Hemoglobin, oxygen, and dissoct.

hyperventilating - think panic attacks with that brown bag. You're breathing really fast, getting a lot of O2 and losing a lot of CO2. Except I think you need CO2 for some stuff which is why they tell you to breath into that bag, so you breath back some of your expelled CO2...I think. I vaguely remember this from a bio class a long time ago so I could be mis-remembering

at high altitudes, there's lower O2 partial pressure so to adapt short term, your metabolic rate may increase, your heart will work harder, stuff like that. People who live at high altitudes long term will have better adaptations but I can't remember them now....

I remember running across cases where we just moved along the curve due to external changes...Does anyone know of such cases?

hmm... all I remember are cases where you have, say, a lot of CO2 at the muscles which creates acidic conditions that lowers affinity of hemoglobin for O2 so the curve shifts. I just can't remember in what direction... lol

O2-Hb Curve Shifts to the Right (think about during exercise - facilitates unloading of O2 into the tissues):
Increase in PCO2 and decreases in pH
Increases in temp
Increases in 2,3-DPG concentration
This also occurs at high altitude



O2-Hb Curve Shifts to the Left (increases Hb-O2 affinity):
Decrease in PCO2 and increases in pH
decreases in temp
decreases in 2,3-DPG concentration

antissa said:

hyperventilating - think panic attacks with that brown bag. You're breathing really fast, getting a lot of O2 and losing a lot of CO2. Except I think you need CO2 for some stuff which is why they tell you to breath into that bag, so you breath back some of your expelled CO2...I think. I vaguely remember this from a bio class a long time ago so I could be mis-remembering

at high altitudes, there's lower O2 partial pressure so to adapt short term, your metabolic rate may increase, your heart will work harder, stuff like that. People who live at high altitudes long term will have better adaptations but I can't remember them now....

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Since you're losing a lot of CO2, you need it back to get equilibirum, your blood is too basic, so you get the CO2 back by inhaling from the bag
C02+h20->h2co3->h+ hco3- and then that breaks back down into co2+h20

Hyperventilating causes a rise in plasma oxygen concentration and a drop in plasma CO2 concentration. People who are hyperventilating can pass out, although this may seem counterintuitive at first. The reason is because although hemoglobin saturation may be at 100%, it cannot release O2 to the tissues without some degree of CO2 present. In other words, CO2 decreases hemoglobin affinity of O2 at the tissue level. Hence, breathing into a brown paper bag will allow more Hb molecules to release their supply of O2.

Hb displays cooperativity, meaning that when one of its binding sites (there are 4 per hb molecule) binds to O2, it facilitates binding of O2 to the other three sites. This is why the dissociation curve is sigmoid shaped. A left-shifted curve signifies increasing affinity of Hb for O2 (at the tissue level), a right shifted curve signifies decreasing affinity of Hb for O2 (at the tissue level). The curve shifts to the right when you need more oxygen, i.e. exercising, etc. Byproducts of respiration are the major trigger for a rightward shift: CO2 (with concomitant pH decrease), and increased 2,3-bpg.

A couple other things to remember: myoglobin is left-shifted in comparison to Hb. Myglobin exists in muscle and must have a higher affinity for O2 in order to pick up O2 from the Hb molecules passing through the muscle capillaries. Also, fetal hemoglobin is left shifted. Just remember, you can't spell "fetal" without L-E-F-T.

One last thing: at high altitudes, the kidneys will produce the hormone erthropoietin to increase the number of red blood cells (i.e., to facilitate tissue oxygenation due to the lower O2 in the air).

wow! *clap clap*

Fusion said:

Hyperventilating causes a rise in plasma oxygen concentration and a drop in plasma CO2 concentration. People who are hyperventilating can pass out, although this may seem counterintuitive at first. The reason is because although hemoglobin saturation may be at 100%, it cannot release O2 to the tissues without some degree of CO2 present. In other words, CO2 decreases hemoglobin affinity of O2 at the tissue level. Hence, breathing into a brown paper bag will allow more Hb molecules to release their supply of O2.

