circulation [part 3]
TRANSCRIPT
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TRANSPORT IN ANIMALS [part 3]
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A) General characteristics of a circulatory system
B) The development of blood systems in animals
C) Composition of blood
D) The circulatory system
E) Formation of tissue fluid
F) The heart
G) Functions of mammalian blood
H) Oxygen dissociation curves –
The Bohr shift
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Oxygen transportHaemoglobin: 4 haem prosthetic
groups linked to 4 globin polypeptide chains
1 Fe2+ in 1 haem
1 Fe2+ can combine loosely with 1 O2 molecule
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four molecules
of oxygen.
One Hb molecule carries:
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Oxygenation: Hb + 4O2 <--> Hb(O2)4 (oxyhaemoglobin)
readily reversible
Fe2+ in haemoglobin must remain Fe2+
certain chemicals can oxidise Fe2+ to Fe3+ producing derivatives of haemoglobin which cannot carry oxygen
e.g. carboxyhaemoglobin contains Fe3+
CO reaction: Hb + CO --------> HbCO (carboxyhaemoglobin)
very high affinity (230X greater than for O2)
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Under which condition does Hb :
LOW partial pressure of O2 : capillaries supplying metabolically active tissues
HIGH partial pressure of O2: lungs
Bonds holding O2 to Hb become unstable & O2 is released
Bind to O2?
Release O2?
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Oxygen dissociation curves
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Oxygen tensiondetermines the amount of oxygen that
can combine with haemoglobin is expressed as a partial pressure is the fraction of oxygen found in the air
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The oxygen dissociation curve is a plot of the:
% oxygen saturation of blood against
the partial pressure of oxygen
What is O2 saturation?
% s
atu
rati
on
of
hae
mo
glo
bin
wit
h o
xyg
en
Oxygen tension /mmHg
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Oxygen saturation is the:fraction or percentage of all the haemoglobin
binding sites that are currently occupied by O2
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A 100% saturation of Hb: is rarely achieved
At a partial pressure of 0%: no O2 is attached to Hb
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tension at which 95% of the pigment is saturated with O2
tension at which 50% of the pigment is saturated with O2
Loading tension: Unloading tension:
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The oxygen dissociation curve shows that when haemoglobin is exposed to a gradual increase in
O2 tension, it absorbs:
1. O2 rapidly at first
2. but more slowly as the O2 tension continues
to rise
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a big fall in Hb saturation, so
that O2 is released to the tissues where
need is greatest
Over the steep part of the curve, a small decrease in O2 tension:
Results in:
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The curve is S (sigmoid)-shaped due to: positive
cooperativity
shape of Hb molecule is altered as each O2 molecule is taken up
each one facilitates much faster uptake of oxygen than the previous one
affinity for O2 INCREASES as each O2 molecule is taken up
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2. When an O2 molecule combines with the iron atom
of a single haem unit, it distorts its shape slightly.
The whole molecule changes shape accordingly.
1. Deoxy-haemoglobin has a relatively low affinity for O2.
3. The 2nd molecule binds more easily, and the 3rd & 4th even more easily.
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When Hb gives up its O2:
The 1st O2 molecule is released: very rapidly
The 2nd, 3rd, 4th molecules are released: less rapidly at a much reduced partial pressure
of O2
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Changes in haemoglobin affinity for oxygen
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The term "affinity"
is used to describe the attraction of oxygen to haemoglobin binding sites
affinity changes with: variation in pH temperature CO2
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Traditionally the curve starts with: pH at 7.4 temperature at 37C partial pressure of
CO2 at 40 mmHg
changes from these values are called "shifts"
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Left shift
Right shift
pH
temperature
pH
temperature
norm
al
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increases oxygen's affinity for haemoglobin
A LEFT shift:
A RIGHT shift:
decreases oxygen's affinity for haemoglobin
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blood holds onto O2
less O2 is released to tissues
A LEFT shift:
A RIGHT shift:
blood releases O2 more readily
more O2 is released to tissues
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minimal effects on O2 loading in the lung:
because the O2 dissociation curve is still fairly flat at a PO2 of 100 mmHg.
Shifts in O2 dissociation curves have:
maximal effect on O2 unloading at the tissues:
because the curve is steep at a vein (e.g. 40 mmHg).
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The Bohr Effect or Shiftis a shift to the RIGHT in the O2 dissociation
curve in regions with an increased partial pressure of CO2 (fig. 21)
Low CO2 tension [2 kPa]
Medium CO2 tension [5 kPa]
High CO2 tension [8 kPa]
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The effect of increased CO2 is to cause O2 to be released from the Hb molecule
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H+ released: combine with haemoglobin and make it less able to carry O2
CO2 has this effect because when it dissolves it forms a weak acid:
How does CO2 make Hb release O2?
