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Diagram 1: The three phases of gas exchange
1. Breathing
• When an animal breathes, a large, moist internal surface is exposed to
air. O2 diffuses across the cells lining the lungs and into surrounding
blood vessels. At the same time, CO2 diffuses out of the blood and into
the lungs. As the animal exhales, CO2 is removed from the body
2. Transport of gases by the circulatory system
• The O2 that diffused into the blood attaches to hemoglobin in red blood
cells. The red vessels in the diagram are transporting O2-rich blood from
the lungs to capillaries in the body’s tissues. CO2 is also transported in
blood from the tissues back to the lungs, shown here by the blue
vessels
3. Body cells take up O2 from the blood and release CO2 to the blood
• O2 functions in cellular respiration in the mitochondria as the final
electron acceptor in the stepwise break down of fuel molecules. H2O
and CO2 are waste product, and ATP is produces to power cellular work
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The part of an animal where gases exchange are exchanged with the environment is called
the respiratory surface. Respiratory surfaces are made up of living cells whose plasma
membranes must be wet to function properly.
Gas Exchange takes place by diffusion. The surface area of the respiratory surface must be
extensive enough to take up sufficient O2 for every cell in the body and to dispose of waste
CO2.
Figure 1: The earthworm is an example of gas exchange through the use of the entire outer
skin. Oxygen diffuses into a dense net of thin-walled capillaries lying just beneath the skin.
Earthworm's and other “skin breathers” must live in damp places or in water because their
whole body surface has to stay moist. Small size or flatness provides a high ratio of
respiratory surface to body volume, allowing for sufficient gas exchange for the entire body
Figure 2: Gills have evolved in most aquatic animals. Gills are extensions, or outfoldings, of
the body surface specialized for gas exchange. A fish has a set of feather-like gills on each
side of its head. O2 diffuses across the gill surfaces into capillaries, and CO2 diffuses into
the opposite direction, out of the capillaries and into the external environment.
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Figure 3: The tracheal system of an insect is an extensive system of branching internal
tubes with the respiratory surface found at their tips. The smallest branches exchange
gases directly with body cells and therefore gas exchange in insects requires no assistance
form the circulatory system.
Figure 4: Most terrestrial vertebrates have lungs which are internal sac lined with moist
epithelium. As the diagram indicates, the inner surfaces of the lungs branch extensively,
forming a large respiratory surface. Gases are carried between the lungs and the body cells
by the circulatory system.
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Diagram 2:
Structure of Fish Gills
There are four supporting gills arches on each side of the body. Two rows of gill filaments
project from each gill arch. Each filament bears many platelike structures called lamellae
(singular, lamella) which are the actual respiratory surfaces. A lamella is full of tiny
capillaries that are separated from the outside by only one of or a few layers of cells.
Ventilation refers to any mechanism that increases the flow of the surrounding water or air
over the respiratory surface (gills, tracheae, or lungs). Increasing this flow ensures a fresh
supply of O2 and the removal of CO2. Swimming fish simply open their mouths and let
water flow over the gills while pumping water across the gills by the coordinated opening
and closing of the mouth and operculum, the stiff flap that covers and protects the gills.
Countercurrent Exchange
The arrangement of capillaries in a fish gill enhances gas exchange. Blood flows opposite
the movement of water past the gills. This makes it possible to transfer oxygen to the blood
by an efficient process called countercurrent. Countercurrent exchange is the transfer of a
substance from a fluid moving in one direction to another fluid moving in the opposite
direction.
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Air contains a much more higher concentration of O2, the air is much lighter and easier to
move than water. Thus, a terrestrial animal expends much less energy than an aquatic
animal ventilating its reparatory surface. The main problem facing any air breathing animal
is the loss of water to the air by evaporation.
Figure 5: Illustrates the tracheal system in a grasshopper. The largest tubes, called
tracheae, open to the outside and are reinforced by rings of chitin, as shown in the blowup
on the bottom right of the figure
Figure 6: An insect in flight has a very high metabolic rate and consumes 10-200 times
more O2 than it does at rest. In many insects, alternating contraction and relaxation of the
flight muscles rapidly pumps air through the tracheal system.
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It now seems clear that tetrapods first evolved in shallow water from what some
researchers jokingly call ”fishapods’. These ancient forms had both gills and lungs. The
adaptations for air-breathing evident in their fossils include a stronger and elongated snout
and a muscular neck that enabled the animal to life the head clear of water into the
unsupportive air. Strengthening of the lower jaw may have facilitated the pumping motion
presumed to be used by early air-breathing tetrapods and still employed by frogs to inflate
their lungs. The recently discovered 375 million year old fossil of Tiktaalik illustrates some
of these air-breathing adaptations.
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Mammalian lungs are located in the chest or thoracic cavity and protected by the
supportive rib cage. The thoracic cavity is separated from the abdominal cavity by a sheet
of muscle called the diaphragm.
Diagram 4 shows the human respiratory. Air enters our respiratory system through the
nostrils. It is filtered by hairs and warmed, humidified, and sampled for orders as it flows
through a maze of space in the nasal cavity. We can also draw in air through the mouth, but
mouth breathing does not allow the air to be processed by the nasal cavity. From the nasal
cavity or mouth, air passes to the pharynx, where the paths for air and food cross. From the
pharynx, air is inhaled into the larynx (voice box). When we exhale, the outgoing air rushes
by a pair of vocal cords in the larynx, and we can produce sounds by voluntarily tensing
muscles in the voice box. From the larynx, or windpipe. Rings of cartilage reinforce the
walls of the larynx and trachea, keeping this party of the airway open. The trachea forks
into two bronchi, one leading to each lung. Within the lung, the bronchus branches
repeatedly into finer and finer tubes called bronchioles.
