Oxygen Disosiation Curve ??????????
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Oxygen Disosiation Curve
??????????
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The oxygen dissociation curve of adult
haemoglobin is a sigmoid curve. The three
main points to indicate on the curve are:
– Arterial point: pO2 100 mmHg with SaO2 = 97.5%
– Mixed venous: pO2 40 mmHg with SaO2 = 75%
– P50: pO2 26.6 mmHg with SaO2 = 50%
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Four other simple points to remember to allow rapid and reasonablyaccurate drawing of the ODC in a viva are:
– pO2 0 mmHg, SO2 0% - the origin
– pO2 10 mmHg, S02 10% - just easy to remember & helps get thesigmoid shape.
– pO2 60 mmHg, SO2 91% - the ‘ICU’ point – pO2 150 mmHg, SO2 98.8% - shows flat upper part of ODC
• The ‘ICU point’ can be considered as the point on the curve thatseparates the steep lower part from the flat upper part.
• This is a bit artificial but a pO2 of 60 mmHg in this sense isconsidered as the lowest acceptable pO2 in an ICU patient becausemarked desaturation occurs at pO2 values below this point.
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What is the mixed venous point?
• This is the point which represents mixed venous blood. The pO2here is 40 mmHg and the haemoglobin saturation is 75%.
• The oxygen content cannot be specified without furtherinformation (eg [Hb] )
• Note that the mixed venous point does NOT really lie on the normalODC as above (& in all the texts).
• The increased pCO2 and decreased pH in mixed venous blood mean
that the mixed venous point must lie on a slightly right shifted ODCrather than the standard ODC.
• This is the Bohr effect.
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What is meant by the term ‘P50’?
• This term is used in reference to the oxygen dissociationcurve. It is defined as the partial pressure of oxygen atwhich the oxygen carrying protein is 50% saturated.
• It is usually used in relation to haemoglobin but can also beused for other oxygen binding proteins such as myoglobin.
• Though often drawn as a point on the dissociation curve,this is incorrect as the P50 is, by definition, a point on the x-axis as it is a particular pO2 value (& not a pO2-SO2 valuepair like the mixed venous point for example.)
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What is the normal value for the P50 of adult
haemoglobin?
– The P50 of normal adult haemoglobin is 26.6
mmHg.
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What is the P50 used for?
Why was this point on the curve chosen for this purpose?
• The P50 is used to specify the position of the oxygen dissociationcurve (or alternatively, the P50 is an index of oxygen affinity of theoxygen carrying protein.
• This is what specifying the position of the curve is really about).
• It is the most useful point for specifying the curve’s positionbecause it is on the steepest part of the curve.
• It is therefore the most sensitive point for detecting a shift of thecurve.
• Specifying the P50 of a curve allows comparison with the positionof other curves under different conditions.
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What does a right shift indicate?
What are the causes of a right shift?
• A right shift indicates decreased oxygen affinity.
The P50 is higher for a right shifted curve.• A right shift can be caused by an increase in 4
factors: – temperature
–
[H+] – pCO2
– red cell 2,3 DPG level.
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Efek Bohr
• Efek Bohr pertama kali dijabarkan oleh ilmuwanDenmark bernama Christian Bohr.
• Beliau menyatakan bahwa peningkatan konsentrasiproton dan/atau CO2 akan menurunkan daya serap
hemoglobin terhadap oksigen.• Peningkatan rasio plasma CO2 juga akan menurunkan
pH darah oleh karena sifat antagonis antara proton dankarbondioksida.
•
Peningkatan CO2 ini akan mempengaruhi kurva oksigenterlarut dalam darah. Pergeseran kurva ke sebelahkanan berarti suatu pengurangan dalam afinitas darihemoglobin untuk oksigen.
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• Efek fasilitas transport oksigen seperti hemoglobin membungkusoksigen di dalam paru-paru, tetapi kemudian melepaskan ke jaringan-jaringan yang paling membutuhkan oksigen.
• Ketika jaringan tersebut metabolismnya meningkat, produksikarbon dioksidanya pun meningkat.
• Karbon dioksida dengan cepat dijadikan molekul bikarbonat danproton asam oleh enzim karbonik anhydrase.
• Hal ini menyebabkan pH jaringan menurun dan juga meningkatkanoksigen terlarut dari hemoglobin, memperbolehkan jaringantersebut memperoleh oksigen yang cukup sesuai kebutuhannya.
• Kurva disosiasi bergeser ke kanan ketika karbon dioksida ataukonsentrasi ion hydrogen meningkat.
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The Bohr Effect
• The Bohr effect was first discovered by a guynamed Christian Bohr.
• He discovered that there were other factors that
affected the loading/unloading of oxygen by Hb.• One of the factors that he discovered was pH.
• He found that if the pH was lower than normal(normal physiological pH is 7.4), then Hb does notbind oxygen as well.
• This is shown in the following % Saturation graph.
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• This plot includes the % Saturation of Hb at normal pH(7.4) and at a lower pH value (7.2).
• You should notice that the curves are not identicalsuggesting that pH affects the binding of oxygen to Hb.
• Let us examine the % saturation at the lungs and at thetissues when only the pH is changed.
• In the lungs (pO2 = 100 mm Hg), Hb is 98% saturated atboth pH 7.4 and 7.2.
• Thus the binding of oxygen to Hb in the lungs is notaffected by changing the pH and oxygen will loadnormally.
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• The situation is different at the tissues.
• The change in the pH results is a lower % saturation of Hb eventhough the pO2 in the tissues has remained at 40 mm Hg for thisexample.
• Reading from the graph, the % saturation at pH 7.4 with a pO2 of 40
mm Hg is about 70% (meaning 30% of the oxygen coming to thetissue is released).
• At a pH of 7.2, the value is close to 60% (meaning 40% of theoxygen coming to the tissue is released).
Thus more oxygen is delivered to tissues at a lower pH even when thepO2 remains unchanged.
Lowering the pH appears to shift the entire red curve to the right.
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• Another indicator that a tissue has a highmetabolic rate (and thus a need for increasedoxygen delivery) is the production of carbondioxide.
• When a tissue is more active, the amount ofcarbon dioxide produced will be increased(pCO2 is higher).
• Carbon dioxide reacts with water as shown in the
following equation:
CO2+ H2O <---------> H+ + HCO-3
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• What this means is that as the amount of carbon dioxide increases,more H+ are formed and the pH will decrease.
• Thus, a lower pH in the blood is suggestive of an increased carbondioxide concentration which in turn is suggestive of a more activetissue that requires more oxygen.
• According to Bohr, the lower pH will cause Hb to deliver moreoxygen.
• Note that if pO2 and pH should drop together, even more oxygen
will be delivered then if only one of the parameters were changed• (for example, pH = 7.2, pO2 = 30 mm Hg compared to pH = 7.4,
pO2 = 30 mm Hg).