chloride shift phenomenon

[SamarEsawy]

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surfing google books I found this

I marked my questions with red
1-first what makes the RBC membrane relatively impermeable to cations?
2-what makes the total number of ions inside the RBCs increase despite the fact that we exchange HCO3 for Cl ion

Thanks in advance and sorry for posting this thread before members told me that this is the right place

 

[coolforschool]

surfing google books I found this

I marked my questions with red
1-first what makes the RBC membrane relatively impermeable to cations?
2-what makes the total number of ions inside the RBCs increase despite the fact that we exchange HCO3 for Cl ion

Thanks in advance and sorry for posting this thread before members told me that this is the right place

1. It might happen along the lines of carbonic anhydrase converting CO2 --> H2CO3 --> H+ and HCO3-. The increased H+ concentration might create a membrane potential difference in such a way as to block cation influx and facilitate anion influx, hence the "relative" impermeability. It might also depend on the other ion concentrations within the RBC or maybe it is because of the lack of specific channels for cations to go through (as opposed to the Cl-/HCO3- exchanger "band 3"). This paper dives into the whole thing: http://jgp.rupress.org/content/50/1/171.full.pdf

2. I think you're forgetting about CA converting intracellular CO2 --> H2CO3 --> H+ and HCO3-. If you exchange bicarb for Cl-, you still have H+ and Cl- inside the cell while HCO3- goes out --> net gain (RBC membrane is impermeable to H+).

That's my take on it. We have a thread called "Step 1 Complicated Concepts" where we post things like this if you're interested.

 

[SamarEsawy]

2. I think you're forgetting about CA converting intracellular CO2 --> H2CO3 --> H+ and HCO3-. If you exchange bicarb for Cl-, you still have H+ and Cl- inside the cell while HCO3- goes out --> net gain (RBC membrane is impermeable to H+).

but this means that the osmotic pressure of RBCs is already higher than that of plasma even before chloride shift right?
it's written in my textbook that the osmotic pressure of RBCs increases due to increase of both HCo3 and Cl inside them
Can you explain this for me?

 

[coolforschool]

but this means that the osmotic pressure of RBCs is already higher than that of plasma even before chloride shift right?
it's written in my textbook that the osmotic pressure of RBCs increases due to increase of both HCo3 and Cl inside them
Can you explain this for me?

No, the RBC has a net gain of one ion (the H+) at the end of all of this. Remember that CO2 diffuses into the RBC from the plasma; it is a gas, but carbonic anhydrase combines CO2 and H2O to eventually form H+ and HCO3-, which are two ions. The HCO3- inside the RBC is exchanged for a Cl- from the plasma, but the H+ stays inside the RBC membrane because it is impermeable, so you get a net gain of one ion, which increases osmotic pressure inside the RBC.

Basically, imagine the RBC having a baseline osmotic pressure. Now, CO2 from the plasma comes into it, gets combined with water to form H+ and HCO3-. HCO3- can get out of the RBC, but H+ has to stay in there, so the osmotic pressure has to increase.

 

[Anastomoses]

No, the RBC has a net gain of one ion (the H+) at the end of all of this. Remember that CO2 diffuses into the RBC from the plasma; it is a gas, but carbonic anhydrase combines CO2 and H2O to eventually form H+ and HCO3-, which are two ions. The HCO3- inside the RBC is exchanged for a Cl- from the plasma, but the H+ stays inside the RBC membrane because it is impermeable, so you get a net gain of one ion, which increases osmotic pressure inside the RBC.

Basically, imagine the RBC having a baseline osmotic pressure. Now, CO2 from the plasma comes into it, gets combined with water to form H+ and HCO3-. HCO3- can get out of the RBC, but H+ has to stay in there, so the osmotic pressure has to increase.

I gave her this explanation but you've made it even more clear. Nice.

 

[SamarEsawy]

great explanation thank you guys all thank you 🙂

 

[palbar1]

No, the RBC has a net gain of one ion (the H+) at the end of all of this. Remember that CO2 diffuses into the RBC from the plasma; it is a gas, but carbonic anhydrase combines CO2 and H2O to eventually form H+ and HCO3-, which are two ions. The HCO3- inside the RBC is exchanged for a Cl- from the plasma, but the H+ stays inside the RBC membrane because it is impermeable, so you get a net gain of one ion, which increases osmotic pressure inside the RBC.

Basically, imagine the RBC having a baseline osmotic pressure. Now, CO2 from the plasma comes into it, gets combined with water to form H+ and HCO3-. HCO3- can get out of the RBC, but H+ has to stay in there, so the osmotic pressure has to increase.

But at the end of the day, you still have a two ion gain, no? CA-->H(+) + HCO3(-) that gets us to a gain of two ions in the beginning, then the exchange of Cl- for HCO3- is no change in the net ion count because you are exchanging one anion for another. Did I go wrong somewhere?

 

[coolforschool]

But at the end of the day, you still have a two ion gain, no? CA-->H(+) + HCO3(-) that gets us to a gain of two ions in the beginning, then the exchange of Cl- for HCO3- is no change in the net ion count because you are exchanging one anion for another. Did I go wrong somewhere?

Haha you're right. Completely missed that. Thanks!

 

[ketosisevangelist]

all CHLORIDE SHIFTS are electrical gradient only!!!!

only electrical gradient!

 

[Anastomoses]

But at the end of the day, you still have a two ion gain, no? CA-->H(+) + HCO3(-) that gets us to a gain of two ions in the beginning, then the exchange of Cl- for HCO3- is no change in the net ion count because you are exchanging one anion for another. Did I go wrong somewhere?

yes, causing osmotic increase. I think the RBCs have 3% more liquid in them in the venous circulation as well.

 

[SamarEsawy]

But at the end of the day, you still have a two ion gain, no? CA-->H(+) + HCO3(-) that gets us to a gain of two ions in the beginning, then the exchange of Cl- for HCO3- is no change in the net ion count because you are exchanging one anion for another. Did I go wrong somewhere?

this made it much more clear thanks