Physiology question, respiratory system

[kov82]

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PO2 concentration of blood to the lungs is 40 and arterial blood leaving lungs is PO2=100,

PCO2 concentration of blood to the lungs is 46 and arterial blood leaving lungs is PCO2=40,

I understand why it drops for CO2, because you breadth it out, but why is there such a small range in change for CO2 but a large one for O2?

 

[docelh]

Short answer: CO2 is more soluble in water than O2.

 

[Vassili Zaitsev]

I'm not sure I quite understand what you're asking, but from my understanding you want to know why oxygen has a larger contribution to the change in partial pressure of blood going in/blood coming out of the lungs. If you look at what air is made up of, you'll see that there's proportionally a lot more O2 than CO2 in the atmosphere. Any time you inhale or exhale, there's a bigger change in your lungs' oxygen concentration than in your carbon dioxide concentration.

Nitrogen N2 78.084%
Oxygen O2 20.947%
Argon Ar 0.934%
Carbon Dioxide CO2 0.033%

I'm not sure I completely understood your question, so sorry if that doesn't make any sense.

 

[ACSheldor]

In the lungs I think the PCO2 is around 40 or something and the PO2 in the lungs is 110 ish. You have a stronger gradient so that's why the change in O2 is greater. So these are the numbers inside the alveoli, not the capillaries. You posted the number for the capillaries.

I think the person above me is saying the same thing.

Their soluibility in blood (as gases) is negligible.

 

[docelh]

Their soluibility is blood (as gases) is negligible.

Do you know what causes partial pressure in fluids? Are you aware that CO2 is 24 times as soluble as O2? Sounds more than negligible to me.

 

[ACSheldor]

True but we're talking about blood and the respiratory system. We don't get most of our oxygen from oxygen dissolved in plasma, we get it from oxygen attaching to hemoglobin by way of a gradient. A small percentage of the oxygen that goes to our tissues is from oxygen dissolved in plasma, most of it is from hemoglobin that picked up oxygen from a concentration gradient between the alveoli and capillaries.

OP, think of it this way: oxygen could be 1000 times more soluble than CO2 but nothing would happen if there wasn't a gradient. So the increased solubility isn't directly responsible for the bigger change in oxygen, it's the bigger gradient that's responsible. Between the lungs and capillaries the gradient for CO2 is 40 in lungs to 46 in blood while for O2 it is 110 in lungs to 40 in blood. The huge gradient allows a significant amount of O2 to be transferred, while the small gradient for CO2 allows for some but not as much.

 

[docelh]

It's just too complicated to explain my answer, so I'm gonna do a half-pass.

The first post strongly suggests that questioner understands the partial pressure differences in various parts of the body, and probably understands the concentration gradient drives the exchange. On my interpretation, the questioner is hinting at why the body tolerates such a large shift in O2 versus CO2; this is the right way to look at it as the body is responsible for the why those differences in pO2 and pCO2 exist at various points.

Consider this. On a glucose diet, the body will breathe out 5 millivolumes percent of CO2 for each 5 millvolumes percent of O2. That is a 1-to-1 ratio. Equal quantities are being inspired and expired, yet the partial pressure difference for each gas is wildly different. What explains this?

It's too complicated to lay it out, so I'm gonna let it go. My last post on this topic. This topic really should be transferred to Q&A so the rest of 'em can get a crack at it.

 

[ACSheldor]

I'm not saying you're wrong, you're probably right but I'm just trying to give a bare bones perspective. I'm not sure you would have to know all that for the MCAT, maybe it would help, I don't know. I'm not sure if the OP does understand the partial pressure differences in the lungs. He stated the in capillary pressure and out capillary pressure for each on the capillary side but not the alveolar side, maybe that's what's confusing him.

Just to make it clear, I'll just give my caveman interpretation of the question and my answer:

PCO2 change from 46 to 40 in capillary. Change little. PO2 change from 40 to 100 in capillary, change big. Why change big for O2 and little for PCO2?

My response is: PCO2 change little because Pressure difference little between capillary and lungs (46 to 40 respectively). PO2 change big because pressure difference big between capillary and lungs (40 to 110 respectively).

Imagine the cavemen from geico having that conversation.

I'm sure you're right though because nothing in the body is caused by just one factor. Every fricking thing is interrelated.

 

[ACSheldor]

Okay, I see what's happening here. You're answering why the body TOLERATES the large change and I'm answering why there is such a large change to begin with. We seem to be answering two different questions. Lets just blame to OP for not being clear and not responding after posting the question.

 

[SN2ed]

Thread moved. These types of questions belong in the Study Q&A forum.

 

[fizzgig]

i think i wanna add to this (by asking stuff, not answering stuff).

i don't quite understand is if normal air has SO little co2 in it, why do we 'keep' the co2 in our alveoli at 5%? we breathe out 5% co2, but we can't get rid of it enough to create a steeper gradient during exchange.

is the gradient from alveoli to outside for co2 (~40mmHg to ~0mmHg) due to the same stuff that causes the 02 gradient from outside to alveoli (~160mmHg to ~100mmHg)?? namely that not all the air in the lungs is going in and out with each breath by a long shot, so a gradient develops from alveoli to outside based on how much air you're moving and how much exchange is going on?

hope that makes sense...

 

[UCB05]

i think i wanna add to this (by asking stuff, not answering stuff).

i don't quite understand is if normal air has SO little co2 in it, why do we 'keep' the co2 in our alveoli at 5%? we breathe out 5% co2, but we can't get rid of it enough to create a steeper gradient during exchange.

is the gradient from alveoli to outside for co2 (~40mmHg to ~0mmHg) due to the same stuff that causes the 02 gradient from outside to alveoli (~160mmHg to ~100mmHg)?? namely that not all the air in the lungs is going in and out with each breath by a long shot, so a gradient develops from alveoli to outside based on how much air you're moving and how much exchange is going on?

hope that makes sense...

You have the right idea and it might help to know a little lung physiology to answer that question. One pertinent part to this question is that even if you breathe out as much air as you can, there is still a bit of air in the lungs, called the residual volume, that you can not exhale. This residual volume is CO2 rich and O2 poor compared to the air, and in your next breath, that inspired air mixes with that left-over volume so that the air that reaches your alveoli has more CO2 and less O2 than what was inspired.

Also keep in mind residual volume (about 1/5 of total lung capacity) is what's left after maximum exhalation, so during normal breathing, that volume of uncirculated air (called functional residual capacity) that gets mixed with inhaled air is much higher. The partial pressures of gases at the alveoli reflect that mixing (160 vs 140, 40 vs negligible).

 

[RogueUnicorn]

not an MCAT question in the least

 

[fizzgig]

no it's not a question on the mcat, but i assumed since physiology is on there the lungs might be included, and once a 'hmmm i don't quite understand that' thing pops up for me, it can grow until it screws me up royally.