One bulk element, three places of residence and a single sentence the European Union has authorised for it. Chloritschein keeps the two apart: descriptive physiology on one side, the quoted regulatory text on the other.
On to the orientationThe four sections that follow build on one another. Anyone starting at point three does not have the orders of magnitude from point one to hand — and without them the rest reads like a string of technical terms.
In an adult body by far the greatest share of this element lies in bone tissue and in the musculature. What is travelling in blood plasma at the same time makes up only a very small fraction of that — and it is precisely that fraction which a laboratory captures in an ordinary blood draw.
Chemically it is a doubly positively charged particle that surrounds itself with a strikingly large shell of water molecules. This shell helps determine which passages in the body are open to it and at which points it needs a route of its own.
Under Regulation (EU) No 432/2012
What the filter in the renal corpuscle lets through is by no means lost. Along the stretch through the tubules that follow, by far the greatest part is taken back into the blood; only a narrow remainder reaches the bladder. The amount that finally appears in the urine is therefore not a picture of intake but the result of a calculation from two very large figures and one small one.
The fine control sits at precisely this point: it is not absorption in the gut but recovery in the tubule that determines from day to day how much stays in the body.
Between two neighbouring tubule cells there is no open gap; there lies a strip of interlocking proteins. Two members of this family, listed as claudin-16 and claudin-19, form a pore in the strip that lets charged particles through.
If either of the two proteins is missing, the return route collapses at that point. The specialist literature carries findings from human genetics on this; on this page they remain descriptive text and are not presented as an authorised claim.
In plasma the element is present in three forms side by side. About three fifths move freely, just under a third hangs on proteins, and the small remainder has combined with acid residues into uncharged compounds.
The protein-bound fraction stays behind in the blood because it does not pass the filter. The other two appear in the filtrate — and with that begins the stretch these pages are about.
Almost every depiction of a transport process in the body shows a cell, two membranes and a channel that moves something through. For this element that picture does not hold on the most important section. The greatest share of recovery runs not through the cells but past them — through the narrow gap two neighbouring cells leave between them.
For a particle to cross such a gap in a particular direction at all requires two things: a pore that lets it pass, and a driving force that gives it direction. Both are present at this point of the tubule, and both can be described separately.
In the ascending part of the loop of Henle the cell wall takes sodium, potassium and chloride together out of the urine flowing past. Part of the potassium taken up returns immediately to the lumen. Because more positive than negative charge thereby reaches the outside, the lumen remains slightly positively charged relative to the surrounding tissue — on the order of a few millivolts.
This voltage is the driving force. It pushes doubly positively charged particles out of the lumen towards the blood without any pump of its own being needed. At this point the process costs the cell no additional energy; that was already paid when the voltage was built up.
The pore in the sealing strip only arises when two different proteins come together. Claudin-16 provides the passage point, claudin-19 ensures that it is installed at the right place in the strip and stays there. If the partner is missing, the site remains unoccupied and the voltage in the lumen runs to nothing.
From this construction follows a peculiarity described in the specialist literature since the 2000s: the permeability of the strip is not a material property of the membrane but the result of an assembly that can be shifted.
After the loop comes a short, convoluted section in which transport again runs through the cells themselves. Measured against the total amount, this section moves little; measured against the amount that would be lost without it, it moves a great deal. It is the last place at which anything can be corrected, for in the collecting duct nothing is retrieved any more.
The annex to the regulation holds a single sentence for this element and this keyword. It is nowhere reworded on these pages and always appears in the same version:
Everything else on this page is editorial factual text on physiology. It is deliberately not marked as an authorised claim, and it makes no statement about diseases, their prevention or their course.
This page traces the route in outline. The file goes through it section by section: a table on the shares per tubule section, a second on the contents in commercially available goods, the source for every figure, and a questions section at the end.
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