Gaseous exchange in mammals

12/13/2015DEPARTMENT OF BIOLOGY1
Gaseous exchange in Mammals

Introduction
 No work is possible without a supply of energy in a
useful form.
 Living organisms need energy in a useful form all the
time they are alive.
 They need energy in useful form for the following
reasons:
1. A living cell is active . Chemical processes are constantly
taking place in it, and must continue to do so for the cell to
remain alive. This means every cell in the body of a multi-
cellular organism requires a supply of energy. These
processes are called metabolism

2. Energy for growth, repair and replacement of
damaged tissues
3. Energy rquired for movement and keep the organism
active
4. To maintain a constant body temperature in mammals
and birds
 So plants and animal obtain most of their energy
they need from the aerobic respiration of food
molecules. This chemical reaction is called
Cellular(internal) respiration
 C6H12O6 + 6O2 + 6H2O → 12H2O + 6 CO2 +Energy(ATP)

 Oxygen is essential for aerobic respiration
 It is obtained from the air and transported in the blood
 Co2 is produced as waste product of respiration
 It is transported in the blood, and is breathed out into
the air.
 The exchange of O2 and Co2 between their and the
blood is called Gaseous exchange(Breathing) or
external respiration.
 Breathing is the active exchange of gases btn an
animal and its surrounding.
 It includes inspiration(breathing air in) and
expiration (breathing air out)

 Gaseous exchange
always occurs by
diffusion over part or all
of the body surface.
This is called
respiratory surface.
 These are where
gaseous exchange with
the environment
actually takes place .

Characteristics of respiratory surface
 In order to maintain the maximum possible rate of
diffusion of gaseous exchange across the respiratory
surface and thus an efficient gaseous exchange,
they must posses the following characteristics:
1. Large surface area to volume ratio
 Large surface area in relation to the size of the body
 Some animals are small and shaped in a way that allows
the surface of the animal to be gas-exchange eg
 Other animals are complex and have a specialized gas
exchange surface

2. Permeable so that gases can pass through easily
3. Thin walls
 Diffusion is only efficient over distances up to 1mm sine rate of
diffusion is inversely proportional to the square of the distance
between the concentrations on the two sides of the respiratory
surfaces
4. A moist inner surface
 Respiratory surface must be moist so gases can diffuse across
cell membranes
5. A rich blood supply- this will provide an efficient
transport system to maintain a diffusion gradient

 Some animals in moist environments lack
specialized respiratory structures
 Gases diffuse short distances in smaller animals to
reach cells –
 Gas exchange optimized by long, flat bodies with greater
surface area
 Examples: flatworms
 Respiratory systems facilitate more effective
exchange of gases between the environment and an
animal’s body

 In summary, the diffusion of gases across the
respiratory surface is proportional to the difference
in partial pressure, proportional to the area of the
membrane and inversely proportional to the
thickness of the membrane.
 This is Fick’s Law of diffusion of gases
 In mammals the gaseous exchange takes place in
the lungs
 Organisms can obtain their gases from the air or
from water.

Respiratory structures of various
animals
 Large animals cannot maintain gas exchange by
diffusion across their outer surface.
 They developed a variety of respiratory surfaces that
all increase the surface area for exchange, thus
allowing for larger bodies.
 A respiratory surface is covered with thin, moist
epithelial cells that allow oxygen and carbon dioxide
to exchange.
 Those gases can only cross cell membranes when
they are dissolved in water or an aqueous solution,
thus respiratory surfaces must be moist.

 Sponges and jellyfish lack specialized organs for
gas exchange and take in gases directly from the
surrounding water.
 Flatworms and annelids use their outer surfaces as
gas exchange surfaces.
 Arthropods, annelids, and fish use gills;
 Terrestrial vertebrates utilize internal lungs.
 Tracheal Systems in insects

Various Respiratory Surfaces

Various Respiratory surfaces

Gaseous exchange in Mammals
 Lungs are the sites of gaseous exchange in
mammals.
 The lungs consists of a system of tubes of ever
decresing size which end in microscopic bulbous sacs
called Alveoli.
 This is where gaseous exchange takes place.

