Internal Distribution

Potassium Lives Inside Cells

Total body potassium is roughly 3000 mEq, or 50 to 75 mEq/kg of body weight.

About 98 percent of that sits inside cells. Only the remaining sliver circulates in the extracellular fluid where you can measure it.

That is why a serum potassium of 5.5 mEq/L tells you almost nothing about total body stores on its own.

Potassium Versus Sodium

Sodium and potassium are mirror images across the cell membrane.

Sodium is the major cation of the extracellular fluid and sits at a much lower concentration inside cells. Potassium is primarily an intracellular cation.

Holding these two gradients in opposite directions is the whole point of the system you are about to study.

The 140 to 5 Gradient

Intracellular potassium runs near 140 mEq/L.

Extracellular potassium runs only 4 to 5 mEq/L. That is roughly a thirtyfold difference across a single membrane.

Small shifts of potassium out of cells therefore move the serum value a great deal, which is exactly the trap in tumor lysis or crush injury.

The Pump That Holds It

The gradient does not maintain itself.

The Na-K-ATPase pump in the cell membrane moves sodium out of the cell and potassium into the cell in a 3:2 ratio.

It runs continuously, at metabolic cost, against both concentration gradients. Every cell membrane in the body is doing this right now.

From Gradient To Voltage

The ratio of intracellular to extracellular potassium is the major determinant of the resting membrane potential.

That resting voltage sets the stage for the action potential.

Action potentials are what make normal neural and muscle function possible, including cardiac conduction.

Why The Ratio Matters More

The determinant is a ratio, not an absolute number.

Because extracellular potassium is the small number, a change of 1 mEq/L outside the cell shifts the ratio far more than the same change inside a 140 mEq/L pool.

This asymmetry explains why the extracellular compartment is the fragile one clinically.

Both Directions Are Dangerous

Disturb the ratio in either direction and the same two systems fail.

Hyperkalemia and hypokalemia can each cause muscle paralysis and potentially fatal cardiac arrhythmias.

Neither is a benign laboratory abnormality. The shared endpoint is loss of a normal resting membrane potential.

The Serum Value Is A Window

A serum potassium samples only the extracellular fluid.

That compartment holds about 2 percent of the roughly 3000 mEq of total body potassium.

So the number on your chemistry panel reports the ratio's vulnerable side, not the size of the reservoir behind it.

Carry This To The Bedside

Three anchors are worth holding cold for exams and for the unit.

Distribution: 98 percent intracellular, roughly 140 mEq/L inside against 4 to 5 mEq/L outside, from about 3000 mEq total.

Mechanism: Na-K-ATPase pumps sodium out and potassium in at 3:2. Consequence: the ratio sets the resting membrane potential, and both hyperkalemia and hypokalemia threaten muscle and heart.

Approximately 98% of total body potassium is located inside cells.

• True
• False

Answer: True

This statement is correct. Only about 2% of total body potassium is in extracellular fluid, where serum potassium is measured.

Sodium is the major cation of the _____, whereas potassium is primarily an _____ cation. The _____ moves sodium _____ of cells and potassium _____ to cells.

Answer: Sodium is the major cation of the extracellular fluid, whereas potassium is primarily an intracellular cation. The Na-K-ATPase pump moves sodium out of cells and potassium in to cells.

Sodium and potassium have opposite distributions across the cell membrane. The Na-K-ATPase maintains these gradients by moving sodium outward and potassium inward.

Which description of the Na-K-ATPase pump is correct?

• It moves 3 sodium ions out and 2 potassium ions in
• It moves 2 sodium ions out and 3 potassium ions in
• It moves 3 potassium ions out and 2 sodium ions in
• It passively allows equal sodium and potassium movement

Answer: It moves 3 sodium ions out and 2 potassium ions in

The Na-K-ATPase pump continuously moves sodium out of cells and potassium into cells in a 3:2 ratio, using metabolic energy.