About 60% of an adult's body weight is water, and the salts dissolved in it carry the electrical charges that nerves and muscles depend on. Keeping water, electrolytes and pH in range is essential for life.
Body water compartments
About two thirds of body water is inside cells and one third is outside, split between blood plasma and the fluid between cells. Water follows sodium, so sodium largely decides how much fluid stays outside cells.
Main electrolytes
Sodium is the main positive ion outside cells and potassium the main one inside. Calcium is needed for muscle contraction and clotting, and magnesium helps keep the heart rhythm stable.
pH balance
Normal blood pH is 7.35 to 7.45. Below that range is acidosis and above it is alkalosis.
Three lines of defense
Buffers such as bicarbonate act in seconds, and the lungs adjust CO2 within minutes. The kidneys excrete acid and keep bicarbonate over hours to days.
| Value | Normal adult range | Main role |
|---|---|---|
| Sodium | 135 to 145 mmol/L | Fluid balance, nerve signals |
| Potassium | 3.5 to 5.0 mmol/L | Heart rhythm, muscle function |
| Bicarbonate | 22 to 28 mmol/L | Buffers acid |
| pH | 7.35 to 7.45 | Acid-base status |
At the bedside
Abnormal potassium can cause dangerous heart rhythms, so get an ECG when it is high or low. Vomiting, diarrhea and diuretics are common causes of fluid and electrolyte loss.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).