K+ is the chemical symbol for the potassium ion, the form of potassium that exists in your body and plays a critical role in keeping your cells, nerves, and muscles working. The “K” comes from kalium, the Latin word for potassium, and the “+” sign indicates it carries a positive electrical charge. You’ll most often see “K+” on blood test results, where the normal range is 3.5 to 5.5 mEq/L.
Why K+ Matters in Your Body
Potassium is the most abundant positively charged particle inside your cells. Of the roughly 3,500 milliequivalents of potassium in your body, about 98% sits inside cells at concentrations of 140 to 150 mEq/L, while only 2% floats in the fluid outside cells at much lower concentrations (3.5 to 5 mEq/L). That steep difference between inside and outside is not a quirk of biology. It’s the engine that powers electrical signaling throughout your body.
Your cells maintain this gradient using a pump embedded in every cell membrane. This pump constantly pushes potassium in and sodium out, consuming energy with every cycle. The resulting electrical charge across the membrane is what allows nerve cells to fire signals and muscle cells to contract on command.
How K+ Powers Your Nerves and Heart
Every time a nerve fires or your heart beats, potassium is involved. At rest, your cells hold a slight negative charge inside (about -85 millivolts), largely because of that high potassium concentration. When a nerve signal arrives, sodium rushes in and flips the charge positive. Then potassium channels open, potassium flows out, and the cell resets to its resting state. This cycle repeats millions of times a day across your nervous system and heart.
Because the heart depends on precise electrical timing, even small shifts in blood potassium can disrupt its rhythm. A departure of less than 1 mEq/L from the normal range is associated with significant health risks, which is why doctors pay close attention to K+ levels on blood work.
How Your Body Controls K+ Levels
Your kidneys are the primary regulators. They filter all the potassium in your blood, reabsorb most of it in the early parts of the kidney’s filtering tubes, and then fine-tune how much gets excreted at the end. Two types of specialized cells in the kidney’s collecting ducts handle this: one type secretes potassium into urine, the other reabsorbs it back.
A hormone called aldosterone is the main dial your body turns to adjust potassium excretion. When potassium levels rise, aldosterone increases, which ramps up the kidney’s potassium pumps and channels to flush more out. When you eat a lot of potassium-rich food, this system kicks into higher gear. When your sodium intake drops, the system shifts to prioritize sodium retention, which can slow potassium excretion.
What Normal K+ Levels Look Like
On a standard blood test, normal serum potassium falls between 3.5 and 5.5 mEq/L. That range is narrow for good reason. Levels above 5.5 mEq/L need prompt evaluation. Levels above 6.5 mEq/L are treated as an emergency because the risk of dangerous heart rhythm problems climbs steeply.
On the low end, potassium below 3.5 mEq/L is considered hypokalemia, and levels under 3 mEq/L can cause serious complications including muscle paralysis that starts in the legs and can progress upward to affect breathing.
High K+ (Hyperkalemia)
The most common causes of elevated potassium include kidney disease (particularly when kidney filtration drops below about 30% of normal), certain medications, and conditions that cause widespread cell damage. Crush injuries, severe burns, and the breakdown of tumor cells during cancer treatment can all release large amounts of intracellular potassium into the bloodstream at once. Insulin deficiency, as seen in diabetic emergencies, also drives potassium out of cells.
Symptoms of high potassium include weakness, fatigue, palpitations, and in severe cases, muscle paralysis or fainting. The heart is especially vulnerable. As levels rise, characteristic changes appear on an electrocardiogram in a predictable sequence: tall, peaked T-waves at 5.5 to 6.5 mEq/L, loss of P-waves at 6.5 to 7.5 mEq/L, widening of the main heart signal at 7 to 8 mEq/L, and dangerous rhythm disturbances or cardiac arrest at levels above 8 mEq/L.
It’s worth knowing that not every high reading on a blood test is real. The most common apparent cause of hyperkalemia is “pseudohyperkalemia,” where the blood sample itself was damaged during collection. Squeezing the fist too hard, leaving a tourniquet on too long, or rough handling of the sample can all break open red blood cells and release their potassium, giving a falsely elevated result.
Low K+ (Hypokalemia)
Low potassium most often results from losing too much rather than eating too little. Chronic diarrhea is the most common non-kidney cause. Vomiting, excessive sweating, and certain diuretics (water pills) are also frequent culprits. Eating disorders involving laxative or diuretic misuse are a recognized cause in younger patients. Rare genetic conditions like Bartter syndrome and Gitelman syndrome cause the kidneys to waste potassium continuously.
Symptoms include muscle weakness, cramping, fatigue, palpitations, and constipation (because the smooth muscle lining the gut slows down). In severe cases, weakness starts in the legs and climbs upward, a pattern called ascending paralysis, and can eventually compromise the muscles used for breathing. On an electrocardiogram, low potassium produces flattened T-waves, prominent U-waves (an extra bump after the T-wave), and can trigger dangerous fast heart rhythms.
Medications That Shift K+ Levels
Diuretics are the most common drug-related cause of potassium imbalance. Loop diuretics and thiazide diuretics tend to lower potassium by increasing kidney excretion. On the other side, ACE inhibitors (commonly prescribed for blood pressure) can raise potassium, especially in people with reduced kidney function, adrenal problems, or those also taking anti-inflammatory drugs like ibuprofen. Anti-inflammatory drugs on their own can occasionally push potassium up in people with kidney issues. If you’re on any of these medications, routine blood work typically includes a K+ check for this reason.
How Much Potassium You Need Daily
The National Academies of Sciences set adequate intake levels for potassium in 2019. Adult men need about 3,400 mg per day, and adult women need about 2,600 mg. During pregnancy, the target rises to 2,900 mg, and during breastfeeding, to 2,800 mg. Children’s needs are lower, ranging from 2,000 mg at ages 1 to 3 up to 2,300 to 3,000 mg during the teen years depending on sex.
Potassium-rich foods include bananas, potatoes, spinach, beans, avocados, and yogurt. Most people can meet their needs through diet alone. These intake targets do not apply to people with kidney disease or those taking medications that impair potassium excretion, since their bodies handle potassium differently.