Oxygen Dissociation Curve

The oxygen dissociation curve plots the % saturation against the partial pressure of oxygen, and its contribution to the total oxygen content. This is an S shaped curve due to the alterations in hemoglobin's affinity for oxygen in response to other physiologic factors. Please note the dotted line at the bottom of the graph. This represents the dissolved O2. Dissolved O2 is a linear relationship to its partial pressure and results in a straight line.

 

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PO2 can fall from 100 to 60 mm Hg and the hemoglobin will still be 90 percent saturated with oxygen

Excellent safety zone, this corresponds to the flat upper portion of the curve. Also indicates that the hemoglobin can load a fair amount of oxygen at the lungs even if there is a diffusion problem.

As the Hb moves through the A-C system, a significant partial pressure difference continues to exist between the alveolar gas and blood, even after most O2 has transferred.

Oxygen that diffuses from the alveolus into the capillary plasma passes into the RBC to bind with hemoglobin where it no longer exerts a partial pressure. This process facilitates and enhances the diffusion of oxygen by maintaining the pressure gradient between the alveolus and the plasma.

However, once the hemoglobin molecules are saturated, increasing PO2 beyond 100 mm Hg adds very little O2 to the blood

Effects dissolved O2 only (PO2 x 0.003 = dissolved O2)

A reduction of PO2 below 60 mm Hg causes a rapid decrease in amount of O2 bound to hemoglobin.

However, diffusion of oxygen from hemoglobin to tissue cells is enhanced by this process. This corresponds to the steep portion of the curve.

P50

The P50 represents the partial pressure at which hemoglobin is 50 percent saturated with oxygen.

Normal P50 is 27 mm Hg

P50 provides a means of quantifying the hemoglobin's affinity (willingness to bond) with oxygen. Reflects what are called shifts of the dissociation curve.

Right shift – hemoglobin has decreased affinity, increased P50 – takes more oxygen to reach 50% (higher partial pressure to get 50% saturated)

Left shift – increased affinity, decreased P50 – less oxygen to reach 50% (less partial pressure to get 50% saturated)

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Factors that influence hemoglobin's affinity for oxygen include

changes in

• pH

• Temperature

• Carbon Dioxide

• 2,3-DPG

and the presence of hemoglobin variants

• Fetal Hemoglobin

• Carbon Monoxide Hemoglobin

• Hemoglobin S (sickle cell)

• Methemoglobin

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Bohr Effect – effect of changes in pH and PCO2 on oxygen binding to hemoglobin

Enhances unloading @ cells - pH 7.40 to pH 7.37 results in a right shift, decreased affininity and release of oxygen

Enhances uptake @ lungs – pH 7.37 to 7.40 results in a shift back to normal, normal affinity

Temperature is related to metabolism

Increased temperature causes increased metabolism and results in

An increased cellular need for O2

Decreases hemoglobin's affinity and helps unload O2

Hypothermia results in a decrease metabolic rate

Decreased Oxygen need

Increases affinity and decreases unloading

2,3 diphosphoglycerate (organic phospate) – stabilizes the Hb molecule in its deoxygenated state and decreases affinity for O2

Increase 2,3 DPG results in a R shift of the curve

Causes of increased 2,3 DPG include anemia, alkalosis, chronic hypoxemia

Decrease 2,3 DPG results in a L shift of the curve

Causes by acidosis, administration of stored blood

Blood stored one week loses 2/3rds of DPG. So even though we give blood to increase hemoglobin levels and improve oxygen content the O2 is not released easily @ the cellular level until the body restores the DPG levels

HbS causes a R shift : decreases affinity

Methemoglobinemia (pronounced as Met Hemoglobinemia not Meth) is the change of the iron molecule from Ferrous 2+ to Ferric 3+

Ferric ion is unable to combine with O2 therefore greatly decreases hemoglobin's affinity – R shift of curve

Seen in nitrate poisoning, shellfish toxins and algae blooms

A good first indicator is BROWN (chocolate syrup) blood – treated with methylene blue or ascorbic acid

Like rusted blood!

Carboxyhemoglobin

Hb affinity for CO is 200 times that of O2!

Strong bond with site prevents O2 from bonding

Also increases hemoglobin affinity for the oxygen that is present and prevents the release of O2 from those sites it has bonded to

Results in the bright red color of the blood and tissues (cherry red appearance to lips) which may be misidentified as good oxygenation when in fact the tissues are severely compromised. PaO2 will be normal!

L shift of curve

Treatment is removal from the environment where CO is present and 100% oxygen to create a large pressure gradient to compete for binding sites in an attempt to drive off the CO which will be released back into the lungs for exhalation.