kurva oksigen.pptx

8/11/2019 kurva oksigen.pptx

http://slidepdf.com/reader/full/kurva-oksigenpptx 1/18

Oxygen Disosiation Curve

??????????



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%





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.



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.



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.)



What is the normal value for the P50 of adult

haemoglobin?

– The P50 of normal adult haemoglobin is 26.6

mmHg.



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.



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.





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.



• 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.



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.





• 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.



• 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.



• 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



• 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).

Documents

kurva oksigen.pptx