20A20: Exam Report

Outline the distribution, absorption, elimination, regulation and physiological role of phosphate.

29% of candidates passed this question.

The answer structure should have utilized the headings provided in the question. Many candidates described the physiology of calcium, which while related, did not attract marks. The distribution section required not only the sites of distribution but also the percentages found in each. The regulation should have included both primary and secondary mechanisms and an
outline on the factors affecting renal excretion, intestinal absorption and release from bone etc. An outline of the physiological role of phosphate required a broad knowledge of physiological processes.

I1iii / 20A20: Outline the distribution, absorption, elimination, regulation and physiological role of phosphate


Phosphorus is a chemical element with symbol P and atomic no. 15


  • Major anion of intracellular fluid
    • ICF = 20-45mmol/L
    • ECF = 0.7mmol/L
  • Total body Phosphate = 320mmol/kg or 700g
    • 85% bound to hydroxyapatite bone matrix
    • 14% in soft tissues
    • 1 % extracellularly


  1. Combine reversibly w co-enzyme systems and compounds necessary for metabolism
  2. Bone Matrix
  3. Acid-base buffering
  4. Cell structure (phospholipids, nucleic acids)
  5. Cellular signalling

GI Absorption

  • Daily intake 800-1500mg
  • 65% ingested phosphate absorbed (active & passively) in duodenum and jejunum; brush border luminal membrane → cell cytoplasm → basolateral epithelium
  • Influenced by Vit D
  • Inhibited by Ca2+, Mg2+, Al3+
  • ECF phosphate then exchanges with bone pool; 200mg phosphate is being exchanged in remodelling processes w skeleton
  • Remaining absorbed phosphate is given to the kidneys to offload


  • In urine
  • In faeces


  • ECF Serum Phosphate is closely regulated 2.5-4.5mg/dl


  • Intake: 700-2000mg/day
  • Uptake: 200mg/day (skeletal exchange)
  • Output: 700-900mg/day
  • Pi excreted = Pi filtered – Pi reabsorbed
    • Pi filtered at glomerulus
    • 70% reabsorbed at PCT (energy dependant w Na+)
    • Relies on availability of cotransporters on apical memb of PCT cells (availability influenced by hormones)

Increase Phosphate Absorption

Low phosphate diet

1,25 Vit D3

Thyroid hormone

Metabolic alkalosis

Decrease Phosphate Absorption


Phosphatonins (FGF23 – fibroblast growth factor 23 produced in osteoblasts in response to high serum Pi reduces the sodium phosphate cotransporters on PCT)

High-phosphate diet

Metabolic acidosis (because phosphate is a titratable acid, metabolic acidosis stimulates phosphaturia)

K deficiency