Physiology: Chloride Shift

[Typo]

Another question on what looks like a key error: on the 1979 Apr/Dec test, Biochem/Physio section, #89:

89. Which of the situations below characterizes the exchange of chloride and bicarbonate ion between the red cell and plasma in the lungs?

1. The passage of chloride ion into the cell from the plasma and the passage of bicarbonate ion from the cell into the plasma.
2. The passage of chloride ion into the plasma from the cell and the passage of bicarbonate ion from the plasma into the cell.

The test key says that 1 is the answer, which correctly describes the "chloride shift" (the exchange of chloride for bicarbonate in the RBC). From what I understand, though, #2 would occur because in the lungs, bicarbonate ion needs to be converted to CO2 (and water) for exhalation, and this happens in the RBC. Am I understanding it wrong, or is the question keyed wrong?

---

Members don't see this ad.

 

[wise146]

Typo I think that the key is correct for that question. Bicarbonate leaves the RBC in exchange for Cl- as part of the cholride shift. Carbonic anhydrase is only present in the RBC and not in the plasma, however the rxn. occurs in the plasma at a slower rate. Let me know if this makes sense or if I am wrong.

 

[ToothMonkey]

Typo, I believe you are right. The answer is #2. CO2 is primarily transported in the blood as bicarbonate, which re-enters the RBC in pulmonary capillaries, where it recombines with hydrogen to reform H2O & CO2, which then diffuse out of the RBC to be exhaled. That would mean Cl- moves out of the RBC in the lungs in order to balance the net influx of negatively-charged bicarbonate. Someone can correct me if I'm wrong but I'm fairly sure that's how it works.

Actually answer #1 *does* correctly describe the chloride shift, but the chloride shift itself takes place in tissue capillaries. Pulmonary capillaries are the site of the reverse chloride shift, which is described by answer #2.

 

[Typo]

Typo, I believe you are right. The answer is #2. CO2 is primarily transported in the blood as bicarbonate, which re-enters the RBC in pulmonary capillaries, where it recombines with hydrogen to reform H2O & CO2, which then diffuse out of the RBC to be exhaled. That would mean Cl- moves out of the RBC in the lungs in order to balance the net influx of negatively-charged bicarbonate. Someone can correct me if I'm wrong but I'm fairly sure that's how it works.

Actually answer #1 *does* correctly describe the chloride shift, but the chloride shift itself takes place in tissue capillaries. Pulmonary capillaries are the site of the reverse chloride shift, which is described by answer #2.

I actually just read it in BRS Physio - you describe it exactly right. After describing the chloride shift (#1), the book says this:

"In the lungs, all of the above reactions occur in reverse. HCO3- enters the RBCs in exchange for Cl-. HCO3- recombines with H+ to form H2CO3, which decomposes into CO2 and H2O. Thus, CO2, originally generated in the tissues, is expired."

So, either the key was wrong, or our understanding respiratory physiology has changed in the past 30 years. Thanks for the help.

 

[mam6701e]

Agree w you toothmonkey.
The key words are
Tissue capilares (system)..........................HCO3 get out cell Cl get in
Pulmonary capilaries (lungs).......................HCO3 get in cell Cl get out

- CO2 has to go in a trip to be eliminated from tissues to enviroment through lungs.
- Plasma cannot transport CO2 as CO2, it has to be transform in HCO3 in order to travel in plasma,
- but CO2 cannot be transform into HCO3 in plasma cos plasma doesnt have the enz carbonic anhydrase required to convert CO2 into HCO3,
- Therefore CO2 get into the cell (which have enz CA) and it is converted into HCO3, now, it is ready to travel trough the plasma to lungs as a detination.
- CO2 has to leave the cell and get into plasma to travel to lungs, and as a compensation Cl get in cell from plasma.
- Once it get to lungs, HCO3 has to be reconverte into CO2 in order to get the alveoli to be eliminated to the exterir
- Plasma cannt conver HCO3 into CO2 cos doest have CA enz,
- So, HCO3 has to get in the cell again, as compensation Cl get ot cell to plasma/
- In the cell, HCO3 becomes CO2 and is sent to alveoli of lungs to be eliminated