A factual text about buffers, kidney tubules and a single authorised sentence
Every metabolic day leaves protons behind. The greater part of them leaves the body as a gas via the lungs; a smaller part remains as fixed acid and is excreted. Beryllschicht describes this process, names the orders of magnitude and keeps separate from it what the European Union has authorised for zinc.
Go to the guideWhat is meant is the sum of the reactions in which protons arise, are bound, shifted and released again in the organism. The term therefore denotes an ongoing turnover and not a state that could be ascribed to a person. The blood plasma of a healthy adult moves within a very narrow band between pH 7.35 and 7.45 — a span of one tenth of a pH unit, while the stomach contents lie two storeys lower and pancreatic juice above it.
Two sources feed this turnover. Carbon dioxide arises from the burning of carbohydrates and fats; reckoned in millimoles that is by far the larger amount, but it leaves the body again with every breath. Alongside it comes a markedly smaller load that cannot be exhaled: sulphuric acid from the breakdown of sulphur-containing amino acids, phosphoric acid from phospholipids and nucleic acids, plus organic acids from incompletely burned intermediates.
This second load is why a text about acid and base ends at the kidney tubule and not in the lung. On a mixed diet it is of the order of about one millimole per kilogram of body weight per day. How this figure comes about, which items it is made up of and by what methods it is captured is set out in the guide.
Physiology textbooks order the participants by the speed at which they intervene. The three levels work alongside one another, not in sequence, and they achieve different things.
A buffer catches a free proton by binding it to an anion. In blood plasma bicarbonate takes on the greatest share of this task, inside the cell phosphates and the side chains of proteins do it, and in bone mineral there is a reserve that is only called on under prolonged demand. These reactions run in fractions of a second. They do not remove the proton, however, but park it — the day's balance remains untouched by them.
Because dissolved carbon dioxide with water stands in equilibrium with carbonic acid, every change in the depth of breathing also shifts the acid position of the blood. The carbon dioxide partial pressure is therefore the only quantity in this system that can be controlled through an organ within minutes. This route too, however, only clears the volatile share.
Everything that can be neither buffered nor exhaled goes via the kidney. It does two things in the process: it retrieves the filtered bicarbonate from the primary urine — arithmetically several thousand millimoles a day — and it hands protons to two carriers, to phosphate and to ammonia. Only these two items together give the amount that actually leaves the organism.
The step in which carbon dioxide and water become bicarbonate and a proton runs too slowly on its own for the tempo of a cell. It is accelerated by an enzyme family that carries a single zinc ion in its active site, held by three histidine side chains. In humans around fifteen forms of this family are described, twelve of which are catalytically active; two of them sit in the cells of the proximal tubule, one in the cytosol and one anchored on the outer face of the brush border.
This description comes from the biochemical literature and is editorial content of this page. Legally quotable within the meaning of the health claims regulation is only the following sentence, and it is tied to this one element:
The sentence names a contribution to a process. It names no amount, no finding and no complaint, and it also does not say that a larger intake would achieve more. It may be used only for products containing a significant amount of zinc within the meaning of nutrition labelling.
Three things regularly get muddled in this subject area: the element, the units of measurement and the values from a laboratory report. They therefore stand separately.
A trace element with atomic number 30, almost always present in metabolism in bound form — as a reaction partner inside an enzyme or as a clamp that gives a protein chain its shape. Among the zinc-containing enzymes is the family that accelerates the turnover of carbon dioxide and water.
Under Regulation (EU) No 432/2012
Milligrams denote a mass in a food, millimoles an amount of substance in the body, per cent the share of a labelling reference value. For zinc, Annex XIII of Regulation (EU) No 1169/2011 lists ten milligrams per day as the reference value. The three kinds of figure cannot be converted into one another.
Labelling information. No health-related statement is connected with it.
A blood gas analyser outputs three measured and several calculated quantities. They describe the state of a sample at the moment it was taken. Neither the composition of a meal nor the intake of a mineral can be derived from them; assessing them belongs in medical hands.
A description of measured quantities, not a guide to interpreting your own results.
The written-out version calculates a day's acid load in millimoles, sets the laboratory quantities alongside with their reference ranges, and names the source in the specialist literature for every figure.
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