Effects of Altitude Question (Biology)

[fastnfurious]

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Here's a question outta my workbook that I need help with...



Most of the CO2 in the blood travels as HCO3- and H+ ions in the plasma. Which of the following would a physician find upon examining a person with Acute Mountain Sickness (Altitude Sickness)?

A Increased levels of O2 in the plasma
B Increased levels of CO2 in the plasma ----> my answer
C An increased red blood cell count -----> right answer
D An increased white blood cell count


The work book said B was Incorrect because the levels of CO2 in the plasma would be lower since the pressure differential in CO2 is greater.



What does that mean? 😕 I thought there would be increased levels of CO2 because PCO2 increases with altitude

 

[pitt1166]

They might be trying to trip you up on the fact that there won't actually be more CO2 in the blood plasma, but rather, more HCO3-.

That's how I'd think about it, anyway. Their reasoning wouldn't seem very intuitive to me.

Plus, you've got to figure that RBC counts would increase at higher elevations. I think that's kind of an understood fact.

 

[FutureInternist]

As far as I remember pCO2 does not increase with altitude. The air being "thinner" at higher altitude means that the total partial pressure of gas decreases BUT the relative percentage of each gas remains the same. So at sea level & Mt. Everest, O2 is 21% but ofcourse 21% of a smaller number (at high altitude) is a smaller number.

I would pick B as well but only because I do not agree with their answer since increased RBC count (secondary to EPO) would take weeks (and hence NOT be present in an acute situation)

Most likely the correct answer is "E - None of the above" 🙂

 

[TieuBachHo]

I am just taking a shot here, so feel free to add or correct me if I am wrong. The explanation might be off LOL but the best answer out of the 4 is C (Not that it is a right answer)... Since the partial pressure of CO2 drops at high altitude (not the percentage), it creates a larger gap between blood CO2 pressure and the atmospheric CO2 pressure called the pressure differential of CO2 😕... Therefore, the plasma CO2 level must be lowered to maintain a homeostasis (guessing). I think that what they meant🙄

EDIT:
Thanks guys,
I think it's the hyperventilation in response to the environmental changes (lack of O2) to cause a drop in plasma CO2 level (not the homeostasis / body equilibrium).

 

[161927]

Here's a question outta my workbook that I need help with...



Most of the CO2 in the blood travels as HCO3- and H+ ions in the plasma. Which of the following would a physician find upon examining a person with Acute Mountain Sickness (Altitude Sickness)?

A Increased levels of O2 in the plasma
B Increased levels of CO2 in the plasma ----> my answer
C An increased red blood cell count -----> right answer
D An increased white blood cell count


The work book said B was Incorrect because the levels of CO2 in the plasma would be lower since the pressure differential in CO2 is greater.



What does that mean? 😕 I thought there would be increased levels of CO2 because PCO2 increases with altitude

At high altitude your body needs more oxygen. It responds by emphasizing hemoglobin release of oxygen. Hemoglobin is directly linked to red blood cell activity. As you decrease hemoglobin's binding potential for oxygen, you increase the concentration of free hemoglobin, which signals the body to produce more red blood cells.

 

[FutureInternist]

I am just taking a shot here, so feel free to add or correct me if I am wrong. The explanation might be off LOL but the best answer out of the 4 is C (Not that it is a right answer)... Since the partial pressure of CO2 drops at high altitude (not the percentage), it creates a larger gap between blood CO2 pressure and the atmospheric CO2 pressure called the pressure differential of CO2 😕... Therefore, the plasma CO2 level must be lowered to maintain a homeostasis (guessing). I think that what they meant🙄

Actually with higher altitudes the pCO2 of atmosphere approaches that of the body --> a decreased drive for CO2 to be blown off which would --> an increased plasma CO2 level

 

[Vihsadas]

I think you guys might be confusing cause and effect here.

At higher altitudes, barometric pressure decreases, so alveolar PO2 decreases = less oxygen transported to the blood => eventual compensation by activation of erythropoiesis, and an increase in red blood cell count and blood hematocrit.

