The mineral potassium is essential to human body chemistry. Blood levels of potassium are tightly regulated, with a normal level between 3.5 and 5.0 mEq per liter for adults (reference values may vary somewhat between labs). These levels are tested as part of routine blood work and are monitored closely in cardiac and kidney patients.
Potassium in the Cells
Potassium is the predominant cation (positive ion) inside the cells of the body. The amount within the cells is more than 36 times that outside of the cells, which creates a difference in charge, or electrical potential, across the cell membrane. Potassium, sodium and calcium, along with other minerals, exist in balance on opposite sides of the membrane, and their movement across it creates energy for the heart beat, skeletal and smooth muscle contraction, and other cellular processes.
Potassium and the pH of the Body
Hydrogen is the main cation of acid-base balance, with pH being a measurement of hydrogen concentration. When the blood becomes too acidic (increased cation concentration, known as acidosis), hydrogen moves into the cells, forcing potassium into the blood. When the blood becomes too alkaline (not enough cations, known as alkalosis), the opposite shift occurs. The kidneys can also excrete or retain potassium to maintain acid-base balance. Any illness that interferes with excretion, or causes abrupt losses of minerals through vomiting or diarrhea, can upset this balance.
Potassium and the Heart
Potassium is vital to the movement of the heart muscle. When serum potassium is lower or higher than the normal range, the contractions of the heart become inefficient and arrhythmias occur. Low potassium levels affect the action of digitalis, and increase the potential for reactions to this drug. Potassium levels should be monitored regularly in anyone taking digitalis or being treated for cardiac symptoms.
Potassium and the Kidneys
The kidneys excrete potassium, and levels of the mineral are regulated through their actions. If serum levels are too high, more potassium will be excreted; if too low, more will be retained. In cases of renal failure, excretion of potassium decreases, and hyperkalemia results. Diuretics can cause excessive loss of potassium through the kidneys. Potassium-sparing diuretics, on the other hand, conserve potassium while forcing the excretion of sodium, one of its balancing cations. Kidney patients and anyone taking diuretics should have potassium levels monitored regularly.
Potassium in Food
Since blood levels of potassium are so tightly regulated, a lack of the mineral in the diet can have dire consequences. Bananas are the most famous dietary source of potassium. Other sources include avocado, potatoes, dark green leafy vegetables, and legumes. Those taking potassium-sparing diuretics should consult a nutritionist regarding regulation of potassium intake.