The role of potassium in cardiovascular health, like calcium, is to regulate heart rhythm, smooth muscle tone in the arterial system, and lower blood pressure. Under normal circumstances, dietary potassium deficiency does not occur. However, a good deal of potassium can be lost through the kidneys (especially when diuretics are used on a regular basis), the gastrointestinal tract (in people with protracted vomiting, diarrhea, or laxative abuse) or by profuse sweating. Excessive potassium loss can cause tiredness, weakness, poor appetite, nausea, listlessness, or irrational behavior. It can also cause severe irregularities of heart rhythm.
Three recent studies showed that higher potassium intake is associated with a modest drop in systolic (by three points) and diastolic (by two points) blood pressure. African Americans and older persons are especially responsive to increased potassium intake.
(Source: Every Heart Attack is Preventable, by Michael Mogadam, M.D.)