🧪 Potassium (K⁺) is a vital intracellular electrolyte, essential for nerve transmission ⚡, muscle contraction 💪 (including the heart ❤️), fluid balance 💧, and acid-base regulation ⚖️. Around 98% of total body potassium is intracellular — small shifts can cause dangerous changes in plasma K⁺.

📊 Distribution of Potassium

~98% inside cells, only ~2% extracellular.
Intracellular concentration: 140–150 mmol/L.
Extracellular concentration: 3.5–5.0 mmol/L (tightly regulated!).
Maintained by the Na⁺/K⁺ ATPase pump (pumps 3 Na⁺ out, 2 K⁺ in).

⚙️ Functions

Resting Membrane Potential: Keeps cells excitable ⚡ — critical for muscle & nerve function.
Action Potentials: Repolarisation phase relies on K⁺ efflux.
Muscle Contraction: Skeletal, smooth, and cardiac muscle require stable K⁺.
Fluid Balance: Maintains intracellular osmotic pressure 💧.
Acid-Base Buffer: K⁺ exchanges with H⁺ across membranes (important in acidosis/alkalosis).

🛠️ Regulation of Potassium

Kidneys (main regulator):
- Filtration at glomerulus → reabsorption (proximal tubule, loop of Henle).
- Final secretion in distal tubule & collecting duct under aldosterone.
Hormones:
- Aldosterone: ↑ K⁺ secretion in kidney.
- Insulin: Drives K⁺ into cells (after meals or in DKA treatment).
- β₂-agonists (adrenaline, salbutamol): Shift K⁺ into cells.
Acid-Base status:
- Acidosis → K⁺ moves OUT of cells (↑ plasma K⁺).
- Alkalosis → K⁺ shifts INTO cells (↓ plasma K⁺).

⚠️ Clinical Relevance

Hypokalaemia (K⁺ < 3.5):
- Causes: GI loss (vomit/diarrhoea), diuretics, insulin, alkalosis.
- Symptoms: Weakness, cramps, ileus, arrhythmias (e.g. U-waves on ECG).
- Tx: Oral/IV K⁺ replacement + treat cause.
Hyperkalaemia (K⁺ > 5.5):
- Causes: Renal failure, acidosis, tissue breakdown (burns, rhabdomyolysis), drugs (ACEi, spironolactone).
- Symptoms: Muscle weakness, tingling, arrhythmias (peaked T-waves, wide QRS, sine-wave → cardiac arrest 🚨).
- Tx: Calcium gluconate (stabilises heart), insulin+glucose, salbutamol neb, diuretics/dialysis if severe.
DKA: Total body K⁺ is LOW (loss in urine) despite normal/high serum K⁺. Starting insulin → rapid shift into cells → life-threatening hypokalaemia unless corrected.

🧾 Summary

Potassium is key for electrical stability, muscle activity, and fluid balance. Too little (hypokalaemia) or too much (hyperkalaemia) can cause life-threatening arrhythmias ⚡❤️. Regulation depends on kidneys, hormones (aldosterone, insulin), and pH. Careful monitoring and correction of K⁺ are essential in acute medicine and critical care 🏥.

📚 References

The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms.

Potassium Physiology