Sodium and Water

Describe the function, distribution, regulation and physiological importance of sodium, chloride, potassium, magnesium, calcium and phosphate ions

Normal total body Na+ is 60mmol.kg-1, 70% of which is exchangeable. Total body Na+ is distributed as:

  • 50% in ECF
    Sodium is the dominant extracellular cation.
    • Typical ECF [Na+] of 140mmol.L-1.
  • 45% in bone
  • 5% in ICF
    A minor intracellular cation.
    • ICF [Na+] varies with cell type, but is typically 12-20mmol.L-1.
    • Concentration is kept low by the action of the 2Na+-3K+ ATPase exchange pump and the low permeability of the cellular membrane to Na+

Function of Sodium

  • Regulation of ECF volume
    Principal ECF cation. Changes in sodium levels cause compensatory fluid shifts. Loss of sodium content will result in hypotension/hypovolaemia, with consequent baroreceptor stimulation and activation of the RAAS. Baroreceptors will activate with a 7-10% change in volume.

  • Osmolarity
    Changes in sodium concentration affect osmoreceptors and will affect ADH and thirst mechanisms. Osmoreceptors will activate with a 1-2% change in osmolality.

  • Acid-Base balance
    Na+-H+ exchange pumps in the kidney are stimulated in acidosis.

  • Resting Membrane Potential
    Alterations in sodium concentration will affect intracellular potassium to a similar degree, which will alter the RMP.

Regulation of Sodium and Water

Regulation of any system is typically a balance between input and output:

  • Sodium intake is essentially unregulated
  • Therefore, sodium concentration is a function of:
    • Sodium elimination
    • Sodium reabsorption
    • Water homestasis
      Control of total body water is a major mechanism to regulate sodium concentration.

Sodium Elimination

Sodium is eliminated in:

  • Sweat and GIT
    Obligatory and not amenable to regulation.
    • Acclimatisation to hot environments improves the efficiency of sweating by reducing its tonicity, reducing sodium loss
  • GIT
  • Urine
    • Adjust renal elimination is the main mechanism to regulate sodium concentration
      Can be performed in two ways:
      • Changes in GFR
        Changes in GFR due to hyper or hypovolaemia will (indirectly) adjust sodium elimination. Increased plasma volume increases GFR, and vice versa.
      • Changes in sodium reabsorption
        This is the main mechanism for controlling sodium in euvolaemia, and is mediated primarily by aldosterone.

Sodium Reabsorption

Given that:

  • Normal glomerular filtrate is ~180L.day-1
  • The dominant osmole in glomerular filtrate is sodium
  • Normal urine output is ~1.5L-1

The majority of filtered sodium must be reabsorbed. This is called bulk reabsorption and occurs in the PCT and LOH:

  • 60% of total reabsorption is by the Na+-K+ ATPase pump in the PCT
  • 30% of total reabsorption is by the Na+-K+-2Cl- co-transporter in the LOH

The remaining 10% of sodium reabsorption occurs in the DCT and CT. As it is under the influence of aldosterone, it is the component which is important in regulation. Aldosterone increases Na+ reabsorption by increasing the number or activity of these pumps:

  • Na+-Cl- pumps in the DCT
  • Na+-K+ ATPase pumps in principal cells of the DCT
  • Na+-H+ pumps in intercalated cells of the CT

Water Homeostasis

Body water homeostasis involves:

  • Sensors
    Osmoreceptors present in the:
    • Macula densa
    • Circumventricular organs
      Subfornical organ and the vascular organ of the lamina terminalis.
      • Change in cellular volume secondary to changes in osmolality alter hormone secretion.
  • Effectors
    Predominantly hormonal:

References

  1. Brandis K. The Physiology Viva: Questions & Answers. 2003.
  2. CICM September/November 2014
  3. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
  4. National Research Council. Recommended Dietary Allowances. 10th Ed. 1989. National Academies Press.
Last updated 2018-06-25

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