Internal structure of lung

The internal structure of the lungs
BRONCHI
 Tubes that carry air from the trachea to the lungs
 After passing the larynx, the air moves into the
bronchi that carry air in and out of the lungs.
 Bronchi is a large, hollow tube made of hyaline
cartilage and lined with ciliated pseduostratifed
epithelium
 Hyaline cartilage of the bronchi forms incomplete
rings shaped like letter “C”
 The rigid hyaline cartilage prevents the bronchi from
collapsin and blockin airflow to the lungs.

 Pseudostratified epithelium lines the inside of the
hyaline ring and connects the unfinished ends of the
ring to form a hollow tube shaped like the letter “D” with
the flat part of the tube facing the posterior direction
 Each lung receives air from a single, large primary
bronchus
 As the primary bronchi enter the lungs, they branch
off into smaller secondary bronchi that carry air to
each lobe of the lung
 Thus, the right bronchus branches off into 3 secondary
bronchi while the left lung branches off into 2
secondary bronchi

 The secondary bronchi further branch into many
smaller tertiary bronchi within each lobe.
 The secondary and tertiary bronchi improve the
efficiency of the lungs by distributing air evenly within
each lobe of the lungs.
 The pseudostratified epithelium that lines the bronchi
contains many cilia and goblet cells.
 Cilia move together to push mucus secreted by the
goblet cells away from the lungs.
 Particles of dust and even pathogens like viruses, bacteria and
fungi in the air entering the lungs stiick to the mucus and are
carried out of the respiratory tract.

 Goblet cells are specialized epithelial cells that secrete
mucus to coat the lining of the bronchi and also to trap
dusts and pathogens thus help keeping the lungs clean
and free from disease
Bronchioles
 Bronchi branch into smaller and smaller tubes known
as bronchioles
 Many small bronchioles branch off from the tertiary
bronchi.
 They are made of elastic fibres and smooth muscle
tissues
 This allows the diameter of bronchioles walls tochange
to a significant degree depending on the amount of air
needed to enter the lungs, eg during exercise

 The bronchioles further branch off into many tiny
terminal bronchioles.
 Terminal bronchioles are the smallest air tubes in the
lungs and terminate at the alveoli
Alveoli
 Alveoli are the functional unit of the lungs that permit
gas exchange between the air in the lungs and the
blood in the capillaries.
 They are found in small clusters called alveolar sacs
at the end of the terminal bronchioles.

 Each alveolus is ahollow, cup-shaped cavity
surrounded by many tiny capillaries
 Only about 0.2 µm separate the alveoli from the
capillaries due to the extremely thin walls of both
structures.
 The walls of the alveolus are lined with simple
squamous epithelium cells known as Alveolar cells.
 A thin layer (0.0001mm thick) of connective tissue
underlies and supports the alveolar cells.
 Septal cells and macrophages are also found found
inside the alveoli cells

 Septal cells produce alveolar fluid that coats the
inside surface of the alveoli
 Alveoli fluid is extremely important to lung function, as it
a surfactant
 Surfactant has a greater role to play in the lung
function
1. It moisten the alveoli
2. Helps maintain the elasticity of the lungs,
3. Prevents the thin alveolar walls from collapsing
4. It lowers the surface tension of the fluid layer lining
the alveolus, and thereby reduces the amount of
effort needed to breathe in and inflate the lungs.

5. It also speed up the transport of O2 and CO2 btn the
air and the liquid lining the alveolar
6.Helps to kill any bacteria which reach the alveoli
 Surfactant is constantly being secreted and
reabsorbed in the healthy lungs
 It is first made in the lungs of a foetus which is 23
weeks old. Before 24 weeks, the baby is incapable of
independent existence
 Permanent born babies are at risk of being deficient in
surfactant and suffers the condition known as
respiratory distress syndrome.
 They experience difficulty in breathing and hence premature
death occurs.

The Pleura
 The pleura are double-layered serous membranes
that surround each lung
 Thin sheets of epithelium (pleura) separate the inside
of the chest cavity from the outer surface of the lungs.
 Attached to walls of the thoracic cavity, the parietal
pleura forms the outer layer of the membrane
 The visceral pleura forms the inner layer of the
membrane covering the outside surface of the lungs

 Between the parietal and visceral pleura is the
pleural cavity, which creates a hollow space for the
lungs to expand into during inhalation
 Serous fluid secreted by the pleural membranes
lubricates the inside of the pleural cavity to prevent
irritation of the lungs during breathing.

