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From Pre examination procedures in laboratory diagnosis by Walter Guder :

...It consists of three parts by weight of ammonium oxalate which causes swelling of erythrocytes balanced by two parts of potassium oxalate which causes shrinkage...

This is regarding use of oxalates as anticoagulants. To me, it makes sense that potassium oxalate causes shrinkage of erythrocytes but i don't understand why Ammonium oxalate causes swelling. Is it because it converts to Ammonia in alkaline pH of blood, diffuses into the RBC and converts back to ammonium.

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After a lot of search I think the following seems a satisfactory answer.

You are correct in thinking that outside the RBC, $\ce{NH_4^+}$ converts to $\ce{NH_3}$ and once inside the RBC, it again converts to $\ce{NH_4^+}$. But this does not explain how $\ce{NH_4^+}$ swells up the RBCs. For that, let us look at the process from the beginning.

As described by Hemker et al, 2003, an important part of this process is the enzyme carbonic anhydrase (an absolutely important constituent of RBCs). Hemker et al, while studying how the Rh complex in RBCs exports $\ce{NH_4^+}$ ions out of the cell (although they used $\ce{NH_4Cl}$ in their study, it doesn't matter since blood already contains enough $\ce{Cl^-}$) they came up with the following scenario:

  • $\ce{NH_4^+}$ breaks into $\ce{NH_3}$ and $\ce{H^+}$ and enters inside the RBC. $\ce{H^+}$ combines with $\ce{OH^-}$ (coming from $\ce{HCO_3^}-$, we'll talk about this later) to give $\ce{H_2O}$.

  • Due to increased osmotic pressure (by $\ce{NH_3}$), $\ce{H_2O}$ enters inside the RBC where it is again broken into $\ce{H^+}$ and $\ce{OH^-}$. $\ce{NH_3}$ combines with $\ce{H^+}$ to give $\ce{NH_4^+}$ again (and we're done with $\ce{NH_4^+}$ here, though Hemker et al took this further).

  • The $\ce{OH^-}$ (from water) combines with $\ce{CO_2}$ (via carbonic anhydrase) to give $\ce{HCO_3^-}$. This $\ce{HCO_3^-}$, via Hamburger shift, is exchanged with a $\ce{Cl^-}$ while the $\ce{HCO_3^-}$ outside converts into $\ce{H^+}$, $\ce{OH^-}$ and $\ce{CO_2}$ (providing $\ce{OH^-}$ to $\ce{NH_4^+}$). See the diagram below for visual representation:

diagram

So, where does the swelling effect come in? It turns out, it is due to the increasing concentration of $\ce{NH_4^+}$ and $\ce{Cl^}-$ inside the RBC which keeps on generating higher and higher osmotic pressure. Hemker et al also found that Rhnull RBCs (lacking Rh complex) became victims to osmolysis much faster than the normal RBCs, suggesting that increased concentration of intracellular $\ce{NH_4^+}$ is the main cause of swelling and osmolysis of RBCs (in their research, they concluded that Rh factor indeed exports $\ce{NH_4^+}$ outside the RBCs).

Reference: Hemker, M. B., Cheroutre, G., Van Zwieten, R., Maaskant-van Wijk, P. A., Roos, D., Loos, J. A., Van Der Schoot, C. E. and Von Dem Borne, A. E. G. Kr. (2003), The Rh complex exports ammonium from human red blood cells. British Journal of Haematology, 122: 333–340. doi:10.1046/j.1365-2141.2003.04425.x

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    $\begingroup$ What is an answering rampage anyway? :-) $\endgroup$
    – user24284
    May 16 '17 at 4:54
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    $\begingroup$ @gerardofurtado WYSIWYG et al coined this term (see The Biosphere chat room) for the frequency at which AliceD et al was answering questions (most probably about 10-12 answers per day, they seem to have slowed down now ;). But since my monthly/quarterly repo gain always seemed to be higher than AliceD's, I considered myself a part of this competition. Now you seem to have taken over me :P $\endgroup$
    – another 'Homo sapien'
    May 16 '17 at 4:58
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    $\begingroup$ I imagined it'd be something like this! Good luck! $\endgroup$
    – user24284
    May 16 '17 at 5:02
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    $\begingroup$ @another'Homosapien' Since the OP is inactive, can I ask you why potassium oxalate causes shrinkage? Doesn't the addition of potassium oxalate, increase the intracellular K+ levels? $\endgroup$
    – JM97
    May 18 '17 at 15:31
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    $\begingroup$ @jm97 the point is, potassium is outside the RBC and the effect is thus reverse of what you think. You might be interested in this. Why hesitate in asking a question, its the most beautiful thing one can do...;) $\endgroup$
    – another 'Homo sapien'
    May 18 '17 at 16:16

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