For PCO2 however, this is not the case.
The decrease in arterial PO2 prompts a compensatory response in respiration by promoting hyperventilation. Hyperventilation always causes a decrease in arterial PCO2 by releasing excess CO2 from the lungs, but it increases PO2 (cause the body needs more oxygen at high altitudes). Counterintuitive, yes?

Actually with higher altitudes the pCO2 of atmosphere approaches that of the body --> a decreased drive for CO2 to be blown off which would --> an increased plasma CO2 level

In reality, the PCO2 of the atmosphere is about 0.30mmHg, while the PCO2 of the blood is around 40mmHg. What happened was that you mixed up cause and effect here. It's not the approach to CO2 equilibrium that is driving an increased plasma CO2, rather it's the hyperventilation which reduces arterial PCO2 that is causing the drive to equilibrium.

Hope this makes sense.

 

[FutureInternist]

I see what you're saying but I still think you would NOT have an increase in RBC production in an acute situation.
The acclimatization that happens also tends to take a couple of days, but for me raised pCO2 is the better of two answers.

 

[Vihsadas]

I see what you're saying but I still think you would NOT have an increase in RBC production in an acute situation.
The acclimatization that happens also tends to take a couple of days, but for me raised pCO2 is the better of two answers.

Well B is clearly wrong though. PCO2 just simply doesn't go up. It goes down due to compensatory hyperventilation at rest which is aimed at increasing PO2. When we see a similar question like this, we should remember that "raised arterial/plasma PCO2" should be the first thing we cross off. A is also wrong, as is D. The only remaining feasible answer choice is C. Is it a great answer choice? No, I don't think it is, and I can see why people have trouble with this question. It's simply not a very good question.

That being said, can we rationalize why it would be C?
Well, the problem comes from the discrepancy between the meanings of "Acute" and "Chronic". The Chronic form of mountain sickness is a condition only really seen in people who have been living at high altitude for many years or cultures that have always lived at high altitudes. They definitely have polycythemia, as well as some other physiological effects.
But at what length of time does something switch from being Acute to chronic? 1 year? 6 months? 2 weeks?
In my mind, I just thought of how climbers acclimatize themselves to the higher altitudes. They ascend (over a period of a few days or weeks) in order to let their bodies increase their RBC count and improve their blood's O2 carrying capacity. Because its an effect that occurred only over a few days, and is reversed when the climber comes down, I'd say that could be considered "acute".
Think of other acute pathophysiologies that last a few days. What about the flu? That's not a chronic illness and it can last a few weeks. It's an acute illness.

To me, C is clearly the best answer. If for no other reason than I'm reasonably sure that the other answers are wrong.

 

[TieuBachHo]

Actually with higher altitudes the pCO2 of atmosphere approaches that of the body --> a decreased drive for CO2 to be blown off which would --> an increased plasma CO2 level

Hi!
The pressure of CO2 in the plasma always greater than the pressure of CO2 in the air making diffusion possible. So when the pressure of CO2 in the air drops, it creates a larger gap in the pressure differential of CO2, right? Due to diffusion taking place in the lung, more CO2 diffuse out causing the lower level of CO2.

Now I have a second thought about whether hyperventilation decreases the level of CO2 in the plasma but it's actually the pressure differential in CO2 because it's a diffusion process. 😕

 

[Vihsadas]

Now I have a second thought about whether hyperventilation decreases the level of CO2 in the plasma but it's actually the pressure differential in CO2 because it's a diffusion process. 😕

You've got it right. It's actually due to both hyperventilation and the pressure differential that you are talking about. Both concepts work together to reduce plasma PCO2. Hyperventilation, meaning larger tidal volume and respiration rate than at rest (read faster and deeper breaths) actually clears out more of the latent (reserve) air in the lungs which has a higher pressure of CO2 than atmospheric air. So when you hyperventilate you are moving more new air into the lungs which allows the pressure differential between the plasma and the environment to more efficiently move CO2 from the lungs to the environment. So Plasma CO2 decreases.