Gaseous exchange in the alveoli
 Gaseous exchange takes mainly in the alveoli.
 The blood is the transport medium that carries
oxygen and carbon dioxide between the lungs and
the body cells.
 These gases are exchanged between the air and the
blood in the alveoli.
 Alveoli are in close contact with a vast network of
blood capillaries; each alveolus has its own blood
supply.

 Gases dissolve in the fluid on the cell surface
membrane and diffuses through the thin walls of the
alveolus and its neighboring capillaries into the
blood. Oxygen enters the blood in this way.
 CO2 leaves the blood and diffuses into the air in the
alveolus.
 Oxygen diffuses across the thin barrier represented
by the epithelium of the alveolus and the
endothelium of the capillary.
 O2 passes first into the blood plasma and then
combines with haemoglobin in the red blood cells to
form oxyhaemoglobin.

 Co2 diffuses in the
reverse direction from
the blood to the
alveoli
 .

 The site of gas exchange in MAMMALS is the
epithelium of the alveoli.
 To ensure a constant supply of oxygen to the body, a
diffusion gradient must be maintained at the alveolar
surface
 Alveoli adaptations
 The diffusion of gases across the alveoli is efficient
because:
 Alveoli have a large surface area due to presence of a large
number
 The gases have a short distance to travel i.e. the alveolar
an the capillary walls are very thin and close together
minimizing the diffusion distance
 Surfactant is present thus preventing the alveoli from
collapsing during exhalation
 A steep diffusion gradient is maintained by ventillation, a

Exchange and Transporting of gases
 Diffusion primary accounts for the exchange
between the air in the alveoli and the blood in the
pulmonary capillaries

Transport of O2
 Most oxygen entering the blood combines with
hamoglobin(Hbo2)
 Hb + O2 HbO2
oxyhaemoglobin
 Oxygen is carried in blood
1. Bound to haemoglobin (98.5 % of all O2 in the blood)
2. Dissolved in Plasma(1.5%)
 Because almost all O2 in the blood is transported
by Hb, the relationship between the conc’n(partial
pressure) of O2 and Hb saturation( the % of Hb
molecules carrying oxygen) is an important one.

Haemoglobin(Hb)
 Hemoglobin (Hb), an intracellular protein, is the
primary vehicle for transporting oxygen in the blood.
 Oxygen is also carried (dissolved) in plasma, but to a
much lesser degree. Hemoglobin is contained in
erythrocytes, more commonly referred to as red blood
cells.
 Under certain conditions, oxygen bound to the
hemoglobin is released into the body tissue, and
under others, it is absorbed from the tissue into the
blood.

 It is the iron contained in the Hb that forms a loose
association with O2
 The combination of O2 and Hb to form HbO2 occurs
under the condition when the conc’n of O2 is
high, such as in the lung alveolar capillaries
 When the conc;n of O2 is low, the bond holding
HbO2 becomes unstable and O2 is released. This
diffuses in solution into the surrounding cells.
 This release of O2 from Hb is called Dissociation
 HbO2 Hb + O2

Structure of Hb

 Each hemoglobin molecule has a limited capacity for
holding oxygen molecules.
 How much of that capacity that is filled by oxygen
bound to the hemoglobin at any time is called the
oxygen saturation.
 Expressed as a percentage, the oxygen saturation is
the ratio of the amount of oxygen bound to the
hemoglobin, to the oxygen carrying capacity of the
hemoglobin.
 The oxygen carrying capacity is determined by the
amount of hemoglobin present in the blood.

 The amount of oxygen bound to the hemoglobin at
any time is related, in large part, to the partial
pressure of oxygen to which the hemoglobin is
exposed.
 In the lungs, at the alveolar-capillary interface, the
partial pressure of oxygen is typically high, and
therefore the oxygen binds readily to hemoglobin that is
present.
 As the blood circulates to other body tissue in which
the partial pressure of oxygen is less, the hemoglobin
releases the oxygen into the tissue because the
hemoglobin cannot maintain its full bound capacity of
oxygen in the presence of lower oxygen partial
pressures.

