Inner Nature: Sodium and Potassium — The Yin and the Yang

By Vidya Rajan, Columnist, The Times

In previous articles, I have examined the various roles that oxygen and iron play in living organisms. These elements are linked in an energy-cycling relationship – iron moves oxygen and electrons around the body, as well as around the inside of the cell itself to harvest energy from food [1]. In this article, I will examine another dynamic relationship: that of sodium and its alter-ego, potassium. Although we tend to demonize sodium in the diet and glorify potassium, they operate together like yin and yang in many physiological functions in animals.

Sodium and potassium are both essential nutrients. We do not usually lack sodium in our diet because it is often used as a preservative and taste-enhancer. Consider that farmers provide a salt lick for cows, sweat bees lick salt from sweat and tears, and the recommended treatment for cholera is rehydration with water with an added pinch of salt and sugar. But we are advised not to drink sea water, and to limit salt levels in food. The absence of dietary salts can wreak havoc with animal metabolism, but too much salt can cause dehydration, madness and death. Both in the absence and presence of sodium, cramping and ultimately death can occur due to myriad problems: pathological changes in blood volume and pressure, kidney failure, muscle cramping, confusion, seizures, and coma.

Both sodium and potassium are alkali metals (group 1 on the periodic table), named because they making strong alkalis, such as sodium hydroxide. Sodium is lighter than potassium, with an atomic number of 11. Both are corrosive and reactive metals, potassium marginally more so than sodium. But when they are combined with the highly reactive and poisonous gas, chlorine (Cl), they make the familiar, unreactive table salt, sodium chloride (NaCl), or the less familiar potassium chloride (KCl). In animals, their intake is assured in marine organisms because they live in water in which these salts are present. The challenge for marine animals is to remove excess salt from their bodies so as not to become dehydrated or pickled. Freshwater and terrestrial organisms on the other hand, must actively ingest these electrolytes and avoid losing too much in urine and sweat. That begs the question: What role do these elements play, such that such that excess or paucity are both detrimental?

Sodium and potassium ions have a single positive charge, and their yin and yang arise from their concentrations inside and outside the cell and their movement from one location to another. Since the movement of positive charges is exactly the reciprocal of the movement of negative charge, their movement relates to the movement of electricity in the opposite direction. The body hacks this property of ions to underpin some of their most important physiological functions, such as the electrical action of nerve firing and the heart’s pacemaker action.

So what constitutes a nerve cell firing? Also known as an action potential, this event underlies how nerves work to stimulate outcomes as diverse as memory and muscle movement. The key feature of conducting cells is the difference in charge (called the voltage or potential difference) due to concentration of these ions inside and outside the cell. The cell’s surroundings are positive and the cell contents are negative, and function like the terminals of a battery. Such a cell is called a polarized cell, with the negative pole on the inside and the positive pole on the outside. When a nerve is stimulated, little passages open up in the membrane and allow the positive sodium ions outside the cell to flood into the cell causing a depolarization; the key to the firing. Depolarization stimulates a wave of movement of ions right down the nerve which acts like a wire, conducting this information down its length. When it reaches the nerve terminal, the nerve releases neurotransmitters to communicate the information the next cell. Then the nerve takes a pause to reset the polarity and is ready to fire again. This is the yin and yang of sodium and potassium. Sodium is necessary for depolarization or the action of nerve firing and potassium to restore the nerve’s polarity in order to fire again.

When there is either a paucity or an excess of either sodium or potassium, the fine balance of polarization and depolarization is upset, and nerve firing is compromised. Sodium and potassium also have high osmotic pressure and draw water towards themselves. If there are high electrolyte levels in blood, blood volume increases, causing high blood pressure. High electrolyte levels in tissues can cause water to pool, a condition known as edema, often seen in people with poor circulation or kidney disease. That’s why is is important to lower your salt consumption if you have heart or kidney disease. On the other hand, if electrolyte levels are too low, then the reverse can happen, with blood pressure dropping and nerve transmission faltering. So, it’s important to keep the water-electrolyte balance within a narrow band, not too much but not too little.

That, then, is the rationale for not drinking salt water when you are marooned on the ocean. You will go mad with your nerves firing all over the place, your kidneys overloading, your blood pressure skyrocketing and your heart beating erratically.

Just ask Coleridge’s Ancient Mariner:

“Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink.”