Potassium & Sodium钾 · 钠

Place · Level 3

一个守细胞内 · 一个守细胞外 · 共同维持血压、神经、肌肉的电力网

K in, Na out

Stop 1/6

内钾外钠 · 全身的电池

Every cell in the body maintains the same gradient: K⁺ inside (~140 mmol/L), Na⁺ outside (~140 mmol/L). This mirror-image relationship is the power source for every excitable tissue.

The worker maintaining this gradient is the Na⁺/K⁺ ATPase pump: it works thousands of times per second on every cell; each cycle uses 1 and pumps out 3 Na⁺ and pumps in 2 K⁺. Body-wide, this pump consumes 20–30% of resting energy — second only to the respiratory chain itself.

The outcome: a resting membrane potential of ~ -70 mV (inside negative, outside positive) — the 'standby state' for nerves and muscles; during an action potential, Na⁺ channels open and Na⁺ rushes in, the membrane potential flips to +30 mV, the signal propagates; immediately after, K⁺ channels open and K⁺ rushes out, the membrane returns to baseline, and the cell readies for the next firing.

So K and Na aren't a 'pair of electrolytes' — they're the positive and negative terminals of the same electrical system. Anything that throws the gradient off (heavy sweating + lots of plain water = dilutional hyponatremia; severe diarrhea = loss of potassium) simultaneously affects nerve conduction, muscle contraction, and cardiac rhythm.

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