22B14: Exam Report

Describe the physiological role, distribution and regulation of potassium ions (K+ )

51% of candidates passed this question.

The best answers demonstrated an appreciation of the multiple roles of potassium in normal physiology and described the integrated regulation of potassium concentration/distribution as opposed to many answers that seemed to focus purely on the renal handling of a filtered potassium load.

I1iii / 22B14: Describe the physiological role, distribution & regulation of K+

Defintion

Chemical element with atomic no. 19. Second most common cation in the body

Quantify

  • ICF 150mmol/L
  • ECF 3.5 – 5mmol/L
    • 90% ICF
    • 8% bone (ion-exchange)
    • 2% ECF
  • Total body K+ = 45mmol/kg

Role

  1. Intracellular tonicity
  2. Major determinant RMP → muscle & nerve excitability
  3. Na/K/ATPase of all PM
  4. Intracellular processes (pr & glycogen synthesis)

Importance of Regulation

  • ECF K+ is closely regulated
  • ↑K+ ECF = ↓RMP (approaches zero, ∴potential across PM becomes less)

              = ↑excitability & ↓conduction velocity

              = potential for life threatening arrhythmias

  • ↓K+ ECG = ↑RMP

                  = ↓excitability & ↑conduction velocity

Movement of K+ Across PM

K+ IN

Adrenaline

  • ↑Na/K/ATPase activity

Insulin

  • ↑Na/K/ATPase activity

K+ OUT

Acidosis

  • H+/K+ exchange

 Cell lysis

 Exercise/cell depolarisation

  • Sux ↑ECF K+ 1mmol/L
  • Muscle cells release K+ into T-tubules but with disturbed muscle architecture

Regulation

  • Intake: 5000mg/day → not regulated
  • Output
    • Sweat 10mmol/day
    • Faeces 10mmol/day
    • Renal = major regulation of K+ balance
  • K excreted = K filtered – K reabsorbed + K secreted
    • K freely filtered by glomerulus
    • 15% load excreted
    • Distal segments most important for tweaking K+ secretion

PCT

  • Reabsorbs 55%
  • PARACELLULAR DIFF. due to SOLVENT DRAG

Thick Asc. LoH

    • Reabsorbs 30%
    • Co-transporter Na/K/2Cl (driven by Na/K/ATPase)
    • Paracellular diff. → due to +ve transtubular potential

Collecting Duct

  • PRINCIPLE CELLS
    • 70% cells
    • Secrete K+ depending on dietary load
    • Active transporter Na/K/ATPase
Principle Cells
  • TYPE A-INTERCALATED CELLS
    • 30% of cells
    • Reabsorbs K+
    • Active transport via K/H/ATPase
Type A

Regulation

1. Plasma K level

    • Na/K/ATPase highly sensitive to plasma K
    • ↑↑↑upregulated by hyperK
    • K+ secretion in collecting duct

2. Aldosterone

  • Stimulated by hyperK
  • Directly stimulated by adrenal cortex
  • ↑no Na/K/ATPase channels in distal nephron

3. Na+ Delivery to Distal Nephron

    • ↑Na+ delivery
    • ↑Na+ reabsorption
    • ↑K secretion

4. Flow Rate to Distal Nephron

  • K secretion ∝ flow rate
  • ↑K+ delivery = ↑K+ secretion