Editing Potassium (section)

====Renal filtration, reabsorption, and excretion====
Renal handling of potassium is closely connected to sodium handling. Potassium is the major cation (positive ion) inside animal cells (150{{nbsp}}mmol/L, 4.8{{nbsp}}g/L), while sodium is the major cation of [[extracellular fluid]] (150{{nbsp}}mmol/L, 3.345{{nbsp}}g/L). In the kidneys, about 180{{nbsp}}liters of plasma is filtered through the [[Glomerulus (kidney)|glomeruli]] and into the [[renal tubules]] per day.<ref name="Potts1964">{{cite book |author=Potts, W. T. W. |author2=Parry, G. |date=1964 |title=Osmotic and ionic regulation in animals |publisher=[[Pergamon Press]]}}</ref> This filtering involves about 600{{nbsp}}mg of sodium and 33{{nbsp}}mg of potassium. Since only 1–10{{nbsp}}mg of sodium and 1–4{{nbsp}}mg of potassium are likely to be replaced by diet, renal filtering must efficiently reabsorb the remainder from the plasma.

Sodium is reabsorbed to maintain extracellular volume, osmotic pressure, and serum sodium concentration within narrow limits. Potassium is reabsorbed to maintain serum potassium concentration within narrow limits.<ref>{{cite journal |last1=Lans |first1=H. 's. |last2= Stein |first2=I. F. |last3= Meyer |first3=K. A. |title=The relation of serum potassium to erythrocyte potassium in normal subjects and patients with potassium deficiency |journal=American Journal of the Medical Sciences |volume=223 |issue=1 |pages=65–74 |year=1952| pmid=14902792 |doi=10.1097/00000441-195201000-00011}}</ref> [[Sodium pump]]s in the renal tubules operate to reabsorb sodium. Potassium must be conserved, but because the amount of potassium in the blood plasma is very small and the pool of potassium in the cells is about 30 times as large, the situation is not so critical for potassium. Since potassium is moved passively<ref>{{cite journal |last1=Bennett |first1=C. M. |last2= Brenner |first2=B. M. |last3= Berliner |first3=R. W. |title=Micropuncture study of nephron function in the rhesus monkey |journal=Journal of Clinical Investigation |volume=47 |issue=1 |pages=203–216 |year=1968 |pmid=16695942 |doi=10.1172/JCI105710 |pmc=297160}}</ref><ref>{{cite journal |last1=Solomon |first1=A. K. |title=Pumps in the living cell |journal=Scientific American| volume=207 |pages=100–8 |year=1962 |pmid=13914986 |doi=10.1038/scientificamerican0862-100 |issue=2 |bibcode=1962SciAm.207b.100S}}</ref> in counter flow to sodium in response to an apparent (but not actual) [[Donnan equilibrium]],<ref>{{cite book |last=Kernan |first= Roderick P. |title=Cell potassium (Transport in the life sciences) |publisher=[[John Wiley & Sons|Wiley]] |location=New York |date=1980 |pages=40, 48 |isbn= 978-0-471-04806-0}}</ref> the urine can never sink below the concentration of potassium in serum except sometimes by actively excreting water at the end of the processing. Potassium is excreted twice and reabsorbed three times before the urine reaches the collecting tubules.<ref>{{cite journal |last1=Wright |first1=F. 's. |title=Sites and mechanisms of potassium transport along the renal tubule |journal=Kidney International |volume=11 |issue=6 |pages=415–432 |year=1977 |pmid=875263 |doi=10.1038/ki.1977.60 |doi-access=free}}</ref> At that point, urine usually has about the same potassium concentration as plasma. At the end of the processing, potassium is secreted one more time if the serum levels are too high.{{citation needed|date=August 2017}}

With no potassium intake, it is excreted at about 200{{nbsp}}mg per day until, in about a week, potassium in the serum declines to a mildly deficient level of 3.0–3.5{{nbsp}}mmol/L.<ref>{{cite journal |last1=Squires |first1=R. D. |last2= Huth |first2 = E. J. |title=Experimental potassium depletion in normal human subjects. I. Relation of ionic intakes to the renal conservation of potassium |journal=Journal of Clinical Investigation |volume=38 |issue=7 |pages=1134–48 |year=1959 |pmid=13664789 |doi=10.1172/JCI103890 |pmc=293261}}</ref> If potassium is still withheld, the concentration continues to fall until a severe deficiency causes eventual death.<ref>{{cite book |author=Fiebach, Nicholas H. |author2=Barker, Lee Randolph |author3=Burton, John Russell |author4=Zieve, Philip D.|title=Principles of ambulatory medicine |url=https://books.google.com/books?id=UGVylX6g4i8C&pg=PA748 |date=2007 |publisher=Lippincott Williams & Wilkins |isbn=978-0-7817-6227-4 |pages=748–750}}</ref>

