Electrolytes

Master answer

Phosphate

• Most abundant anion, approx. 1% of total body weight
• Normal serum level 0.8 to 1.3 mmol/L in adults
• daily requirement = 0.4mmol/kg/day

Distribution

• Mostly intracellular.
• Bone & teeth (85%), tissue (14%), ECF (1%)
• In skeleton with calcium, as hydroxyapatite crystals or amorphous calcium phosphate
• In tissue and ECF:
  • 2/3 is tied up in molecules like ATP
  • 1/3 in inorganic phosphate
  • pH dependent

Absorption

Dietary intake 20mg/kg/day → Proximal intestine absorbs 16mg/kg/day

Elimination

• Feces:
  • 3mg/kg/day secreted into intestine via pancreatic, bile, intestinal secretion
  • unabsorbed 4mg/kg/day from dietary intake
• Kidney:
  • 100mg/kg/day in UF.
    • 80% reabsorbed in proximal tubule
    • 5% in DCT
  • rest excreted (~15mg/kg/day)
• Net Intake-Excretion of Phosphate
  • modulated based on physiological needs such as bone remodelling, bone growth, etc
    • bone resorption ~300mg/day
    • absorption ~300mg/day

Regulation

GI – Bone – Renal axis

Regulators:

• Dietary phosphate intake and absorption
• Calcitriol
  • which increases phosphate absorption from the gut and bone
• Parathyroid hormone (PTH)
  • directly causes phosphate resorption from bone and decreases its reabsorption in the proximal tubule
  • indirectly by stimulating the production of calcitriol.
• Phosphatonins, such as fibroblast growth factor-23 (FGF-23):
  • Inhibit renal phosphate reabsorption
  • inhibit synthesis of calcitriol

Factors that alter renal regulation of phosphate:

Physiological Role

• Bone and teeth formation
• Cell wall structure (phospholipids)
• Nucleic acid generation
• Glucose metabolism
• High energy bonds
• O2 transport (2,3 DPG)
• Phosphate Buffer
• intracellular signalling

• Sources
  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3743361/
  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5686387/
  • https://cjasn.asnjournals.org/content/10/7/1257

JC 2020

Answer by JC