Oxyhaemoglobin dissociation curve
 Hb saturation
 It shows the extent to which the Hb in blood is
combined with O2
 Depends on partial pressures of O2 of the blood
 The amount of O2 that can combine with Hb is
determined by the O2 conc’n or Partial pressure.
 O2-binding characteristics of Hb can be studied by
examining oxyhaemoglobin dissociation(saturation)
curve
 This curve indicates the relationship between O2 levels and
Hb saturation

Dissociation curve

 These curves shows the % of O2-binding sites of
Hb that are carrying O2 at various O2 partial
pressures (PO2)
 As the Po2 decreases, Hb give up its O2 more easily
 Both (a) higher temperature and (b) higher acidity
promote this effects
 At the normal partial pressures of O2 in the lungs,
Hb becomes practically saturated with O2, but at the
PO2 in the tissues, oxyhaemoglobin quickly gives up
much of its O2
 HbO Hb + O2

 In the tissues with increased partial pressure of CO2
, the O2-dissociation curve shifts to the right. This is
known as Bohr effect
 This shift has a physiological advantage
 It facilitates the release of O2 from Hb. Where co2
conc’n is higher i.e. in respiring tissues, O2 is released
readily; where co2 conc’n is low i.e. at the respiratory
surfaces, O2 is taken up readily by Hb
 The Bohr Effect allows for enhanced unloading of
oxygen in metabolically active peripheral tissues
such as exercising skeletal muscle

 Increased skeletal muscle activity results in
localized increases in the partial pressure of carbon
dioxide which in turn reduces the local blood pH.
 Because of the Bohr Effect, this results in enhanced
unloading of bound oxygen by hemoglobin passing
through the metabolically active tissue and thus
improves oxygen delivery. Importantly, the Bohr
Effect enhances oxygen delivery proportionally to the
metabolic activity of the tissue.

 As more metabolism takes place, the carbon dioxide
partial pressure increases thus causing larger
reductions in local pH and in turn allowing for greater
oxygen unloading. This is especially true in
exercising skeletal muscles which may also
release lactic acid that further reduces local
blood pH and thus enhances the Bohr Effect.

 At low co2 conc’n, the
curve shifts to the right.
 At higher pH, Hb
would bind to O2 with
greater affinity
 At high Co2 conc’n, the
curve shifts to the left.
 at lower pH (more acidic
environment, e.g., in
tissues), hemoglobin would
bind to oxygen with less
affinity.

 As the level of CO2 becomes high, Bohr effect would
cause the dissociation curve to shift to the right
reducing the Hb’s affinity for O2. O2 is then released
 The more the dissociation curve of Hb saturation is
shifted to the right, the less readily it picks up O2,
but the more easily it releases it.
 The more the dissociation curve is shifted to the left,
the more readily it picks up O2, but less readily it
releases it .
 An example of this is seen in fetal Hb

Fetal Hb and O2-Hb dissociation curve
 Fetal hemoglobin (Hb F) more avidly binds oxygen
than adult hemoglobin (Hb A).
 This binding of oxygen shifts the oxygen –
hemoglobin disassociation curve to the left and
less oxygen is unloaded.
 HbF has a greater affinity for O2 than HbA to enable
it to obtain O2 from the mother’s Hb in the placenta
 At birth the production of HbF gives way to that
of the adult type( HbA)

 The larger affinity for
O2 of HbF in
comparison with HbA
facilitates O2 transfer
from the mother to
foetus

 Because fetal blood displays a higher affinity for
oxygen than maternal blood, oxygen will diffuse from
the pregnant maternal to the fetal circulation within
the placenta, allowing for oxygenation of fetal
tissues.

Factors that affect the Oxygen-
Hemoglobin Dissociation Curve
 Factors favoring
rightward shift include
 temperature
 Pco2(Bohr effect)
 The effect of Pco2 +H ion
on oxyhaemoglobin
dissociation curve
 H ion concentration
( pH)
 Reduces Hb affinity for
O2, thus improves O2
unloading.

 Factors favouring leftward shift ie. Increases High
affinity of Hb for O2 include:
 temperature
 Pco2
 H ion conc’n ( pH)
 2, 3 diphosphoglycerate (2,3-DPG)
 A by product of glycolysis in RBCs, BPG binds to Hb and
stabilizes the low affinity structure.
 DPG associated with stored blood interferes with release of O2
to tissues

Transport of Carbon dioxide
 Gaseous Co2 is
produced in large
quantities by
metabolically active
tissues and diffuses
into the blood stream
where it is transported
to the lungs for
elimination
 It must not be allowed
to accumulate in the
body because it forms
an acid in solution
and could lead to fatal
changes in blood pH.

