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35 Posts
Discussion Starter · #1 ·
Can somebody please explain the differences between these, and the associated pathological changes.
O2 content (CaO2)
Dissolved O2 (PaO2) (is this the same as arterial PO2?)
%O2 sat of Hb (SaO2)

Not sure if I missed any other associated O2 values as well.


242 Posts
This one is a classic confusion for first and second year medical might be glad to know that even I suffered from this bug!

Lets relax and start with a deep breadth,

A deep breadth results in the incoming of oxygen to the lungs creating PAO2(BIG A) which is basically the partial pressure of oxygen in the lung alveoli. It's about 100 mm of Hg.

This PAO2 drives oxygen into the incoming blood from the pulmonary artery giving rise to PaO2(small A) which basically the partial pressure of oxygen in the artery.

This PaO2(oxygen in the artery) remains in dissolved form but dissolved fraction is very low and would not had been able to have sufficient oxygen for the survival of man. So god created Hemoglobin in the RBCs so that our blood could carry even more oxygen bound to this Hemoglobin.

So this PaO2 gives rise to two fraction,
1>Dissolved Oxygen in Blood
2>Oxygenates the hemoglobin(which holds maximum oxygen in comparison to dissolved fraction)

We like to refer 1(dissolved fraction as PaO2) and 2(as SaO2)

SaO2 actually comes from the PaO2 itself but since its much much much greater than PaO2 we gave it a new term SaO2.

SaO2 is the saturation% of Hemoglobin in blood. We are usually 97% saturated in sea levels which corresponds to the PAO2 of 100 mm of Hg in the alveoli, when PAO2 falls to 60 mm of Hg in the alveoli SaO2 becomes 90% and if it falls below 90% we call the person to be hypoxic!

So PAO2 leads to PaO2...PaO2 leads to SaO2

Now coming to oxygen content!

Oxygen Content = (PaO2) + (SaO2 x Hb conc) roughly

so oxygen content depends on:

PaO2 - Oxygen dissolved in plasma (very very low)
SaO2 - % Saturation of oxygen attached to Hemoglobin (major player)
Hb Conc - obviously more the hemoglobin more oxygen it can hold!

Last but not the least to remember is that SaO2 falls in methhemoglobinemia and CO poisoning while PaO2 remains unchanged because in both cases there is some thing wrong with the Hemoglobin and not the plasma.

Extra point is that when someone is suffering from hypoxia due to respiratory inhibition due to overdose of opiates,then PaCO2(partial pressure of arterial CO2) becomes high but our body can't detect it...this high PaCO2 leads to lowering of PAO2 thus reducing PaO2,SaO2 down the line, if we give this patient oxygen to correct PAO2 and thus PaO2,SaO2 he will go into respiratory failure as his oxygen deficit will be manged and his aortic center for respiration which senses PaO2 will get satisfied but his CO2 will still be high and will remain undetected in the medullary center of respiration.

By the same logic respiration is not stimulated in an anemic although his oxygen content falls...

again, Oxygen Content = (PaO2) + (SaO2 x Hb conc) roughly

in anemia Hb conc falls so oxygen conc must fall but since the respiration centers only sense PaO2 and not oxygen content of the blood so respiration will not be stimulated!


25 Posts
O2 content

Was looking for answers for my own questions. stumbled upon this thread...

In my book, O2 content = (O2 binding capacity x % saturation) + dissolved O2

so lets say PaO2 is dissolved O2; and that SaO2 is % saturation...

but Hb concentration can't be o2 binding capacity

also in my book, O2 binding capacity is (1.34 x Hb concentration)

so i'm totally lost.

242 Posts
When we are measuring Hemoglobin, the unit is "gm/dL" i.e how much hemoglobin you have!

Some scientist discovered that a 100% saturated hemoglobin(of 1gm/dL) can carry 1.34mL of O2

So a normal person with 14gm/dL hemoglobin with a saturation of 95% is going to have oxygen = [(Hb conc x 1.34) x 95] = [(14 x 1.34) x 95]

Next we add the PaO2 that is the "oxygen DISSOLVED in the PLASMA"(not Hb bound) to complete the equation

Oxygen Content = Hemoglobin carrying O2 + Plasma carrying O2

N.B. - The equation is so complex because we are playing with units,unit of plasma carrying oxygen is different from amount of hemoglobin, one is mL and the other is grams
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