Phosphorus
[Ref: AV6:chp10; WG21:chp21]
Normal store and distribution
Total body phosphorus
= 500 - 800g
= 16.1 - 25.8 mmol
85-90% in bones.
Forms of phosphorus in blood
Plasma phoshorus level
= 12mg/dL
- 2/3 in organic compunds
- 1/3 in inorganic compound (Pi)
* Mostly PO4
Daily exchange
About 3mg (97micromol)/kg enters and leaves bone daily
Renal handing of phosphorus
Filtration
5-10% of plasma phosphate is protein bound, so that 90-95% is filterable at the glomeruli.
Reabsorption
75% of the filtered phosphate is actively resorbed.
[WG21:p386] 85-90% reabsorbed
Almost all resorption occurs in the proximal tubule
* via Na-PO4 symporter
* Inhibited by PTH
Properties of PO4 renal resorption
PO4 resorption system is a Tm system
- Normal physiological filtration rate of phosphate is a bit higher than Tm
- --> Resorption system is normally saturated
- Any increase in filtered load of phosphate will simply be excreted.
Acidosis
In acidosis,
--> increase release of Ca2+ and PO4 from bone
--> Increased PO4 excretion
* Acidosis also increase Ca2+ excretion by inhibiting resorption
--> More titrable buffer in filtrate to help excretion of excess H+
GIT handling of phosphorus
- Pi absorbed in duodenum and small intestine
- Both by active transport and passive diffusion
- Absorption is linearly proportional to intake
* Unlike Ca2+ absorption in GIT
- 1,25-dihydroxycholecalciferol increases Pi absorption
Regulation
Not directly regulated.
Factors increasing [PO4]
- Increased 1,25-dihydroxycholecalciferol
* Increased GIT absorption and bone resorption
- Increased dietary intake
Factors decreasing [PO4]
- Increased PTH
* Increase PO4 --> increased PTH
* Increased PTH --> Increase PO4 excretion
Overall, net effect may be unchanged, elevated still, or decreased
Other notes
Chronic renal failure
Elevated phosphate (????due to decreased GFR)
--> Stimulate PTH
--> Elevated PTH
--> Excessive bone resorption
Also, there is reduced ability to produce calcitriol
--> Reduced calcium absorption from GIT and kidney
Treatment
Give calcitriol
--> suppress PTH