H 6 - Gas exchange - IBDPBiology-Dnl

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Assessment Statements
 H.6.1
 H.6.2
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Define partial pressure.
Explain the oxygen dissociation curves of adult
hemoglobin, fetal hemoglobin and myoglobin.
H.6.3 Describe how carbon dioxide is carried by the
blood, including the action of carbonic anhydrase, the
chloride shift and buffering by plasma proteins.
H.6.4 Explain the role of the Bohr shift in the supply of
oxygen to respiring tissues.
H.6.5 Explain how and why ventilation rate varies with
exercise.
H.6.6 Outline the possible causes of asthma and its
effects on the gas exchange system.
H.6.7 Explain the problem of gas exchange at high
altitudes and the way the body acclimatizes.
Define the term partial pressure
 partial pressure is the
pressure exerted by a given
gas in a mixture
 the symbol for partial
pressure is P, and the partial
pressure for a gas x is Px. So,
PO2 denotes the partial
pressure of oxygen
 what is the partial pressure
of the oxygen in the air
Atmospheric air is a mixture of gases;
around us?
nitrogen, oxygen, carbon dioxide, water
 At sea level, the atmospheric
vapour & inert gases.
pressure is typically about
At sea level, the atmospheric pressure
101.3 kPa of which 21.0% is
is about 101.3 kPa.
O2, So, PO2 is given by:
What proportion of atmospheric
101.3  21.0
pressure is due to oxygen?
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= 21.3 kPa
100
Role of haemoglobin
 1 molecule of oxygen will
combine with each haem group,
meaning, each haemoglobin
molecule is able to transport 4
molecules of oxygen:
 oxyhaemoglobin is the form in
which oxygen is transported
from the lungs to the respiring
 haemoglobin occurs in the
body tissues
red cells
 at respiring tissue cells,
 haemoglobin molecule is
oxyhaemoglobin breaks down,
built of four interlocking
releasing oxygen & haemoglobin
subunits
 each subunit is composed of a  oxygen is used up by tissue cells
while haemoglobin is returned
large globular protein with
to the lungs to pick up more
a non-protein haem group
oxygen
attached, containing iron
Oxygen dissociation curve
 the affinity of haemoglobin for
oxygen is measured experimentally by
finding the percentage saturation with
oxygen of blood exposed to air
mixtures containing different partial
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pressures of oxygen
the result is called an oxygen
dissociation curve
oxygen dissociation curve is S-shaped,
the amount of oxygen held by
haemoglobin depends on the partial
pressure of oxygen
in the body, too, the amount of oxygen
held by haemoglobin depends on the
partial pressure
in respiring tissues, the oxygen partial
pressure is much lower than that in the
lungs
at lower partial pressures,
oxyhaemoglobin breaks down,
releasing oxygen in solution and this
rapidly diffuses into the surrounding
tissues
Oxygen dissociation curve of adult
haemoglobin
 oxygen dissociation curve for
oxyhaemoglobin is S/sigmoid-shaped
 it shows how the saturation of
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Shift to the right;
(decreased affinity)
low pH,
increased CO2,
increased lactic acid
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haemoglobin with oxygen varies with
partial pressure of oxygen
haemoglobin has an increasing affinity
for oxygen, initial uptake of one oxygen
molecule by haemoglobin facilitates the
further uptake of oxygen molecules
low partial pressure of oxygen
corresponds to the situation in the
tissue, when partial pressure of oxygen is
low, oxygen is released
low pH, increased carbon dioxide &
increased lactic acid causes the curve to
shifts the to the right and oxygen is more
readily released to respiring tissues –
this is known as the Bohr effect
high partial pressure of oxygen
corresponds to the situation in the
lungs, when partial pressure of oxygen is
high, oxygen is taken up by haemoglobin
Oxygen dissociation curve of fetal
haemoglobin
 like adult haemoglobin, fetal haemoglobin
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 between foetal & adult
haemoglobin, which one has a
higher affinity for oxygen?
 why it is advantageous that
fetal haemoglobin higher
affinity for oxygen than adult
haemoglobin?
