Though all anions deserve love to the same equal extent, phosphate has risen to become exceptional in receiving attention in the form of an entire chapter. This is not because of any intrinsic merits of phosphate as an anion, but rather because of the fact that historically on this website the coverage of phosphate and its metabolism has been very poor. In the true intensive care fashion, the author has responded to this with a massive overcorrection. Ergo, an extensive rambling exposition follows.
Phosphate anion has been largely ignored by the CICM examiners. Question 20.1 in the first paper of 2017 potentially counts as a "phosphate question" because it had a hyperphosphataemic patient, but the college did not require the candidates to perform anything in the way of a detailed analysis ("interpret the biochemical results, giving underlying reasons" they asked). The Part I questions have repeatedly probed calcium metabolism, to which much of the phosphate material is linked, but phosphate itself has remained unmolested by the examiners. In short, the time-poor exam candidate can safely omit this topic of discussion to focus on sodium and potassium.
The exam candidate with near-infitine time resource may continue reading. If they for some reason insist on exploring peer-reviewed publications on the topic of phosphate regulation and metabolism, one could do no better than Takeda et al (2004), which at the moment appears to be available as a free full text PDF.
Total body phosphate content
Phosphate is damn near everywhere. Apart from dissolved inorganic phosphate anions, phosphate is a part of bone mineral, cell membrane, DNA and RNA nucleotides, cell signaling secondary messengers (IP3) and numerous protein molecules. It is the P in ATP, and the most convenient handle by which to activate and deactivate enzymes (by phosphorylation and dephosphorylation). It generally seems to have a central role in life as we know it.
How much phosphate is in you, right now? All textbooks seem to have a slightly different value for intracellular and total body phosphate content. Guyton and Hall (11th ed. ) give the intracellular concentration as 11 mOsm/L (p. 294), saying that "approximately 85 per cent of the body’s phosphate is stored in bones, 14 to 15 per cent is in the cells, and less than 1 per cent is in the extracellular fluid". Koam and Walsley's A Primer on Chemical Pathology gives the the figure of total body phosphate as 25 moles, and says that "80% is complexed with calcium in bones, 10% incorporated into organic compounds and 10% combined with carbohydrates, proteins and lipids" (p.103). Ganong's Review of Medical Physiology (23rd ed.) gives the total body phosphorus mass as "500 to 800 g (16.1–25.8 mol), 85–90% of which is in the skeleton" (p.365). None of these textbooks give anything in the way of references. Having said that, the 85-14-1% breakdown seems to be repeated with such seductive consistency that the author was compelled to reproduce these numbers here.
- Total body phosphate: 1% of body mass, 320 mmol/Kg, or 22.5 mol in a 70kg person (700g).
- Of this, the vast majority (85%, 19.1 moles or 595g) is bound in the hydroxyapatite matrix of bone.
- Of this bony phosphate, 300 mg o is exchanged every day as part of normal bone turnover. The other 15% is dissolved, and exists largely as the phosphate ion, PO43-.
- The overwhelming majority of this 15% (say, 14.99%) is intracellular, which makes about 3.4 moles.
- Extracelllular phosphate is about 0.7mmol/L, or 12mmol in the total extracellular fluid.
So where do these numbers come from, originally? They may sound authoritative, but in fact the basis for them is not completely scientific. Generally speaking, whenever a textbook mentions some value for body tissue or fluid composition, they are probably quoting this source, even when they don't realise it. Iyengar, Kollmer & Bowen published their "Normale Konzentrationen verschiedener Elemente tin Organen und Körperflüssigkeiten" in 1978. The book immediately came under fire for containing essentially unfiltered experimental data without much of an attempt to perform a synthesis of the findings. E.I Hamilton, a reviewer, wrote, "the data presented in this compilation covers several decades of reporting but it is only really during the last five or so years that serious attention has been paid to quality control of analytical data; hence, as there appears to be no selection of preferred values, how accurate are the data?" This, as far as one can tell, is the source for the typically quoted textbook values for phosphate.
There is indeed significant variation in the peer-reviewed literature with regards to body fluid and total body electrolyte content, and phosphate is severely affected by this issue, though perhaps not as much as something like cadmium or molybdenum, which would vary by factors of magnitude depending on how close your sample subjects live to the molybdenum-smelting plant. Henry Lukaski (1987) offers an excellent discussion of the various means of body content measurement, and the ways in which it changes the results. In short, depending of what you measure and how, you might come to very different conclusions. Because many of the studies come from a dark age of biochemistry, analytic quality controls and methodology are often unsatisfactory.
A favourite method of measuring total-body elemental content is to remove all volatile components and measure just the "dry" mass of the organism. For example, one of the more famous estimates for mammalian phosphate content is the historical 1926 study by Sherman and Quinn, who presented tabulated information regarding the Average Phosphorus Content of Normal Male Rats . It turned out to be abut 0.75% of total dry body mass, close to the 1% quoted above. The authors described their methods:
"For the determination of phosphorus, the body of the rat was placed in a silica dish, charred slowly over a free flame, and burned at a dull red heat in an electric muffle to a white or light gray ash. The ash was dissolved with nitric or hydrochloric acid, the solution filtered and made up to volume."