Respiratory Problems
Alveoli are so small that specialized secretions called surfactants are required to keep them
from sticking shut due to the surface tension of their moist surface. A lack of lung
surfactant is a major problem for babies born very prematurely.
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One of the worst sources of lung-damaging air pollutants is tobacco smoke, which is mainly
microscopic particles of carbon coated with toxic chemicals. Tobacco smoke irrigates the
cells that line the respiratory tract, inhibiting or destroying their cilia. Smoke’s toxins also
kill the macrophages that reside in the respiratory tract and engulf fine particles and
microorganisms.
Figure 8: Illustrates a cutaway view of a pair of healthy human lungs (left) and the lungs of a
cancer victim (right), whose lungs are black form the long-term buildup of smoke particles.
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Breathing is the alternate inhalation of air. This ventilation of our lungs maintains high O2
and low CO2 concentrations at the respiratory surface.
Diagram 5: Shows the changes that occur in our rib cage, chest cavity, and lungs during
breathing.
Inhalation - During inhalation, the rib cage expands as muscles between the ribs contract.
At the same time, the diaphragm contracts, moving downward and expanding the chest
cavity, The volume of the lungs increases with the expanding chest cavity during inhalation,
which lowers the air pressure in the alveoli to less than atmospheric pressure. Flowing from
a region of higher pressure to one of lower pressure, air rushes through the nostrils and
down the breathing tubes to the alveoli. This type of ventilation is called negative pressure
breathing.
Exhalation – The rib muscles and diaphragm both relax, decreasing the volume of the rib
cage and chest cavity, which increases the air pressure inside the lungs, forcing air out.
Notice that the diaphragm curves upward into the chest cavity when relaxed.
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Although we can voluntarily hold our breathe faster and deeper, most of the time our
breathing is under automatic control. Breathing control centers are located in parts of the
brain called the pons and medulla oblongata. The control center in the pons smooths out
the basic rhythm of breathings set by the medulla.
Diagram 6:
1. Nerves from the medulla’s control center signal the diaphragm and rib
muscles to contract, making us inhale. When we are at rest, these
nerve signals result in about 10 to 14 inhalations per minute. Between
inhalations, the muscles relax, and we exhale
2. The control center regulates breathings rate in response to changes in
the CO2 level of the blood. When we exercise vigorously, for instance,
our metabolism speeds up and our body cells generate more CO2 as a
waste product. The CO2 goes into the blood, where it reacts with water
to form carbonic acid. The acid slightly lowers the pH of the blood and
the cerebrospinal fluid. When the medulla senses this pH drop, its
breathing control center increases the breathing rate and depth. As a
result, more CO2 is eliminated in the exhaled air, and the pH returns to
normal
3. Secondary control over breathing is exerted by sensors in the aorta and
carotid arteries that monitor concentrations of O2 as well as CO2. When
the O2 level in the blood is severely depressed, these sensors signal the
control center via nerves to increase the rate and depth of breathing.
This response may occur, for example, at high altitudes, where the air is
so thin that we cannot get enough O2 by breathing normally.
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Diagram 7: Illustrates the main components of our circulatory system and their role in gas
exchange. One side of the heart handles oxygen-poor blood (colored blue). The other side
handles oxygen-rich blood (red). As indicated in the lower left of the diagram, oxygen poor
blood returns to the heart from capillaries in body tissues. The heart pumps this blood to
the alveolar capillaries in the lungs. Gases are exchanged between air in the alveolar spaces
and blood in the capillaries (top of diagram). Blood leaves the alveolar capillaries, having
lost CO2 and gained O2. This oxygen-rich blood returns to the heart and is pumped out to
body tissues.
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Oxygen is not very soluble in water, and most animals transport O2 bound to proteins called
respiratory pigments. Most of these molecules have distinctive colors, hence the name
pigment. Many mollusks and arthropods use a blue, copper-containing pigment. Almost all
vertebrates and many invertebrates use hemoglobin, an iron-containing pigment that turns
red when bound with O2.
A hemoglobin molecule consists of four polypeptide chains of two different types, depicted
with two shades of purple in the above figure. Attached to each polypeptides is a chemical
group called a heme (colored green in the figure), at the center of which is a iron atom
(black). Each iron atom can carry one O2 molecule. Thus, every hemoglobin molecule can
carry up to four oxygen molecules. Hemoglobin loads up with O2 in the lungs and
transports it to the body’s tissues. There, hemoglobin unloads some or all of its cargo,
depending on the O2 needs of the cells.
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Figure 9 shows the human fetus inside the mother’s uterus. The fetus literally swims in a
protective watery bath, the amniotic fluid. Its lungs are full of fluid and are nonfunctional. A
composite organ that includes tissues from both the fetus and the mother. A large net of
capillaries fans out into the placenta from blood vessels in the umbilical cord of the fetus.
These fetal capillaries exchange gases with the maternal blood that circulates in the
placenta, and the maternal circulatory system carries the gases to and from the mother’s
lungs. Aiding O2 uptake by the fetus is fetal hemoglobin, a special type that attracts O2
more strongly than does adult hemoglobin.
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