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