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I'm fairly certain that you pass out because your body goes into alkalosis. Losing CO2 drives the reaction CO2 + H2O --> H2CO3 backwords. CO2 concentrations do change the hemoglobin binding curve, but not significantly enough to be the cause of any sort of major physiological event.

Medikit said:

I'm fairly certain that you pass out because your body goes into alkylosis, not because you are having trouble delivering Oxygen to tissues. Losing CO2 drives the reaction CO2 + H2O --> H2CO3 backwords. CO2 concentrations do change the hemoglobin binding curve, but not significantly enough to be the cause of any sort of major physiological event.

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Well, the effect that CO2 has on the curve is more of a secondary effect. Elevated levels of CO2 in the plasma will actually drive the CO2 + H20 --> H2CO3 --> HCO3- + H+ reaction forward. So, elevated CO2 levels actually cause you to become acidotic. What you are saying about elevated O2 levels and becoming alkalotic is correct. However, the fact remains that hemoglobin requires a certain level of CO2 in the blood in order to function properly (i.e., release O2 to the tissues). This is actually not a well-known fact, and not likely to pop up on the MCAT. But I guess anything's fair game with the 'CAT!

This is actually something I've never understood. If you dissociate something like H2C03-->HCO3- +H+ you are creating an acid and a base. So by dissociating into these components you are becoming acidic?
I saw this same concept on another question about H+ + 0H - --> H20 + heat
so if you increase heat you drive the reaction to the left producing more H+ and OH- but you end up decreasing the pH overall.

Fusion said:

Well, the effect that CO2 has on the curve is more of a secondary effect. Elevated levels of CO2 in the plasma will actually drive the CO2 + H20 --> H2CO3 --> HCO3- + H+ reaction forward. So, elevated CO2 levels actually cause you to become acidotic. What you are saying about elevated O2 levels and becoming alkalotic is correct. However, the fact remains that hemoglobin requires a certain level of CO2 in the blood in order to function properly (i.e., release O2 to the tissues). This is actually not a well-known fact, and not likely to pop up on the MCAT. But I guess anything's fair game with the 'CAT!

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Hb-O2 affinity is usually only decreased in active tissues so it can release O2 where it is needed. The production of H+ by carbonic anhydrase occurs in the RBCs. Deoxyhemoglobin buffers the H+ that is produced so that the pH of RBCs and venous blood remains within the physiological range.

Fusion said:

Well, the effect that CO2 has on the curve is more of a secondary effect. Elevated levels of CO2 in the plasma will actually drive the CO2 + H20 --> H2CO3 --> HCO3- + H+ reaction forward. So, elevated CO2 levels actually cause you to become acidotic. What you are saying about elevated O2 levels and becoming alkalotic is correct. However, the fact remains that hemoglobin requires a certain level of CO2 in the blood in order to function properly (i.e., release O2 to the tissues). This is actually not a well-known fact, and not likely to pop up on the MCAT. But I guess anything's fair game with the 'CAT!

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You do not pass out because the oxygen can't dissociate properly from hemoglobin, you pass out because alkalosis can cause arteriolar constriction, decreasing the blood supply to the brain.

Medikit said:

You do not pass out because the oxygen can't dissociate properly from hemoglobin, you pass out because alkalosis can cause arteriolar constriction, decreasing the blood supply to the brain.

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actually thats not entirely true.
You pass out because your brain (one of your body's tissues) is not recieving Oxygen. As a built in defense mechanism, you pass out, ie fall down. By falling down, your circulatory system doesnt need to overcome gravity to get blood to your most vital organ. The problem is that you are not lacking Oxygen but rather you have plenty of Oxygen and it just does not dissociate from the hemoglobin. One of the effects of passing out is that if the cause for your HY is psychological, it is over come when you pass out.

You do not have plenty of oxygen in the case of high altitudes (isn't that what we were talking about)? At high altitudes (lower pressure), the partial pressure of O2 in alveolar gas will be reduced. So the diffusion of O2 into capillary blood from the lungs will be reduced, and equilibration with this blood will occur more slowly along the capillary and at a later point along the capillary. You therefore have an arterial PO2 of about 50 mm Hg at high altitudes when you would normally have an arterial PO2 of about 100 mm Hg.

