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Why doesn't pneumonia (or other issues with gas diffusion) lead to increased pCO₂?
If one mechanism of hypoxemia in pneumonia is decreased diffusion of O₂ through fluid-filled alveoli, shouldn't CO₂ be similarly affected, leading to hypercapnia
Let's examine why not!
Why doesn't pneumonia (or other issues with gas diffusion) lead to increased pCO₂?
If one mechanism of hypoxemia in pneumonia is decreased diffusion of O₂ through fluid-filled alveoli, shouldn't CO₂ be similarly affected, leading to hypercapnia
Let's examine why not!
2/
What explains this difference (i.e., why don't pneumonia or pulmonary edema lead to an increase in pCO₂ if they DO lead to a decrease in pO₂)
[Hg = hemoglobin]
What explains this difference (i.e., why don't pneumonia or pulmonary edema lead to an increase in pCO₂ if they DO lead to a decrease in pO₂)
[Hg = hemoglobin]
3/
To understand the answer (CO₂ is more soluble than O₂), we must first revisit Henry’s Law (see picture and video).
The key point: the higher the solubility coefficient (aka Henry's Law constant), the more soluble the gas.
khanacademy.org/science/health…
To understand the answer (CO₂ is more soluble than O₂), we must first revisit Henry’s Law (see picture and video).
The key point: the higher the solubility coefficient (aka Henry's Law constant), the more soluble the gas.
khanacademy.org/science/health…
4/
And here are the solubility coefficients for CO₂ and O₂:
CO₂ = 0.57
O₂ = 0.024
As you can see, CO₂ is 24 times more soluble, in water, than O₂.
elsevier.com/books/guyton-a…
And here are the solubility coefficients for CO₂ and O₂:
CO₂ = 0.57
O₂ = 0.024
As you can see, CO₂ is 24 times more soluble, in water, than O₂.
elsevier.com/books/guyton-a…
5/
The rate of DIFFUSION of a gas is dependent on both solubility and molecular weight. CO₂ is heavier than O₂, narrowing the difference a bit.
With O₂ used as reference, the diffusion coefficient of CO₂ is 20.3.
➜ the rate of diffusion of CO₂ in H₂O is 20x that of O₂.
The rate of DIFFUSION of a gas is dependent on both solubility and molecular weight. CO₂ is heavier than O₂, narrowing the difference a bit.
With O₂ used as reference, the diffusion coefficient of CO₂ is 20.3.
➜ the rate of diffusion of CO₂ in H₂O is 20x that of O₂.
6/
What this means is that even when O₂ diffusion is severely limited (as with pneumonia, pulmonary edema, or interstitial lung disease), CO₂ diffusion continues unabated.
The lungs continue to expire CO₂ leaving pCO₂ unchanged.
What this means is that even when O₂ diffusion is severely limited (as with pneumonia, pulmonary edema, or interstitial lung disease), CO₂ diffusion continues unabated.
The lungs continue to expire CO₂ leaving pCO₂ unchanged.
7/
But, there is a problem.
Although CO₂ is more soluble than O₂, it’s not THAT soluble.
In fact, only 7% of CO₂ is transported in solution. This means that the CO₂ being produced by cellular respiration is largely carried in other forms.
Particularly as HCO₃⁻.
But, there is a problem.
Although CO₂ is more soluble than O₂, it’s not THAT soluble.
In fact, only 7% of CO₂ is transported in solution. This means that the CO₂ being produced by cellular respiration is largely carried in other forms.
Particularly as HCO₃⁻.
8/
Once HCO₃⁻ is returned to the lungs, it must be converted back to CO₂
Here's the thing: the reaction of CO₂ with H₂O requires minutes for equilibrium. If gas exchange had to wait for this reaction, it would be too slow and CO₂ could not easily be eliminated.
Once HCO₃⁻ is returned to the lungs, it must be converted back to CO₂
Here's the thing: the reaction of CO₂ with H₂O requires minutes for equilibrium. If gas exchange had to wait for this reaction, it would be too slow and CO₂ could not easily be eliminated.
9/
How does the body manage the fact that the reaction that converts HCO₃⁻ back to CO₂ is very slow?
How does the body manage the fact that the reaction that converts HCO₃⁻ back to CO₂ is very slow?
10/
Fortunately, we have carbonic anhydrase (CA), one of the fastest enzymes.
➜without CA, the reaction creating CO₂ occurs once every 5 seconds.
➜with CA, the reaction occurs 1,000,000 times each second!
Result: RBCs readily replenish CO₂ for diffusion across alveoli.
Fortunately, we have carbonic anhydrase (CA), one of the fastest enzymes.
➜without CA, the reaction creating CO₂ occurs once every 5 seconds.
➜with CA, the reaction occurs 1,000,000 times each second!
Result: RBCs readily replenish CO₂ for diffusion across alveoli.
11/
In fact, if enough acetazolamide is administration, the pCO₂ may rise.
This was seen in a study published in 1961 when acetazolamide was administered to anesthetized dogs.
ncbi.nlm.nih.gov/pubmed/13630836
In fact, if enough acetazolamide is administration, the pCO₂ may rise.
This was seen in a study published in 1961 when acetazolamide was administered to anesthetized dogs.
ncbi.nlm.nih.gov/pubmed/13630836
12/
Another mechanism that may contribute to the rarity of diffusion-mediated hypercapnia: hypoxemia-mediated stimulation of ventilation, leading to CO₂ washout.
Another mechanism that may contribute to the rarity of diffusion-mediated hypercapnia: hypoxemia-mediated stimulation of ventilation, leading to CO₂ washout.
13/
Before summarizing, let's look again at a version of the original question.
What explains why pneumonia and pulmonary edema don't lead to an increase in pCO₂ if they DO lead to a decrease in pO₂)
[Hg = hemoglobin]
Before summarizing, let's look again at a version of the original question.
What explains why pneumonia and pulmonary edema don't lead to an increase in pCO₂ if they DO lead to a decrease in pO₂)
[Hg = hemoglobin]
14/14 - SUMMARY
⭐️Why doesn't pneumonia (or other issues with gas diffusion) lead to increased pCO₂?⭐️
➜ CO₂ is 24x more soluble than O₂ and has 20x greater diffusion capacity
➜ this means that even when fluid affects O₂ diffusion, CO₂ remains unaffected
⭐️Why doesn't pneumonia (or other issues with gas diffusion) lead to increased pCO₂?⭐️
➜ CO₂ is 24x more soluble than O₂ and has 20x greater diffusion capacity
➜ this means that even when fluid affects O₂ diffusion, CO₂ remains unaffected
