Question 13

Outline the distribution, clearance and physiologic functions of magnesium in the body.

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College Answer

This question was best answered under the headings distribution, clearance and physiologic functions.
Distribution involved intracellular vs extracellular concentrations, the spread amongst organ systems
and state of ionisation and protein binding. Clearance of magnesium required an accurate description
of its renal filtration and sites and proportion of reabsorption and secretion along the nephron. The
regulatory factors and factors that influence this clearance should also be outlined. This included; Mg
plasma concentrations, other cations, ECF volume and PTH. Physiologic functions should cover its role
as a cofactor of metabolism and enzyme systems with some examples, the role and mechanism in the
musculoskeletal system as a calcium antagonist and inhibitory action in the nervous system including
the action against Ach, nerves and NMDA activity. 


  • Distribution
    • Total: 15 mmol/kg.  60%  in bone, 39% is intracellular, 1% is extracellular.
    • Normal intracellular concentration 15-20 mmol/L, extracellular 0.7-1.1 mmol/L
    • In the plasma:
      • 40% is protein-bound (just like calcium)
      • 5-10% is complexed with phosphate lactate citrate, just like calcium
      • about 50-55% is available as free biologically active ion. 
    • Intracellular pool is tightly regulated
    • Extracellular magnesium is exchanged mainly with bone
  • Elimination
    • Free magnesium ions are ultrafiltered at the glomerulus
    • 10–15% of the filtered magnesium is reabsorbed in the proximal tubule
    • 60–70% is reabsorbed in the thick ascending limb of the loop of Henle
    • 10-15% is reabsorbed in the distal convoluted tubule
    • DCT process is actively regulated by plasma Mg2+ concentration; reabsorption is near-total at normal magnesium levels, and decreases in hypermagnesemia
  • Physiological function
    • Enzyme function: membrane-bound ATPases, kinases, alk phos., enolase
    • Membrane function: cell adhesion, Transmembrane electrolyte flux
    • Structural function: protein folding, polyribosomes, nucleic acids
    • Calcium antagonist: muscle contraction/relaxation, neurotransmitter release (NMDA), action potential conduction, vasodilation/relaxation of smooth muscle
    • Bone metabolism (affects the function of parathyroid hormone)


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