Sodium is the main salt outside cells and potassium is the main salt inside cells. The sodium-potassium pump in every cell membrane keeps them apart and makes nerves and muscles work.
Sodium
Normal blood sodium is about 135 to 145 mmol/L. A low sodium usually means too much water rather than too little salt, and a high sodium usually means too little water.
Sodium symptoms
Abnormal sodium mainly affects the brain, because water moves in or out of brain cells. Confusion, headache and seizures can occur, especially when the change is fast.
Potassium
Normal blood potassium is about 3.5 to 5.0 mmol/L, a tiny share of the body's total. The kidney excretes most of the daily intake, driven by aldosterone.
Potassium shifts
Insulin and beta-agonists move potassium into cells, while acidosis and cell damage move it out. These shifts change the blood level quickly without changing total body potassium.
| Problem | Common causes | Main danger |
|---|---|---|
| Low sodium | Diuretics, heart failure, excess ADH | Confusion, seizures |
| High sodium | Not enough water, no access to drinks | Confusion, coma |
| Low potassium | Diuretics, vomiting, diarrhea | Weakness, arrhythmias |
| High potassium | Kidney failure, ACE inhibitors, cell damage | Arrhythmias, cardiac arrest |
At the bedside
With high potassium, get an ECG at once and look for peaked T waves and wide QRS. Correct chronic sodium problems slowly to avoid brain injury.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).