Of course the use of incinerated or pureed organisms is relatively useless for the determination of body composition, because the most important matter is the distribution of the substance among the body compartments, as opposed to the total content of homogenised ash. Even relatively non-invasive techniques are fairly destructive. For instance, Cohn and Dombrovski (1971) developed a method which requires the investigator to "deliver a moderated beam of fast neutrons to the subject", shot out of a cyclotoron or a lump of plutonium-238. The 3.5-8 MeV neutrons, captured by body atoms, create unstable isotopes which then rapidly revert to a more stable condition by emitting a gamma ray of characteristic energy, which you then measure with "a detector array in a highly shielded facility". This permits a measurement of in vivo body elemental content without the need to incinerate your subject, but it does certainly make them quite radioactive, and so cannot be viewed as a totally benign investigation. The article is ancient, and the only evidence of it now is this abstract in the Office of Scientific and Technical Information of the US Department of Energy. It is therefore impossible to say what became of their volunteers, or what the exact numbers were which they had generated.
Distribution of phosphate in the body fluid compartments
Intracellular phosphate content
Phosphate is the most abundant intracellular anion. Exactly how abundant is a matter of some scholarly debate. The clinical literature is awash with articles where the matter is either totally ignored or swept under the carpet, as by Penido et al (2012), where the authors dismissively claim that the intracellular concentration is "100-fold greater than that in the plasma" giving no reference or justification for the statement.
This is because those references are hard to find. Bevington, Kep & Russell, in their chapter for Phosphate and Mineral Homeostasis (1985, pp 469-478) lamented the lack of attention to phosphate, jealous of the unfair spotlight which constantly seems to be shining on calcium. Without digressing into this, it is possible to say that intracellular phosphate is available in several species. Of these, the species immediately available to use in reactions such as making ATP is Pi, "inorganic phosphate" or orthophosphate as it is known in some publications. This species is the "free" dissolved phosphate anion, which is a mixture of H2PO4- and HPO42-. It can be measured non-destructively by 31P-NMR nuclear magnetic resonance spectroscopy (Bevington et al, 1986). The role of phosphate as an intracellular buffer makes it difficult to generalise intracellular concentration data from disembodied strips of muscle to living organisms, and even within the same organism to different states of health or different tissues. As such, the author will offer a list of different ranges from different organisms and tissues, so that the reader can form their own impression.
- 14 mmol/L (Bowen et al, 1982; Ehrlich ascites tumour cells from cancerous mice)
- 10 mmol/L (Knochel et al, 1980, defrosted leg muscles of a dog)
- 0.78 mmol/L (Noorwali et al, 1982; human erythrocytes)
- 2.01 mmol/L (Noorwali et al, 1982; human leukocytes)
- 4.12 mmol/L (Noorwali et al, 1982; human platelets)
In summary, it's all a bit variable. If one were for whatever reason compelled to average these out to some sort of Representative Human Cell, one might settle for 10 mmol/L as a completely arbitrary value.
Apart from the immediately useful inorganic phosphate, intracellular phosphate is also bound to a variety of other molecular species. Of these, the most numerous is probably phosphocreatine, described as a high-powered intracellular energy reserve. Obviously cells differ in their need for battery storage, and so the levels of phosphocreatine vary from 10 to 30 mmol/L (Saks & Strumia, 1993). ATP, ADP and various other somethingP species of energy molecule are also responsible for intracellular phosphate stores, but because these are in a constant state of flux the exact concentration in any given cell at any given time is a complete guess. if one needed a range to use in some sort of childish cylinder diagram, one could fall back on sources such as Ataullakhanov et al (1997), which quotes the range of concentration for the relatively idle human erythrocyte as being between 1.0 and 5.0mmol/L.
Some of the intracellular phosphate is also bound to glucose as glucose-6-phosphate, which will again be highly variable between tissues and nutritional states (eg. before vs. after doughnut). According to a popular textbook by Stipanuk & Caudill, this concentration is usually 0.05-1.0 mmol/L; however it is probably lower at rest. Rothman et al (1992) used 31P-NMR to demonstrate that even under the effects of insulin the human skeletal muscle cells have a glucose-6-phosphate no higher than 0.22 mmol/L.
Extracellular phosphate content
The extracellular concentration of phosphate is about 1.0mmol/L. In total, no more than 12mmol is available in the extracellular fluid at any given time. This is the only phosphate source to the cells. Most of it is present as a univalent species (i.e. with a negative charge of 1). Even though the phosphate ion, when fully ionized, is a trivalent PO43- at physiological pH the divalent PO42- species is the more prevalent (about 80%), and the univalent PO4- species is the other 20%.
Extracellular phosphate behaves in a manner similar to calcium. An authoritative paper by Baker and Worthley on calcium magnesium and phosphate metabolism reports that about 15% of it is bound to protein, another 53% free and ionised, and another 47% compelled with calcium and magnesium.
Movement of phosphate between the intracellular and extracellular compartments
Phosphate equilibrates easily between the extracellular fluid compartments. There really isn’t anything stopping it. It is strongly anionic, so there is slightly more in the interstitial fluid (as it is repelled by the anionic charge of plasma proteins). Phosphate is sucked into the cell by a sodium-phosphate co-transporter, which is more active in the presence of insulin. This process is powered by the awesomely powerful sodium concentration gradient. There are three types (inventively nominated as Type1, 2 and 3); the Type 3 co-transporter seems to be the ubiquitous one, which all of the human cells express.