The potassium moves passively through pores in the cell membrane. When ions move through [[ion transporter]]s (pumps) there is a gate in the pumps on both sides of the cell membrane and only one gate can be open at once. As a result, approximately 100 ions are forced through per second. [[Ion channel]]s have only one gate, and there only one kind of ion can stream through, at 10 million to 100 million ions per second.<ref>{{cite journal |last=Gadsby |first=D. C. |title=Ion transport: spot the difference |journal=Nature |volume=427 |issue=6977 |pages=795–7 |year=2004 |pmid=14985745 |doi=10.1038/427795a |bibcode = 2004Natur.427..795G |s2cid=5923529}}; for a diagram of the potassium pores are viewed, see {{cite journal |author=Miller, C |title=See potassium run |journal=Nature |volume=414 |issue=6859 |pages=23–24 |year=2001 |pmid=11689922 |doi=10.1038/35102126 |bibcode = 2001Natur.414...23M |s2cid=4423041 }}</ref> Calcium is required to open the pores,<ref>{{cite journal |last1=Jiang |first1=Y. |last2=Lee |first2=A. |last3=Chen |first3=J. |last4=Cadene |first4=M. |last5=Chait |first5=B. 't. |last6=Mackinnon |first6=R. |url=http://einstein.ciencias.uchile.cl/CursoTroncal2007/Biblio/Jiang__MacKinnonNature417_515_2002.pdf |title=Crystal structure and mechanism of a calcium-gated potassium channel |journal=Nature |volume=417 |issue=6888 |pages=515–22 |year=2002 |pmid=12037559 |doi=10.1038/417515a |bibcode=2002Natur.417..515J |s2cid=205029269 |url-status=dead |archive-url=https://web.archive.org/web/20090424074015/http://einstein.ciencias.uchile.cl/CursoTroncal2007/Biblio/Jiang__MacKinnonNature417_515_2002.pdf |archive-date=2009-04-24 |s2cid-access=free }}</ref> although calcium may work in reverse by blocking at least one of the pores.<ref>{{cite journal |last1=Shi |first1=N. |last2= Ye |first2=S. |last3= Alam |first3=A. |last4= Chen |first4=L. |last5= Jiang |first5=Y. |title=Atomic structure of a Na<sup>+</sup>- and K<sup>+</sup>-conducting channel |journal=Nature |volume=440 |issue=7083 |pages=570–4 |year=2006 |pmid=16467789 |doi=10.1038/nature04508 |bibcode =2006Natur.440..570S |s2cid=4355500 |postscript=;}} includes a detailed picture of atoms in the pump.</ref> Carbonyl groups inside the pore on the amino acids mimic the water hydration that takes place in water solution<ref>{{cite journal |last1=Zhou |first1=Y. |last2= Morais-Cabral |first2=J. H. |last3= Kaufman |first3=A. |last4= MacKinnon |first4=R. |title=Chemistry of ion coordination and hydration revealed by a K<sup>+</sup> channel-Fab complex at 2.0 A resolution |journal=Nature |volume=414 |issue=6859 |pages=43–48 |year=2001 |pmid=11689936 |doi=10.1038/35102009 |bibcode = 2001Natur.414...43Z |s2cid=205022645 |url=http://www.acsu.buffalo.edu/~moralesm/Zhou.pdf |url-status=dead |archive-url=https://web.archive.org/web/20211017203255/http://www.acsu.buffalo.edu/~moralesm/Zhou.pdf |archive-date= Oct 17, 2021 }}</ref> by the nature of the electrostatic charges on four carbonyl groups inside the pore.<ref>{{cite journal|last1=Noskov |first1=S. Y. |last2= Bernèche |first2=S. |last3= Roux |first3=B. |title=Control of ion selectivity in potassium channels by electrostatic and dynamic properties of carbonyl ligands |journal=Nature |volume=431 |issue=7010 |pages=830–4 |year=2004 |pmid=15483608 |doi=10.1038/nature02943 |bibcode =2004Natur.431..830N |s2cid=4414885 |s2cid-access=free |url=https://www.physics.uci.edu/~tritz/BP/ionchannel.pdf |url-status=live |archive-url=https://web.archive.org/web/20230326185426/https://www.physics.uci.edu/~tritz/BP/ionchannel.pdf |archive-date= Mar 26, 2023 }}</ref>