Why do we need potassium (K+)?|
Potassium is one of the major intracellular positive ions (cations) in the body where it constitutes about 70% of the positive ions in the cells. Other cations include sodium (NA+) and hydrogen (H+).
Potassium is critically involved in the osmotic balance of the cells (keeping the right amount of water in cells without causing cells to burst or shrink), conduction of action potentials ensuring smooth natural rhythms of heartbeat and synchronised nerve conduction. It also mediates important chemical reactions, which mediate passage of other nutrients into cells.
Deficiency in K+ can impair all these processes. In addition it can increase blood pressure, cause nephropathy, produce muscular weakness leading to paralysis and respiratory failure.
Distribution of potassium in the body
The total potassium content in the body is around 3000mmol. Of this around 98% is contained within the cells of the body. The normal recommended daily intake is around 60-100mmol and most western diets have a value over this. The plasma (the fluid non-cellular content of blood) contains only 2% of the total body potassium (which is around 60mmol). But it is this potassium that affects nerve, muscle and heart function. Any decrease in the concentration of potassium outside cells, mainly plasma potassium, is given the clinical term hypokalaemia.
The average concentrations of potassium for a healthy individual are 150mmol/L within the cells and 4mmol/L in the fluid outside cells. Continuous plasma filtration by the kidney would result in a daily loss of around 800mmol.
However regulation in the tubules of the kidney ensures that 90% of the lost K+ is reabsorbed and net daily loss through the kidneys is only 10% of that filtered. Another 60mmol/day is secreted into the intestinal of which most is reabsorbed. Sweating can also cause a very minute loss of K+. In a normal healthy individual absorption from the gut offsets the loss from the kidneys and into the intestines thereby preventing any imbalance.
What causes a deficiency of potassium in the body?
The possibilities are:
- Reduced intake - this happens in starvation or in a K+ deficient diet, as in the case of individuals with eating disorders such as anorexia nervosa and bulimia nervosa. Increased intestinal loss can occur from chronic diarrhoea and excess vomiting and from use of certain medications such as diuretics, cortisone and laxatives.
- Increased loss through the kidneys - this happens in most forms of renal failure and diabetic acidosis. Treating a dehydrated person after trauma without K+ supplementation can dilute the existing potassium in the body and in terms of concentration would result in its deficiency.
- Increased loss through the GI tract - diarrhoea, vomiting, and other gastrointestinal problems may rapidly reduce potassium. Infants with diarrhea must be watched closely for low blood potassium, termed hypokalemia. Diabetes and renal disease may cause low as well as high potassium levels.
- Treatment of normal fluid loss without K+ supplements - Several drugs can cause hypokalemia-diuretic therapy is of most concern; long-term use of laxatives, aspirin, digitalis, and cortisone may also deplete potassium.
There are other factors associated with fluctuations in the body's potassium content, which include diabetes, and the general process of ageing. The body also has certain co-transporters, which are specialised enzymes or molecules contained within cell walls that transport ions in-to and out-of cells.
Co-transporters are especially numerous in the kidney, which co/counter-transport K+ with Na+ and K+ with H+. Therefore a sodium or hydrogen ion imbalance can affect the potassium status of the body and vice versa. This occurs in adrenal gland hyper-function where there is excess aldosterone.
Aldosterone activates the Na-K co-transpor