GLOSSARY

Potassium is the most important intracellular cation; about 98 percent of the body’s potassium is inside cells (especially muscle), and its blood concentration is only about 3.5–5 mmol/L. This large concentration difference is the basis of the resting membrane potential in nerve and muscle.

Functions

• Membrane potential: the main determinant of the resting potential and heart rhythm.
• Muscle contraction: potassium efflux produces repolarisation after the action potential.
• Metabolism: involved in glycogen and protein synthesis.

Control

• Short term: insulin, adrenaline (β2) and alkalosis move potassium into cells.
• Long term: the kidney, by secreting potassium in the collecting duct under the influence of aldosterone; most potassium in urine comes from secretion.

Disorders