 Co2 is transported from the body cells within the
blood back to lungs in three main ways/methods:
i. Dissolved in the plasma forming aqueous solution
(≈5%). Around 5% of Co2 is transported in physical
solution in blood plasma
ii. In combination with Hb as carbaminoHb. Around
10% combines with the aminogroups(-NH2) in the
Hb molecules to form carbaminoHb.
Hb-NH2 + CO2 Hb-NHCOO + H+
The lesser the O2 carried by Hb, the more CO2 is
transported as carbaminoHb

iii. As bicarbonate (HCO3-) (≈85%)
 Formed when co2( released by cells making ATP)
combines with water (due to the enzymes in red
blod cells called carbonic anhydrase) to form
carbonic acid (H2CO3 ) . The H2CO2 dissociates
into H+ ion and HCO3- ions
H2O + CO2 H2CO2 H+ +
HCO3-
 H+ ion produced combines with Hb which loses
its O2.

 The O2 so released diffuses out of the RBC, through
the capillary wall and tissue fluid into a respiring
tissue cell.
 The HCO3
- ion diffuses out of the RBC into plasma
where they combine with Na+ ions from the
dissociation of Nacl to form NaHCO3.
 Most of the CO2 is transported in the form of
NaHCO3 to the lungs.
 In the lungs, the process is reversed, CO2 is released and
diffuses out of the body.
 Reversal of these reactions in the lungs is enhanced by the
presence of high oxygen partial pressures

 The loss of –vely charged HCO3- ions from RBC is
balanced by the inward diffusion of –ve Cl- ions from
the dissociation of NaCl.
 In this way the electrochemical neutrality of the
RBC is restored. This is known as the Chloride
shift.
 Qn: Summarize how CO2 in the blood is expelled
as gaseous CO2 by the lungs.

Reverse CO2 transport

Adaptations of O2 uptake by high
altitude dwellers
 High altitude dwellers include mountaineers or
mountain climbers.
 The amount of O2 in the atmosphere is the same
at high altitude as it is at sea level, namely 21%.
 The respiratory problems associated with living at
high altitude are a result of reduced
atmospheric pressure.
 The reduced pressure means that it is more
difficult to load Hb with O2
 Above about 6000M, the pressure is inadequate
to load Hb effectively

 The dwellers of these areas have become
acclimatized/adapted to this environment

Aclimatization/adaptation factors
include
1. Adjustment of blood pH:
• The reduced loading of the Hb leads to deep
breathing(Hyperventilation) in order to
compensate for lack of O2 in the blood. This
leads to excessive removal of Co2 and a raised
blood pH.
2. Increased rate and depth of Breathing
• In order to take in more O2 and to have broader
arteries and capillaries thereby allowing much
higher rates of blood flow and subsequently
greater amounts of O2 delivered to their muscle

3. Increased O2 uptake
• More O2 is absorbed by the lungs as a result of an
improved capillary network in the lungs, and a deeper
breathing.
4. Improved transport of O2 to the tissues
• This is the result of:
 Increased RBCs conc’n and capillaries to carry more O2
 From 45% to 60% of the total blood volume
 Increased Hb conc’n in the RBCs- by 20%

 Lungs at sea level:
PO2 of 100mmHg
haemoglobin is 98%
SATURATED
 When the PO2 in the
lungs declines below
typical sea level
values, haemoglobin
still has a high affinity
for O2 and remains
almost fully saturated.
 Lungs at high
elevations: PO2 of
80mmHg,
haemoglobin 95
% saturated
 Even though PO2
differs by 20 mmHg
there is almost no
difference in
haemoglobin
saturation.

6. Increased myoglobin levels in muscles.
 With its high affinity for O2, this facilitates the exchange
of O2 from the blood to the tissues.
7. Increased lung size to facilitate diffusion of O2 and
CO2
8. Increased vascular network of muscle to
enhance the transfer of gases.

