The significance of potassium

The human body contains about 3,500mmol of potassium. Only about 2 per cent of this is in the extracellular fluid at any one time and the rest is intracellular. This 98 per cent is held in cells by a set of complex mechanisms and is pumped in and out of them by Na/K-ATPase pumps. Potassium has many biological functions. It is a co-factor for many enzymes and it is required for insulin secretion, creatine phosphorylation, carbohydrate metabolism and protein synthesis.The ratio of intracellular to extracellular potassium is the major determinant of muscular and neuronal excitability and if this balance is disturbed, various pathological states can develop. Tiny changes in the amount of potassium pumped in and out of cells can profoundly affect the level in the extracellular fluid and hence plasma levels. Such changes are often silent and insidious and, sometimes, it is not until a person collapses and is admitted to hospital that the problem is identified. Panel 1 lists factors causing the movement of potassium into cells. One factor affecting plasma potassium levels is blood pH. Acidosis causes potassium to move out of cells and alkalosis causes potassium to move into cells. Insulin is another important regulator. If the plasma potassium level rises, insulin release is stimulated and this, in turn, causes potassium to move into cells. This effect is due to insulin stimulating activity in Na/K-ATPase pumps.