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have S-shaped oxygen dissociation curves
fetal haemoglobin have a high affinity for
oxygen at high partial pressure of oxygen
fetal haemoglobin always has a higher
affinity for oxygen at corresponding partial
pressures of oxygen than adult haemoglobin,
thus fetal haemoglobin dissociation curve
lies to the left of the adult dissociation curve
in the placenta where maternal and fetal
blood come into close proximity there is a
low oxygen partial pressure
fetal haemoglobin must have a greater
affinity for oxygen otherwise the maternal
oxy-haemoglobin would not dissociate
relationship between fetal and adult
haemoglobin dissociation curves does NOT
change at all partial pressures of oxygen
the difference in adult and fetal haemoglobin
structures lead to differences in affinity
Oxygen dissociation curve of
myoglobin
 myoglobin is specialized for oxygen
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 myoglobin is a respiratory pigment
built of a single haem–globin unit,
similar to the four units in
haemoglobin
 myoglobin is only found in skeletal
muscle cells, where it acts as a reserve
of oxygen
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storage
myoglobin has a higher affinity for
oxygen than haemoglobin, its
dissociation curve is to the left of that for
haemoglobin
in normal conditions, at rest myoglobin
is saturated with oxygen
myoglobin is used during intense muscle
contraction when the oxygen supply is
insufficient i.e. when muscle is very
active its oxygen concentration may fall
below 0.5 kPa
when this happens myoglobin releases
oxygen to muscle cells
myoglobin oxygen dissociation curve is
not sigmoid shaped, it has a steep rise
below 5 kPa with no lag & has slower rise
approaching 100 % above 5 kPa
Oxygen dissociation curves of adult
haemoglobin, fetal haemoglobin and myoglobin
 adult haemoglobin:
 rapid saturation of oxygen in the
lungs
 rapid dissociation of oxygen as the
oxygen concentration decreases
 oxygen released in the tissues where it
is needed
 fetal haemoglobin:
 fetal haemoglobin curve to the left of
adult haemoglobin
 higher affinity for oxygen than adult
haemoglobin
 oxygen moves from adult
haemoglobin to fetal haemoglobin
 myoglobin:
 myoglobin to the left of fetal
haemoglobin
 higher affinity for oxygen than adult
haemoglobin
 only releases oxygen at very low
oxygen concentrations in tissues
 acts as oxygen reserve in muscle cells
How carbon dioxide is carried by
the blood
 carbon dioxide is carried in three
forms in the blood:
 carbon dioxide can be dissolved in
the blood plasma forming carbonic
acid (5 %)
 carbon dioxide can be carried as
dissociated carbonic acid i.e. H+ +
H CO3 − in red blood cells (85 %)
 carbon dioxide can be carried as
carbaminohemoglobin when it is
bound to haemoglobin (10 %)
 carbonic anhydrase is an enzyme
found in red blood cells (erythrocytes)
 carbonic anhydrase speeds up
production of hydrogen carbonate (H
CO3 −)
 chloride shift i.e. movement of
chloride ions into red blood cell,
occurs to balance movement of
hydrogen carbonate ion out
Role of the Bohr shift in the supply
of oxygen to respiring tissues
 hemoglobin carries up to four oxygen
molecules
 Bohr shift promotes the release of oxygen
in respiring heart muscle
 active respiration releases CO2 causing the
partial pressure of CO2 increases
 release of CO2 increases acidity i.e. lowers
the pH due to formation of hydrogen ions
(H+)
 hydrogen ions bind to hemoglobin
decreasing hemoglobin’s affinity for O2 so
O2 is released from the oxyhemoglobin
 this occurs due allosteric effect i.e.
conformational change in hemoglobin
which releases O2 more readily
Bohr shift
How and why ventilation rate
varies with exercise
 during exercise the rate of tissue respiration
increases i.e. more carbon dioxide produced
 carbon dioxide production in the tissues
exceeds the rate of breathing it out
 increase in carbonic acid (H2CO3), increase
in H+ ions , pH drops in the blood plasma
 lactic acid produced during strenuous
exercise reduces pH
 chemoreceptors, located in the carotid &
aortic bodies, detect change in pH, increase
in carbon dioxide & decrease in oxygen
 Increased CO2 in the blood & lower pH are
also detected by chemoreceptors in medulla
 nerve impulses sent to the breathing Centre
in the medulla of the brain from the
chemoreceptors
 nerve impulses are then sent to diaphragm &
intercostal muscles from the breathing
Centre in medulla to increase the rate & the
depth of breathing
 ventilation rate is controlled through
negative feedback mechanism
Possible causes of asthma and its
effects on the gas exchange system
 asthma is a chronic
inflammatory disease of the
airway
 it is caused by allergic
reaction to allergens such as;
dust, mites droppings, pollen,
toxins, pets hairs, fungi etc.
 immune responses releases
histamine which causes:
 constriction of muscles of
wall of bronchioles
 more mucus is produced
 these restricts air flow thus
ventilation is hard & gas
exchange is reduced
Problem of gas exchange at high altitudes
 at high altitudes partial
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pressure of oxygen is lower,
at 7000m PO2 is 8.1 kPa
as air is exchanged in lungs
hemoglobin does not
become fully saturated with
O2
oxygen deprivation of tissues
occurs causing fatigue i.e.
Monge disease
mountain sickness
(increased pulse rate,
nausea, headaches, sore
throat, muscular weakness,
dizziness ) may develop
ventilation rate & depth
increases
How the body acclimatizes to high altitudes
 ventilation rate increases
 red blood cell (erythrocyte)
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concentration in blood
increases
myoglobin concentration in
muscles increases
capillary networks in the
muscles develop greater
density
lung working volume, vital
capacity, increases
people living permanently at
high altitude develops
greater lung surface area
Revision Questions
 Define the term partial
 Explain why ventilation rate
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pressure.
[1]
Explain the oxygen
dissociation curves of adult
haemoglobin, fetal
haemoglobin and myoglobin.
[6]
Describe how carbon dioxide
is carried by the blood.
[4]
Explain, with the use of a
diagram, the role of the Bohr
shift in the supply of oxygen
to respiring heart muscle.
[6]
Explain the Bohr shift of an
oxygen dissociation curve
during gas exchange.
[6]
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varies with exercise.
[6]
Explain how and why
ventilation rate varies with
exercise.
[6]
Outline one possible cause of
asthma and its effect on the
gas exchange system.
[3]
Outline how the body
acclimatizes to high altitudes.
[3]
Explain the problem of gas
exchange at high altitudes
and the way the body
acclimatizes.
[6]
 The oxygen dissociation
curve is a graph that shows
the percentage saturation
of haemoglobin at various
partial pressures of oxygen.
Curve A shows the
dissociation at a pH of 7
and curve B shows the
dissociation at a different
pH.
 (i) State the possible cause
of the curve shifting from
A to B.
[1]
 (ii) On the graph, draw the
curve for myoglobin.
[2]
 Explain the oxygen dissociation of myoglobin,
completing the graph below to support your answer.
Po2 is the partial pressure of oxygen.

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