Sodium and potassium are both cations, which is an ion with a net positive charge. An ion is a group of atoms that have an electrical charge. Sodium is the main cation of extracellular fluid and potassium the major cation of intracellular fluid.
The Sodium Potassium Pump is what maintains the balance. It's constantly working to maintain a normal potassium level of 3.5 to 5 mEq/L in extracellular fluid and to maintain a normal sodium level of 135 to 145 mEq/L in intracellular fluid.
Ann Crawford in her article "Balancing act: Sodium and Potassium" in the July issue of Nursing (pp. 44-50) describes the pump as the main mechanism of moving sodium "from inside cells to the extracellular compartments, and returns potassium from the extracellular compartments into cells using adenosine triphosphate (ATP) as an energy source."
She likewise explains that electrolytes tend to move from areas of high concentrations to lower concentrations. So the body naturally works to maintain potassium inside cells 35 times greater than outside cells so potassium has a tendency to want to get out of cells. Likewise, sodium outside cells is kept 14 times greater than inside cells so sodium has a natural tendency to want to go into cells.
Sodium attracts water. So if sodium levels inside the cell were to get too high the cells would absorb water and would swell and ultimately explode. Obviously this wouldn't be good.
Rene Fester Kratz in his book "Molecular & Cell Biology for Dummies" explains the pump as a protein in cellular walls and "for every round of action, the sodium potassium pump moves three sodium ions out of the cell and two potassium ions into the cell. Thus the pump creates a higher concentration of sodium outside the cell, a higher concentration of potassium inside the cell, and a greater positive charge outside the cell. These differences in ion concentration and electrical charge are important in the functioning or nerve and muscle cells in animals."
Sodium is controlled by a hormone called aldosterone secreted by the adrenal cortex, said Michael Roberts, Michael Reiss, Grace Monger in their book "Advanced Biology" (2000, U.K, page 290). Aldosterone increases the absorption of sodium ions by the gut, and this causes the sodium concentration in the blood to rise.
When sodium in the blood is high, the adrenal cortex is sent a signal to decrease aldosterone production. If sodium in the blood is low, the adrenal cortex is sent a signal to increase aldosterone production. When sodium levels in the blood falls, less fluid is absorbed into the blood and the blood pressure falls.
When this happens, renin is released into the blood stream where "it catalyses the conversion of one of the plasma proteins into a substance called angiotensin. This then stimulate the adrenal cortex to secrete aldosterone," according to Advanced Biology.
A rise in sodium has the reverse effect, where less renin and angiotensin are produced and less aldosterone is produced.
It should likewise be noted here that as the concentration of sodium in the blood rises, the potassium concentration in the cell falls. The sodium potassium pump then works hard to maintain a balance of sodium and potassium inside and outside cells and a positive charge outside cells. This is important for normal body function.
Sodium works to maintain acid base balance by combining with an anion such as chloride to form sodium chloride or bicarbonate to form sodium bicarbonate. These act as buffers to absorbed hydrogen ions to help maintain a normal cellular and blood pH.
Crawford further explains that sodium also functions to:
- Promote transmission of nerve impulses
- Maintain intracellular osmality
- Activate several enzymatic reactions
- Assist with regulation of acid base balance
- Promote mycardial, skeletal, and smooth muscle contractility
Potassium, along with working with sodium to maintain electrical neutrality, plays a major role in cellular metabolism, "especially in protein and glycogen synthesis and in the enzymatic processes necessary for cellular energy, according to Crawford.
It's critical to maintain many body functions, such as:
- Acid base balance
- Nerve impulse conduction
- Maintenance of normal cardiac rhythm
- Skeletal smooth muscle contraction
Other than the sodium potassium pump, potassium levels are regulated by the kidneys and the adrenal cortex, and any excess potassium is excreted by the feces and sweat.
Chris O'Callaghan in his book "The Renal System at a Glance" (2006, UK, page 53) said that other than the sodium potassium pump, potassium levels are regulated by the kidneys and the adrenal cortex, so abnormally high or low potassium is a sign of adrenal cortex or kidney abnormalities.
Callaghan said that a rise in potassium ions in extracellular fluid of the adrenal cortex directly stimulates aldosterone release which ultimately leads to an increase in sodium re-absorption by the kidneys and potassium excretion into the bloodstream by the kidneys.
Crawford explains that there is no mechanism to notify your kidneys of a potassium deficit, so it may continue to excrete potassium even when levels are low.
Crawford also explains that sodium is exchangeable with hydrogen ions, and therefore changes in acide base balance will effect potassium levels. "Acidosis causes an increase in hydrogen ions in extracellular fluid; to maintain pH, some hydrogen ions shift to intracellular fluid. To maintain intracellular electrical neutrality, some potassium ions shift to the extracellular fluid, which may cause hyperkalemia.
"Conversely, in alkalosis, more hydrogen ions are present in the intracellular fluid, so some hydrogen ions move to the extracellular fluid to buffer and maintain pH. This again produces inequities in the intracellular ion electrical charges; potassium ions compensate by moving from the extracellular fluid to the intracellular fluid, causing hypokalemia.