist.
O2
CO2
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Adult amphibians like the leopard frog (Rana
pipiens) use three gas exchange surfaces the
skin, the lining of the mouth and the lungs.
http//www.tvdsb.on.ca/westmin/science/snc2g1/frog
resp.htm

• Breathing actions of the adult frog
• The frog uses its throat to pump air into the
  lungs.  Breathing occurs in four steps
•     The throat moves down causing air to flow
  into the mouth through open nostrils.
•     The nostrils close and the throat moves up,
  forcing air into the lungs.
•     When the throat moves down air flows out of
  the lungs into the mouth.
•     The nostrils open and the throat moves up,
  forcing air out of the mouth.

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Pisces Fish
The act of swimming forward through the water
forces the flow of water over the surface of the
gills when fish open their mouths. Water leaves
through the back of the gill cover. Fish have
four pairs of gill arches bearing a double row of
gill filaments.
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Gill Filaments Increase the Surface Area for Gas
Exchange
Gill filaments resemble the teeth of a comb and
serve to increase the size of the exchange
surface. These filaments are further divided
into smaller perpendicular lamellae to increase
the surface of exchange still further. The
lamella contain small blood vessels called
capillaries which deliver CO2 to and remove O2
from the water flowing between them.
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Lamellae are laced with microscopic blood vessels
called capillaries. Secondary lamellae increase
surface area still further.
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Countercurrent Gas Exchange
Gas exchange in fish gills is highly efficient
because it uses a countercurrent exchange system.
The two currents, the flow of water over the
surface of the gill lamellae and the flow of
blood through microscopic blood vessels
(capillaries) within the lamellae, are in
opposite directions.
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The Countercurrent Exchange System Explained.
Countercurrent heat exchangers are widely used in
air conditioners, heat pumps and air handling
systems in buildings. It is an efficiently
designed system for the transfer of heat from
one fluid, air for example, to another, the
refrigerant. It came as a surprise to learn that
living organisms used countercurrent flow systems
for gas, nutrient, waste and heat exchange long
before engineers designed such systems for
buildings.
O2 0.4
O2 1.6
O2 1.2
O2 0.8
Direction of Blood Flow
Diffusion of O2
O2 0.8
O2 2
O2 1.6
O2 1.2
Direction of Water Flow
The flow of the two exchange fluids in opposite
directions, that is countercurrent flow, assures
that a concentration difference in the O2
concentration is maintained in the two fluids for
the duration of exchange.
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Birds Use Air Sacs to Ventilate the Lungs with
Fresh Air Continually
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Air Sacs are Named Relative to their Location
Most birds have 9 air sacs
1 pair of neck air sacks
1 interclavicular (wishbone) air sac
1 pair of front chest air sacs
1 pair of rear chest air sacs
1 pair of abdominal air sacs
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Air Sacs Act as Bellows Moving Fresh Air Through
the Lungs Continually
Breathing in birds moves air in and out of the
air sacs in a manner that is similar to the
movement of air in and out of the lungs of
mammals. The air sacs are not involved in gas
exchange. Expansion of the chest cavity reduces
air pressure in the air sacs relative to the
atmosphere. Fresh air flows into the airs sacs
and through the lungs. The reduction in volume
of the body cavity causes an increase in air
pressure in the air sacs and air flow through the
lungs is maintained. Only air flow through the
primary bronchus and air sacs is reversed with
each inhalation and exhalation. Air flow through
the lungs via the ventro- and dorso-bronchi is
continually maintained from a rear to the
forwards direction. Hence, fresh air is always
delivered to the gas exchange surface of the lung.
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Morphology of a chicken lung. Light microscopy
(top image) and electron microscopy (bottom two
images) of a chicken lung depicting the
respiratory system of birds. In the bird lung,
air capillaries (Ac) run along with blood
capillaries forming the blood-air barrier that is
typically lt 0.2 µm in thickness. The barrier
(shown in the bottom image) separates the air
space of air capillary (Ac and ) from the red
blood cells (RBC) in the blood capillaries. The
barrier consists of a surfactant layer (arrows),
thin epithelial cells (Ep), a membrane of
connective tissue (Bm), and the endothelial cells
of the blood capillary (En). Surfactant is a
aqueous mixture of lipids and proteins helps keep
the lungs inflated. Magnifications top image -
270 middle image - 1,600 bottom image -
88,000 (Image from Bernhard et al. 2001).
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Comparison of Avian and Mammalian Lung Tissues
Light micrographs of a portion of the lung of a
chicken (above) and rabbit (below). Note the
smaller diameter of the air capillaries in the
chicken lung versus that of the rabbit alveoli at
the same magnification). In the chicken lung
capillaries appear more abundant than in the
rabbit.
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Ultra-Low Oxygen Could Have Spurred Bird
Breathing System Recent evidence suggests that
oxygen levels were suppressed worldwide 175 - 275
million years ago, low enough to make breathing
the air at sea level feel like respiration at
high altitude. Peter Ward, a University of
Washington paleontologist, theorizes that low
oxygen and repeated short but substantial
temperature increases because of greenhouse
warming sparked two major mass-extinction events.
In addition, he believes the conditions spurred
the development of an unusual breathing system in
Saurischian dinosaurs. Rather than having a
diaphragm to force air in and out of lungs, the
Saurischians had lungs attached to a series of
thin-walled air sacs that appear to have
functioned something like bellows to move air
through the body. This breathing system, still
found in today's birds, made the Saurischian
dinosaurs better equipped than mammals to survive
the harsh conditions in which oxygen content of
air at the Earth's surface was only about half of
today's 21. "The literature always said that the
reason birds had sacs was so they could breathe
when they fly. But I don't know of any
brontosaurus that could fly," Ward said.
"However, when we considered that birds fly at
altitudes where oxygen is significantly lower, we
finally put it all together with the fact that
the oxygen level at the surface was only 10 - 11
at the time the dinosaurs evolved. That's the
same as trying to breathe at 14,000 feet. If
you've ever been at 14,000 feet, you know it's
not easy to breathe," he said. Ward presented
his ideas at the 2003 annual meeting of the
American Geological Society  in Seattle. See 
http//www.nature.com/nsu/031103/031103-7.html
http//www.washington.edu
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Diagram of parabronchial anatomy, gas-exchange
region of the bird's lung-air-sac respiratory
system. The few hundred to thousand parabronchi,
one of which is fully shown here, are packed
tightly into a hexagonal array. The central
parabronchial lumen, through which gas flows
unidirectionally during both inspiration and
expiration (large arrows) is surrounded by a
mantle (m) of gas-exchange tissue composed of an
intertwined network of blood and air capillaries.
Several air capillaries coalesce into a small
manifold, i.e., the infundibulum (arrowheads),
several of which in turn open into atria ()
found along the parabronchial lumen. Air moves
convectively through the parabronchial lumen,
while O2 diffuses radially (CO2 diffuses
centrally) into the air capillary network. Blood
flows centrally from the pulmonary arteries (a)
located along the periphery of the parabronchi to
pulmonary veins located along the parabronchial
lumen, which then are drained back to the
peripheral veins (v) (Modified from Duncker 1971
by Brown et al. 1997).
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http//www.sci.sdsu.edu/multimedia/birdlungs/
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The superlative for the highest altitude migrant
 goes to the Bar-headed Goose (Anser indicus),
which has been seen at up to 10175 m (33 382
feet). This bird, which breeds in Central Asia,
migrates through the Himalayan range. The air at
these heights is so thin that helicopters cannot
fly there and kerosene cannot burn. The Bar-head
has a slightly larger wing area for its weight
than other geese, which is believed to help the
goose fly so high.