YML said:

The problem is that you are not lacking Oxygen but rather you have plenty of Oxygen and it just does not dissociate from the hemoglobin.

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Before the curve reaches the point at which oxygen cannot dissociate from hemoglobin to maintain a partial pressure of oxygen in the blood that the tissues need the arterial constriction reduces the blood flow to the brain causing a person to blackout.

There can be major physiological effects that occur from a pO2 concentration over 1 atm. When this happens a person passes out because hemoglobin is an important transporter of CO2 and CO2 can only be transported when O2 has become unbound. In this case a person builds up too much CO2 levels in their tissues and the krebs cycle is dragged too a screeching hault due to Le Chatlier's principle.

oh dear.
I'm so misunderstood.
Hey is this my 100th post???

Actually, Medikit, hemoglobin is not an important transporter of CO2. It does transport it to some degree, but on a very minor scale, and it does so through carbamino molecules that are bound to certain parts of the hb molecule. Most of the CO2 is transported through HCO3- . Some CO2 is also transported dissolved in the plasma, but mostly through bicarbonate ion.

As far as alkalosis and contricting arterioles, I have not heard of this, so you could be right about this. But I don't know. Passing out/lying flat certainly allows more blood to reach the brain, and when a person is unconscious, their breathing returns to normal, thus allowing their hb and the rest of their respiratory chemistry to get back to normal.

Also, one more thing that is good to know about hemoglobin is that it has an extremely high affinity for carbon monoxide, which occupies the O2 binding site and essentially behaves as a competitive inhibitor of O2.

kiahs said:

Help please what happens when:

1.hypervent vs. hypovent?

2. When you're at high alt.?

3. Left shift vs. Right Shift hemolobin curve?

Thanks

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Hyperventilation=Respiratory Alkalosis where the PH increases due to CO2 being excreted. Its compensated for by the kidney which tries to excrete bicarbonate to lower the PH
Hypoventilation=Respiratory Acidosis. Decreases PH due to storing CO2. COmpensated by kidney which generates more bicarbonate to Increase the PH
At high altitutes yer ppl have higher hgb levels due to less O2 at those levels.
Left shift hgb curve means is an increase in hgb affinity thus decrease 02 delivery to tissues
Right shift is decrease in hgb affinity and increase in 02 release to tissues.
I got these from my notes...there not exact but good review for me! I have to take my certification exam in a couple weeks! =)

Oh yeah--at high altitudes as one guy mentioned the decrease in 02 stimulates yer BM to produce erythropoietin which generates more RBC's to and thus more hgb to effectively deliver 02 to tissues...might not be exact....hope this helps!

Fusion said:

Actually, Medikit, hemoglobin is not an important transporter of CO2.

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Perhaps not, I was just reading about the toxicity of oxygen at high partial pressures and it appears that it might be from oxidizing free radicals rather than Carbon Dioxide build up.

kiahs said:

But when you are in alkalosis you are too basic this can be caused by hyperventilation and how can it be cured? You have too much base so you want to become acidic how does that happen?
Also I am still having trouble with the brown paper bag, how does that increase CO2 levels?

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Breathing in CO2 will create bicarbonate which will lower your pH. When you breathe into a paper bag you are creating your own atmosphere that is high in carbon dioxide, since you are expelling atmospheric+metabolic CO2 into the bag.

The compensation for respiratory alkalosis consists primarily of renal compensation. The kidneys decrease their excretion of H+ as titratable acid and NH4+ and decreased synthesis and reabsorption of new bicarb (HCO3-). I think this is out of the scope of the MCAT but here it is anyway.

