Electrolytes in the Body: Sodium, Potassium and Chloride

Sports drinks promise to “replace lost electrolytes”, doctors order “electrolyte panels”, and oral rehydration salts save lives in cholera outbreaks. But what exactly is an electrolyte, and why does the body care so much about a few dissolved ions? The answer comes down to simple chemistry — charged particles in water — put to work in some of the most important jobs your body does: firing nerves, contracting muscles and holding water in the right places.

What is an electrolyte?

In chemistry, an electrolyte is a substance that conducts electricity when dissolved in water or melted, because it breaks up into free-moving ions (see what is an ion?). Table salt is the classic example:

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

In medicine, “electrolytes” usually means the ions dissolved in body fluids. The main ones are:

Ion	Main role
Sodium, Na⁺	Main positive ion outside cells; controls fluid volume, nerve signals
Potassium, K⁺	Main positive ion inside cells; resting membrane potential, heart rhythm
Chloride, Cl⁻	Main negative ion outside cells; balances sodium, stomach acid
Hydrogencarbonate, HCO₃⁻	Blood buffer (acid–base balance)
Calcium, Ca²⁺	Muscle contraction, nerve transmission, blood clotting, bones
Magnesium, Mg²⁺	Enzyme cofactor, especially for reactions involving ATP
Phosphate, HPO₄²⁻/H₂PO₄⁻	Bones, ATP, DNA, buffering inside cells

This article focuses on the big three: sodium, potassium and chloride.

Why these elements?

Sodium and potassium are both alkali metals in group 1 of the periodic table (see sodium and potassium). Each easily loses its single outer electron to form a +1 ion. Chlorine, a halogen in group 17, gains one electron to form Cl⁻ (see chlorine). All three ions are very soluble and very stable in water, which makes them ideal charge carriers in a watery body.

Na⁺ and K⁺ have the same charge but different sizes: potassium has one more electron shell, so K⁺ is larger. That small difference is crucial, because protein channels in cell membranes can tell them apart by size and by how tightly each ion holds its surrounding water molecules. A potassium channel lets K⁺ through roughly a thousand times more readily than the smaller Na⁺.

Two sides of the membrane

The most important fact about sodium and potassium is where they are:

Ion	Outside cells (blood plasma)	Inside cells
Na⁺	about 140 mmol dm⁻³	about 10–15 mmol dm⁻³
K⁺	about 4 mmol dm⁻³	about 140 mmol dm⁻³
Cl⁻	about 100–105 mmol dm⁻³	about 5–15 mmol dm⁻³

Sodium is concentrated outside, potassium inside. These gradients don’t happen by accident. Every cell membrane contains a protein called the sodium–potassium pump (Na⁺/K⁺-ATPase). Each cycle, it uses the energy from one ATP molecule to push 3 Na⁺ out and pull 2 K⁺ in (see ATP). This pump is so important that it uses a large share of the body’s resting energy — in nerve cells, a majority of their ATP.

Why spend so much energy? Because a concentration gradient is a store of energy, like water held behind a dam. Cells release it when they need to.

Electrolytes and the nerve signal

Because potassium is slightly able to leak out of cells through open potassium channels, and negative charges (mainly on proteins) are left behind, the inside of a resting nerve cell is negative — about −70 mV compared with the outside. This is the resting potential.

When a nerve fires:

1. Sodium channels open. Na⁺ rushes in, down its concentration gradient. The inside becomes positive (up to about +30 to +40 mV). This is depolarisation.
2. Sodium channels close, and potassium channels open. K⁺ flows out, returning the inside to negative. This is repolarisation.
3. The pump slowly restores the original concentrations.

This wave of ion movement, the action potential, travels along the nerve fibre. Only a tiny fraction of the ions actually move in each signal, so a nerve can fire thousands of times before the gradients run down. Many toxins work by blocking exactly these channels (see venoms and toxins).

Muscles and the heart

Muscles use the same electrical system. A nerve signal triggers an action potential in the muscle cell, which causes calcium ions to be released inside the cell, and calcium switches on contraction.

