dsDNA
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It says that site 4 O2 affinity is PO2 = 100 mm Hg ….essentially what they’re saying is that you need more PO2 to saturate Hb. But this is contrary to what we’ve been taught. Have we not learned that as Hb binds its first O2, its affinity for 2nd O2 increases. So, if Hb were to bind at Site 1 for PO2 = 50mm Hg, it should bind O2 at Site 2 for PO2 = less than 50 mmHg.
Is my understanding flawed?
Also, can someone tell me the relationship between oxygen content in Hemoglobin and arterial O2 content? I have a hard time understanding - "In polycythemia, arterial O2 content is above NL bcos of an inc Hb conc." I see why the Hb conc is hi, and I see why the overall O2 content is hi. But why is the arterial O2 (which I'm assuming is the O2 that's in blood unbound to anything) hi?
Edited by dsDNA on 03/15/07 - 04:21 PM
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| babydoc4usmle
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ok, one step at a time
Hb site 1 has strongest affinity to O2 and never leaves it in N condition; we need very small pressure to bind O2 to this site
Hb site 2 has lower affinity in a sense that it needs ~25 mmHg of O2 to bind O2 to Hb (this is pressure @ which we have Sat 50% (it's not pressure 50, its' %)
Hb site 3 has even lower affinity and needs higher pressure for binding (~40 mmHg) and correlates to ~75% Sat Hb
Hb site 4 has lowest affinity and needs ~100 mmHg of O2 to bind it
what it means is that if we have low PO2 in the air Sat Hb goes down with decreasing pressure and don't confuse 3 things (though connected ones) Sat and Pressure
now, as for content
there is Hb O2 content= 1.34 [Hb]
and blood O2 content
ad there is simple formula for blood O2 content = 1.34 [Hb] x HbSat% + PaO2
Sat is % of Hb in Er hoked with O2 (to some extent we can say that if only 2 sites have O2 on it we have 50% Sat and when we have 4 sites of Hb with O2 on it we have 100% Sat)
PaO2 is an amount of O2 dissolved in blood (not on Hb or anything else) and this force (which is very small) keeps O2 hooked on Hb
if PO2 goes up - Sat goes up
if PO2 goes down - Sat goes down
in polycythemia Sat Hb is not increased, Hb concentration is increased, so we have increased O2 content of blood or increased O2 carying capacity of blood
hope i didn't confuse you and if someone thinks differently, please make a note of it
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| new_n_lost
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The quaternary structure of hemoglobin determines its affinity for O2. In deoxyhemoglobin, the globin units are tightly bound in a tense (T) configuration which reduces the affinity of the molecule for O2. When O2 is first bound, the bonds holding the globin units are released, producing a relaxed (R) configuration which exposes more O2 binding sites. The net result is a 500-fold increase in O2 affinity. In the tissues, these reactions are reversed, releasing O2.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
The Four Sites of Hb react with O2 in the following way
Site 1 - °2 usually remains attached under physiologic conditions. Under physiologic conditions, only sites 2, 3, and 4 need to be considered.
Site 2 has the greatest affinity of the three sites and thus requires the lowest P02 for oxygen to remain attached. The minimum P02 to keep oxygen attached at this site is about 26 mmHg. As long as the P02 of the plasma is 26 mm Hg or above, oxygen remains on this site.
Site 3: When oxygen attaches to the second site, all sites gain affinity but the ranking does not change. The third site has less affinity than the second. It thus requires a greater P02 to maintain the attachment of oxygen, about 40 mm Hg.
Site 4: The last site has the least affinity for oxygen and thus requires the greatest P02 to maintain attachment. This is just above 100 mm Hg. At a P02 of 100 mm Hg, the hemoglobin is approximately 97% saturated.
Most of the oxygen in systemic arterial blood is oxygen attached to Hb. The only significant form in which oxygen is delivered to systemic capillaries is oxygen bound to Hb. When blood passes through a systemic capillary, it is the dissolved oxygen that diffuses to the tissues. However, if dissolved oxygen decreases, PO2 also decreases, and there is less force to keep oxygen attached to Hb. Oxygen comes off Hb and dissolves in the plasma to maintain the flow
of oxygen to the tissues.
There r 2 types of O2 in blood
1. Dissolved O2 which is the determinant of the O2 binding of Hb
2. O2 bound to Hb itself
O2 content bound to Hb is Oxygen content in Hemoglobulin and Arterial Content is the sum of the O2 bound to the Hb and Dissolved O2.
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| babydoc4usmle
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| babydoc4usmle
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pretty mush the same and looks good to me
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| new_n_lost
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Thanks!!!!!
lets c if the person who posted the query gets the concept
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FORUM RULES-- Those who believe in telekinesis, raise my hand. I get enough exercise just by pushing my luck --P4U World.." The pure and simple truth is rarely pure and never simple."
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| dsDNA
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Thank you for the responses guys. I appreciate it. I look forward to having more convos w/ all of you.
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| GoodGirl
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Babydoc and NNL described it nicely.
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Prioritize & simplify.
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