The brown bag works because when you are hyperventilating, you are blowing off too much CO2. The bag traps this exhaled CO2 and you therefore inhale it again on subsequent inspirations.

kiahs said:

Thanks all for your responses. I am still a little shaky on:
I read that when a person is in acidosis they can compensate and return to normal ph by hyperventilation. This is because of the making of HCO3- and expelling CO2 okay see that. But when you are in alkalosis you are too basic this can be caused by hyperventilation and how can it be cured? You have too much base so you want to become acidic how does that happen?
Also I am still having trouble with the brown paper bag, how does that increase CO2 levels?
One more thing, I keep thinking that at high altitudes you don't have enough oxygen, so why would you want to make more RBC if the ones you have aren't getting enough oxygen?

thanks again

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I think when u are in alkalosis the kidney the major producer of bicarbonate tries to rid itself of the bicarbonate which acts to increase the PH by excreting it in urine etc. Its a much longer process taking a couple days instead of respiration/lungs which can compensate faster.
Less 02, stimulates the BM to produce a hormone called erythropoitein which produces more RBC which have hgb incorporated inside it...thus u have increased hgb....I can't remember exactly if u have more 02...ugh have to go to my notes...i'll get back to u on that.....

kiahs said:

Thanks all for your responses. I am still a little shaky on:
I read that when a person is in acidosis they can compensate and return to normal ph by hyperventilation. This is because of the making of HCO3- and expelling CO2 okay see that. But when you are in alkalosis you are too basic this can be caused by hyperventilation and how can it be cured? You have too much base so you want to become acidic how does that happen?
Also I am still having trouble with the brown paper bag, how does that increase CO2 levels?
One more thing, I keep thinking that at high altitudes you don't have enough oxygen, so why would you want to make more RBC if the ones you have aren't getting enough oxygen?

thanks again

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Brown paper bag, white paper bag, plastic bag...doesn't matter. As soon as a hyperventilating person (who is alkalotic due to too much O2) inhales air with a higher concentration of CO2 (which is present in their breath), they respiratory chemicals start to normalize. The air we breath is 79% nitrogen, 20% oxygen, and less than 1% CO2. A person breathing into a paper bag will breathe in air with much more CO2 than is normally present in air. The CO2 that they inhale will help them to become less alkalotic (i.e., more acidic), thus helping them to bring them back to normal.

The key reaction for all of this: CO2 + H20 --> H2CO3 --> H+ + HCO3-. It probably helps to keep the locations of these chemicals in mind (inside the RBC), but you can consult a text for that.

As far as elevated areas, there is plenty of oxygen for people who are accustomed to high areas (they breath normally). But if someone who lives at sea level moves to a the mountains, their breathing rate is going to be higher for a few days/weeks until they become accustomed to the lower oxygen content in the air. The oxygen carrying capacity of a person is totally dependent on their RBCs. A person w/ more RBCs will have better tissue oxygenation (have you guys heard of athletes injecting themselves w/ "epo" to give themselves an edge? Recent cycling scandal...) Therefore, it's not that there isn't enough oxygen at high elevations, it's just that there is LESS oxygen and the body must compensate by providing more RBCs. Any more detail to this would be going to far, but you can PM me if you want a drawn out explanation.

i don't think i ever read quite so much mental masturbation as what i've read in this thread. kudos to all of you with so much facility with your curves and variables, but these are well beyond the scope of the mcat.

Fusion said:

The key reaction for all of this: CO2 + H20 --> H2CO3 --> H+ + HCO3-. It probably helps to keep the locations of these chemicals in mind (inside the RBC), but you can consult a text for that.

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H+ and HCO3- are produced in the RBC but only H+ stays there and is buffered by deoxyHb. The HCO3- is exchanged for Cl- and thus enters the plasma.

short term high altitude adjustment includes an increase in BPG which decreases O2 affinity. I can't remember if that matters on the mcat though.

It's DPG

2,3 BPG = 2,3 DPG both are the same thing (bi- or di-). at high altitudes, the curve shifts to the left and there is less unloading, so that's why the body produces 2,3bpg, to shift it to the right.

here's something good that i found on a site for med students taking the usmle:

"Here's good mnemonic: CADET

C: CO2 (increase)
A: Acidosis (decrease pH)
D: 23 DPG (increase)
E: Exercise (increase)
t: Temp (increase)

remember everything except pH is increased. Shifts curve to right."