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 homeostasis
    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:
  • ADH
  • RAAS
  • Natriuretic peptides

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.