Overview
This lecture introduces potassium as an essential electrolyte, explains its location and normal values, and details its regulation, especially by the kidneys.
Introduction to Potassium
- Potassium is the most abundant intracellular cation in the body.
- It is primarily located within all cells.
- Normal blood potassium level: 3.5–5.0 mEq/L.
Potassium Regulation in the Body
- Potassium enters the body through the diet and is absorbed by the small intestine.
- Only about 9% is excreted via the gastrointestinal (GI) tract.
- Major potassium regulation and excretion (about 90%) occur via the kidneys.
- A small amount (1–2%) is lost through sweat.
Renal Handling of Potassium
- Potassium from the blood is filtered at the glomerulus in the kidney.
- 65% of filtered potassium is reabsorbed in the proximal convoluted tubule.
- 15% is reabsorbed in the thick ascending limb of the loop of Henle.
- Regulation of potassium secretion and excretion occurs mainly in the distal convoluted tubule and early collecting duct.
- Secretion means moving potassium from the blood into the tubular lumen for urinary excretion.
Cellular Mechanisms in the Kidney
- ENaC channels absorb sodium from the tubular lumen into distal tubule cells.
- The sodium-potassium ATPase pump exchanges 3 sodium out for 2 potassium into the cell from the blood.
- Potassium is then secreted into the lumen via ROMK channels.
- Secretion depends on the body's potassium levels and other factors.
Factors Influencing Potassium Secretion
- Increased blood potassium (hyperkalemia) increases secretion.
- Alkalosis (high blood pH) increases potassium loss; acidosis reduces it.
- Aldosterone hormone increases sodium reabsorption and potassium excretion.
- Negative luminal charge in the tubule enhances potassium secretion.
- Increased sodium and water reaching the distal tubule increases potassium secretion.
- Some diuretics (e.g., amiloride) alter luminal charge and affect secretion.
GI Tract and Sweat Losses
- Diarrhea, laxatives, enemas, and vomiting increase GI potassium loss, potentially causing hypokalemia.
- Excessive sweating can also lead to mild potassium loss.
Summary of Key Regulation Sites
- GI tract: minor excretion (~9%).
- Kidneys: major regulation and excretion (~90%).
- Sweat: minor excretion (1–2%).
Key Terms & Definitions
- Potassium (K+) — Major intracellular cation involved in numerous physiological processes.
- ENaC Channel — Epithelial sodium channel enabling sodium absorption.
- Sodium-potassium ATPase — Pump transporting sodium out and potassium into cells.
- ROMK Channel — Potassium channel on tubular cells secreting potassium into urine.
- Aldosterone — Hormone increasing sodium reabsorption and potassium excretion.
- Secretion — Movement of substances from blood into the renal tubule for excretion.
- Hyperkalemia — Elevated blood potassium.
- Hypokalemia — Low blood potassium.
Action Items / Next Steps
- Review magnesium lecture/video if not already done.
- Study factors influencing potassium secretion in detail for exam preparedness.