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An increase in blood temperature causes a shift to the right
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Myoglobinis a red pigment – has
one polypeptide chain
Location: in skeletal muscles and is the reason
why meat appears red
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Myoglobin
Affinity for O2 :
great Acts as:
a store of O2 in resting muscles
Releases O2: only when supplies of oxyhaemoglobin
have been exhausted
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Myoglobin O2 dissociation curveis displaced well to the left of haemoglobin
Haemoglobin[sigmoidal]
Myoglobin
[hyperbolic]
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0
20
40
60
80
100
0 4 8 12 16
Arterial PO2 (kPa)
Sa
tura
tio
n (
%)
Oxygen dissociation curves for various animals
curves shifted to the right
high metabolism so need O2 to be released readily
Small animals (shrews/mice) & birds have:
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South American Llamas live at high altitudes. Where do you think their
oxygen dissociation curve would lie?
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S-shaped hemoglobin curveReleases much Becomes saturated
O2 at tissues with O2 at lungs
High affinity onlyCan’t release much O2 to tissues
Low affinity onlyDoesn’t hold on to But can’t pick up much O2 at tissues much O2 at lungs
Advantages of “S-shaped” curve for Hb-O2 association
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Question: MAY, 2010
Use your knowledge of biology to describe the selective advantage of each
of the following adaptation.
The haemoglobin of certain mud-dwelling organisms has an oxygen-dissociation curve shifted well to the left of that of human haemoglobin. (5)
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Comparison of the O2 dissociation curves of foetal & maternal haemoglobin
To enable it to obtain O2 from the mother’s haemoglobin in the
placenta.
Foetal haemoglobin has a greater affinity for O2
than adult haemoglobin.
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O2 must easily dissociate from the maternal haemoglobin to the foetal haemoglobin
therefore, easily transferred from maternal to foetal blood
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Question: SEP, 2011The graph in Figure 2 shows the variation in oxygen saturation of foetal and maternal haemoglobin in relation to partial pressure of oxygen.
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1. Based on information given in the graph in Figure 2, describe the activity of maternal haemoglobin under different partial pressures of oxygen. (4)
Haemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the haemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape. At pressures above about 60 mmHg, the standard dissociation curve is relatively flat, which means that the oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure.
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2. How does the activity of foetal haemoglobin differ from that of maternal haemoglobin? (2)
Foetal haemoglobin has a higher affinity for oxygen than the maternal one. It has a higher saturation with oxygen at all partial pressures.
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3. Draw another curve on the graph in Figure 2 showing the probable shape of the analogous curve for maternal myoglobin. (2)
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4. Explain the difference between the curve for maternal haemoglobin and that for maternal myoglobin. (2)
Myoglobin has a higher affinity for oxygen than haemoglobin. Myoglobin releases its oxygen at very low partial pressures of oxygen.
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5. How would the curve for maternal haemoglobin be expected to differ under conditions of high partial pressure of carbon dioxide? (2)
Shifted to the right.
Affinity of haemoglobin to oxygen is reduced so that this gas is released to the cells which need it.
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Carbon monoxide & haemoglobin
Result: O2 does not combine with
Hb & so it is not transported
Hb + CO
Hb combines with any CO available in preference to O2
[a relatively stable compound carboxyhaemoglobin]
HbCO
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Transport of carbon dioxideCO2 must not be allowed to accumulate in the
body
it forms an acid in solution that could lead to fatal changes in blood pH
BECAUSE
Carbonic acid
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CO2 is carried in blood in three ways:
In solution
Combined with haemoglobin
[carbamino-haemoglobin]
As hydrogen carbonate 85%
10-25%
5%
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Carbonic anhydrase in RBC catalyses reaction to form H2CO3
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Some H2CO3 dissociates & the H+ displace the O2 from the haemoglobin
this is the basis of the Bohr effect
By accepting H+, haemoglobin acts as a BUFFER molecule and so enables large quantities of carbonic
acid to be carried to the lungs without any major changes in blood pH
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diffuse out of the RBC into the plasma combine with Na to form NaHCO3
this is called the chloride shift
Majority of the HCO3- ions:
the RBC becomes
positively charged as
it loses negative ions
Cl- ions diffuse into
the RBC
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CO2 forms at the lungs & is exhaled
AT ACTIVE TISSUES AT LUNGS
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THE END