14B23: Exam Report

Describe the regulation of sodium in the body.

19% of candidates passed this question.

This question was generally poorly answered. Total body sodium is regulated within 2% in normal individuals. The vast majority is contained in the extracellular compartment. While any physiological regulation involves a balance of input and output, sodium intake is essentially unregulated in humans. Output is regulated via renal, gastrointestinal and skin losses. Candidates needed to present the renal handling of Na including hormonal control and present factual knowledge about the level of absorption and GFR effects to attain a pass mark. Many candidates focused on osmolality and tonicity and some on the use of diuretics thereby not gaining marks on regulation of sodium. Most candidates didn’t mention either the skin and GIT role in sodium balance.

I1iii / 14B23: Describe the regulation of sodium in the body

Definition

Chemical element with atomic no.11.

Major extracellular cation.

Quantify

  • ECF 140mmol/L
  • ICF 20mmol/L
    • 50% → ECF
    • 5% → ICF
    • 45% → bone (non-exchangeable)
  • Total body Na+ = 60 mmol/kg

Role

  1. Tonicity & ECF volume
  2. Enzyme activity e. Na/K/ATPase
  3. RMP
  4. Neuromuscular excitability
  5. Acid base balance

Regulation

Intake

  • Unregulated
  • Requirements = 500mg/day
  • Absorbed from GI tract as symport with glucose/amino acids

Excretion

  • GI tract ≤ 10% in faeces
  • Skin 5 – 50mmol/day in sweat
  • Renal → Na+ excretion = filtered Na – Reabsorbed Na (GFR + Plasma Na+)
    • PCT

→ 65% reabsorbed

→ Co-transporter with amino acids, glucose driven by Na/K/ATPase

→ Counter transporter with Na/H exchange

→ Passive diffusion by solvent drag

  • Thin Desc LoH

→ Permeable to H2O, impermeable to solutes

  • Thick Asc LoH

→ 25% reabsorbed

Excretion
  • DCT

→ 6% reabsorbed

→ Co-transporter Na/Cl → driven by Na/K/ATPase

  • Principle Cells of Collecting Duct

→ 3% reabsorbed

→ Apical Na+ channel → drives Na/K/ATPase

→ Controlled by Aldosterone

Regulation

Intrinsic Renal → Glomerulotubular Balance

  • Kidneys attempt to ensure 65% of Na+ load is always reabsorbed
  • ↑GFR
  • ↑Na, H2O reabsorption

Hormonal → Aldosterone

  • ↓Na = ↓ECF = detected by BaroR
  • ↑Symp activity
  • ↑RENIN by Granular cells
  • ↑AII = acts on mineralocorticoid receptor of Principle Cells
    1. Upregulates Na/K/ATPase pump
    2. Upregulates Apical Na+ channel of Principle Cells
    3. ↑Na+ reabsorption

Hormonal → Renin

  • Produces AII
    1. Powerful efferent arteriole VC = ↓peritubular hydrostatic P = ↑Na+ reabsorption
    2. Upregulates Na/K/ATPase of all tubule cells
    3. Upregulates PCT counter-transporter Na/H exchange

Hormonal → ANP/BNP

  • Atrial stretch 2° ↑ECF
    1. Inhibits Apical Na+ channel of Principle Cells = ↓Na+ reabsorption
    2. Inhibits ADH stimulated reabsorption

Hormonal → ADH

  • ↑Thick Asc. LoH Na/K/2Cl activity = ↑Na+ reabsorption