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#1
09-08-2016
 USMLE Forums Newbie Steps History: Not yet Posts: 4 Threads: 2 Thanked 1 Time in 1 Post Reputation: 11
Respiratory - O2 help needed!

Hey guys,
Can't understand some of these formulas. Hoping an astute, beautiful/handsome male/female can help me!!

So,
total O2 content = (O2 binding capacity x % saturation) + dissolved O2

o2 binding capacity is normally 20.1 mL O2/dL and to calculate that would be, 1.34 x [Hb] x % saturation ??

therefore,
o2 content = (20.1 x % saturation ) + dissolved O2 ???

why would you multiply saturation again?

also how would you calculate HbO2?

and what is % saturation? i mean i get that it is the % of heme groups bound to O2.... but what does that mean? like does it mean that the hemoglobin molecule has 4 heme groups to which a molecule of oxygen binds to each of these groups? and the amount of heme groups that are bound to o2, is what is saturated?

Looking forward to your replies, my lovelies!
 The above post was thanked by: virgo32 (09-10-2016)

#2
09-08-2016
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 237 Threads: 4 Thanked 160 Times in 114 Posts Reputation: 172

OK I dont know whether I qualify for astute, beautiful/handsome male but I will try

So here is the breakup -

(a) Oxygen Content of blood - BIGger term
(b) Oxygen bound to Hemoglobin - BIG term
(c) Oxygen bound to oxygen - small term

Now a = b + c

Oxygen enters the lung alveolar capillaries as we respire, so oxygen enters the PLASMA from the alveoli, next in the blood the dissolved oxygen enters the RBCs and binds to Hemoglobin - so oxygen is bound to two things in blood - plasma and hemoglobin!

Although at first oxgyen hits the plasma and next is taken up by the Hb but max amount of oxygen is carried around by the Hb and not plasma! Evolution formed Hemoglobin because the oxygen carried by plasma is so miniscule we will die!

So now you need to under stand this -

PAO2-->PaO2-->SAO2

The oxygen in the alveolus diffuses across to the blood vessel to give rise to PaO2(small a) which means the oxygen dissolved in the plasma

This PaO2 in the plasma now next dissolves into the RBCs where they get bind to the hemoglobin present inside the RBCs giving rise to SaO2(saturation of hemoglobin)

Now Saturation of hemoglobin depends upon how much saturated was the plasma at the first place! so PaO2 --> SaO2

But one can have 5gms of hemoglobin and be "100% saturated" but that doesnot mean he is carrying a lot of oxygen!

So the next thing to worry about is the amount of hemoglobin in the blood!(hemoglobin concentration of blood)

Thus the equation -

total O2 content = (O2 binding capacity x % saturation) + dissolved O2

O2 binding capacity = [Hemoglobin conc(gms/dL) x 1.34]

O2 bound to hemoglobin = O2 binding capacity x % saturated = 15gms/dL x 1.34 = 20.1 (the 1.34 is a unit converter as we are measuring hemoglobin in gm/dL and not mL of O2 dissolved)

O2 dissolved in plasma = 0.03 mL

Thus Total O2 carried by blood = O2 carried by hemoglobin + O2 carried by plasma = 20.1 + 0.03 = 20.13

Now the key is to understand the physiology - that all the terms are different
PAO2 - oxygen in alveolus (mm of Hg)
PaO2 - oxyegen in the artery (mm of Hg)
Hemoglobin concentartion - amount of hemoglobin you have which can bind to oxygen if oxygen is there is blood (PaO2 leads to the oxygenation of Hemoglobin as oxygen flows from Plasma to RBC)
Saturation % - how much of the hemoglobin I am carrying is actually saturated

PAO2 - decreased in hypoventilation(Opiod poisoning)
PaO2 - causes of hypoxiMEA (see pathology books)
Hb conc - decreased in anemia
Saturation - decreased in CO poisoning

So depending on the physiology the pathology can be understood, refer to the 1st chapter of PATHOMA(pathology course) if you are still having problems!

GOOD LUCK
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 The above post was thanked by: PurpleUnicorn (09-09-2016), virgo32 (09-10-2016)
#3
09-09-2016
 USMLE Forums Newbie Steps History: Not yet Posts: 4 Threads: 2 Thanked 1 Time in 1 Post Reputation: 11
ohhhhh

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