Adaptations of O2 Uptake by Sea
Divers
 At this environment the pressure is high
 The adaptations include:
1. The distribution and conc’n of protein
myoglobin
 Myoglobin is found mainly in muscle and it binds
with O2.
 The conc’n of muscular myoglobin is higher than
those found in terrestrial mammals. This enables
them to store more O2 than non-divers

2. Cardiac adaptations:
i. Decreased heart rate and bradycardia
ii. Blood has the ability to carry more Hb and, larger RBCs
and is less viscous
iii. Mammalian diving reflex Adaptation
 Slowing of the heart rate is known as bradycardia- the pulse
rate slows down by upto 30%. This adaptation preserve O2
reserves by limiting O2 consumption
 Vasoconstriction reduces O2 from being supplied by blood to
areas that uses O2. This is an adaptation of saving O2.
 It functions in two ways:
 By reducing the flow of blood to those areas. This means
that the heart does not have to work as hard to pump
blood around the body
 O2 is preserved for the most vital organs in the body; the
heart, and the brain, which require a constant supply of
Oxygen.

 Contraction of the spleen
 The spleen’s primary function is the destruction of RBCs.
Due to the large volumes of blood contained within the
spleen, it can also provide a boost to RBC numbers, which
in turn allows for greater O2 carrying capacity.
 So, a spleen can contract by upto 20% while diving causing
a significant increase in the amount of O2 that can be stored
in the blood.
 So the mammalian diving reflex prioritizes the blood to
the heart and brain thus shifting a lot of blood to the
chest. This helps to resist the increased pressure

3. Muscle Physiology adaptation
 Increased fatty acids as source of fuel. Fatty acids release more
energy in the form of ATP than carbohydrates. So these animals
rely on fatty acids as an energy source to conserve O2
 There is also increased myoglobin level in divers. Myoglobin binds
Oxygen reversibly, drawing O2 away from the blood since it has a
greater affinity for O2 than Hb at certain pressures.
 Myoglobin therefore, acts as a temporary O2 store and allows
divers to continue aerobic respiration whilest diving.
 The heart and brain, which require O2 to function, are able to
utilize the stores of O2 maintained by the myoglobin.
 Total myoglobin stores in the body increase as diving abilities
increases

4. Oxygen stores
 Deep divers are able to reach an astonishing depths and
remain submerged for long periods due to their ability to
maintain aerobic respiration for extended durations.
 To accomplish this, they make use of their enlarged spleen
which act as a store of red blood cells. Contraction of spleen
resulted in rapid release of RBCs into the circulatory system.
5. Respiratory adaptations- termination of gaseous
exchange.
 For diving mammals, lungs have the potential to be
problematic b’se they are more efficient stores of nitrogen
than O2.

 The airways become more rigid than those of non-
divers, strengthened by connective tissues and smooth
muscle, enabling gas from the alveoli to e trapped in
the rigid airways, thus preventing gas exchange during
submersion to avoid decompresion sickness and
nitrogen necrosis.

Adaptations of Oxygen uptake by
Fetus
 In the adult, blood circulates from the left ventricle
to the systemic circulation and is returned to the
right side of the heart. From there, it circulates
through the lungs for reoxygenation.
 This serial circulatory design is inappropriate for the
fetus because oxygenation occurs in the placenta;
therefore, a pair of parallel circulations is present.

Fetus
 This is made possible
by anatomical
shunts, which
normally close rapidly
at birth when
circulation
independent of the
mother is required.

Fetus
 The foetus is highly adapted to intrauterine conditions
which include low partial pressures of O2 and
relatively limited supply of other substrates compared
with post-natal life.
 The fetus cannot store oxygen and is wholly
dependent on a steady supply, but the =fetus normally
exists with a surplus of O2 relative to its metabolic
needs. This surplus provides a significant margin of
safety when O2 delivery is impaired.

Adaptations to O2 uptake
1. High blood flow to organs
2. Left shift of the O2 dissociation curve which
increases the capacity to carry O2 and O2
extraction at typical O2 tension.
3. The capacity to significantly reduce energy –
consuming processes;
4. Greater anaerobic capacity in many tissues;
5. The capacity to redistribute blood flow towards
essential organs away from the periphery.

6. Presence of systems of shunts such as the dactus
arteriosus and preferential blood flow streaming in the
inferior vena cava to avoid intermixing of oxygenated
blood from the placenta and deoxygenated blood in the
fetal venous system.
 These features ensures maximal oxygen delivery
to essential organs such as the brain and heart.

Metabolism

Gaseous exchange in mammals

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