🧪 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

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Potassium Physiology