The heart is especially sensitive to potassium. Its rhythm depends on precise timing of ion movements, and blood potassium must stay in a narrow range, roughly 3.5–5.0 mmol dm⁻³. Both too little (hypokalaemia) and too much (hyperkalaemia) can cause dangerous heart rhythms. That’s why doctors monitor potassium carefully in people taking certain medicines or with kidney disease.

Sodium and water: where water goes, sodium leads

Water moves across membranes by osmosis, towards the side with more dissolved particles. Because sodium is the main particle outside cells, the amount of sodium in the body largely controls the volume of extracellular fluid, including blood volume (see water in the human body).

• Eat a salty meal, and you feel thirsty: the body wants water to dilute the extra sodium.
• Hold on to more sodium, and you hold on to more water, raising blood volume and, for many people, blood pressure. This is why high salt intake is linked with high blood pressure.

The kidneys fine-tune sodium balance, guided by the hormone aldosterone, which tells them to reabsorb more sodium (and excrete potassium in exchange). See the kidneys as chemical filters.

Chloride: the quiet partner

Chloride usually follows sodium, keeping the fluids electrically neutral. It also has jobs of its own:

• Stomach acid: cells in the stomach lining secrete hydrochloric acid (H⁺ and Cl⁻), giving gastric juice a pH around 1.5–3.5 (see the chemistry of digestion).
• Chloride shift: in red blood cells, Cl⁻ swaps places with HCO₃⁻ as carbon dioxide is carried from tissues to the lungs.
• Mucus and sweat: chloride channels control the water content of mucus. In cystic fibrosis, a faulty chloride channel (CFTR) makes mucus thick and sticky; the classic diagnostic test measures the high chloride content of sweat.

Where we get them and how we lose them

• Sodium and chloride: mostly from salt. Much of the salt people eat is already in processed foods such as bread, cheese, processed meats and ready meals.
• Potassium: fruit and vegetables (bananas, potatoes, beans, leafy greens), dairy, nuts.

Most electrolytes are lost in urine, with the kidneys adjusting output to match intake. Smaller amounts leave in sweat (mainly sodium and chloride) and faeces. Diarrhoea and vomiting can cause large, rapid losses.

When electrolytes go wrong

• Hyponatraemia (low sodium): often caused by drinking huge volumes of water during endurance events, certain medicines, or illness. Water moves into cells, and brain swelling causes confusion, headache and, in severe cases, seizures.
• Hypernatraemia (high sodium): usually due to dehydration — not enough water rather than too much salt.
• Hypokalaemia (low potassium): from vomiting, diarrhoea or some diuretic medicines; causes weakness, cramps and heart rhythm problems.
• Hyperkalaemia (high potassium): often due to kidney failure; can be life-threatening to the heart.

Oral rehydration: chemistry that saves lives

Severe diarrhoea, such as from cholera, can kill by draining water and electrolytes. Oral rehydration solution (ORS) — a precise mix of glucose, sodium chloride, potassium chloride and a citrate or hydrogencarbonate salt dissolved in clean water — is a strikingly effective treatment. It works because cells lining the gut have a transporter (SGLT1) that pulls sodium and glucose in together. Water follows by osmosis. Neither salt water nor sugar water alone works nearly as well. This simple use of membrane chemistry is credited with saving millions of lives since the 1970s.

Do you need sports drinks?

For most everyday exercise lasting under about an hour, water is enough; a normal diet replaces the electrolytes lost in sweat. For long events in hot conditions, heavy sweaters may lose several grams of sodium, and drinks or foods containing sodium help prevent both cramps and dangerous hyponatraemia.

Key takeaways

• Electrolytes are ions dissolved in body fluids; sodium, potassium and chloride are the most abundant.
• The Na⁺/K⁺ pump keeps sodium mostly outside and potassium mostly inside cells, using ATP.
• These gradients power nerve signals and muscle contraction; the heart is very sensitive to potassium.
• Sodium controls fluid volume because water follows it by osmosis.
• The kidneys balance electrolytes; ORS uses sodium–glucose co-transport to rehydrate.

For how these ions show up in a medical test, see blood chemistry: what a blood test measures.