About 60% of an adult's body weight is water, and that water lives in two main compartments separated by cell membranes. The intracellular fluid (ICF) sits inside cells; the extracellular fluid (ECF) sits outside them. Each compartment has a characteristically different mix of electrolytes, and the body works hard to keep both volumes and concentrations stable. When something pushes that balance — vomiting, dehydration, heart failure — it shows up as edema, low blood pressure, or muscle cramps. Knowing which ions belong where, and which hormones move them, is the entire topic.
The Two Fluid Compartments
In a 70-kg adult, total body water is about 42 liters — roughly 60% of body weight. It splits like this:
- Intracellular fluid (ICF) — about 28 L (two-thirds of total). Everything inside cells.
- Extracellular fluid (ECF) — about 14 L (one-third of total). Everything outside cells, divided into:
- Interstitial fluid — about 11 L, the fluid between cells, bathing them in solution.
- Plasma — about 3 L, the liquid portion of blood.
A small amount of ECF also sits in specialized compartments — cerebrospinal fluid, joint fluid, the fluid inside the eye — collectively called transcellular fluid.
The dividing line between ICF and ECF is the plasma membrane, which is selectively permeable. The dividing line between plasma and interstitial fluid is the capillary wall, which is freely permeable to water and small ions but blocks most proteins. Those two boundaries — and what each lets through — are what generate the electrolyte differences between compartments.
The Major Electrolytes in Each Compartment
An electrolyte is a substance that dissociates in water into ions. The body's main electrolytes are sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphate. Knowing which ones dominate which compartment is half of the topic.
ECF (interstitial fluid and plasma) is high in:
- Sodium (Na⁺) — the dominant cation, about 142 mEq/L. It is the main driver of ECF volume because it pulls water with it osmotically.
- Chloride (Cl⁻) — the dominant anion, about 105 mEq/L.
- Bicarbonate (HCO₃⁻) — about 24 mEq/L, the body's main extracellular buffer.
ICF is high in:
- Potassium (K⁺) — the dominant cation, about 140 mEq/L. Outside cells it sits at only about 4 mEq/L.
- Phosphate (HPO₄²⁻) — the dominant anion, plus negatively charged proteins.
- Magnesium (Mg²⁺) — much higher inside cells than outside.
The contrast is striking: a tenfold sodium gradient (high outside, low inside) and a thirty-fold potassium gradient (high inside, low outside) define almost every cell in the body. These gradients are maintained continuously by the sodium-potassium ATPase pump, which moves three Na⁺ out for every two K⁺ in. The same pump that sets up resting membrane potential in neurons is doing this work everywhere.
What Moves Water Between Compartments: Osmosis
Water moves passively across cell membranes by osmosis — from where solute concentration is lower to where it is higher, until both sides are equal. The combined concentration of all dissolved particles is the osmolarity, and normal body fluid osmolarity is about 290 mOsm/L in both ICF and ECF.
That equality is what keeps cells the right size. If ECF becomes more concentrated (high osmolarity), water leaves cells and they shrink. If ECF becomes less concentrated (low osmolarity), water enters cells and they swell. Brain cells are especially sensitive — large shifts in plasma osmolarity cause confusion, seizures, and death.
Because sodium is the dominant ECF solute, sodium concentration largely determines ECF osmolarity. The body therefore controls fluid balance mainly by controlling sodium and water together.
The Hormones That Run Fluid Balance
Three hormones do most of the regulation.
- Antidiuretic hormone (ADH) — released from the posterior pituitary when plasma osmolarity rises (you are dehydrated). ADH tells the kidney's collecting ducts to reabsorb more water, producing a small volume of concentrated urine. It moves water without sodium.
- Aldosterone — released from the adrenal cortex when blood pressure or sodium falls. It tells the kidney's distal tubule and collecting duct to reabsorb sodium (and water follows it) and excrete potassium. It moves sodium and water together.
- Atrial natriuretic peptide (ANP) — released from the atria when blood volume is high (stretched atria). It does the opposite of aldosterone: it causes the kidneys to excrete sodium and water, dropping blood volume and pressure.
Two extra hormones manage specific electrolytes. Parathyroid hormone (PTH) raises blood calcium; calcitonin lowers it. Both are covered in the endocrine system and its hormones.
What Happens When Balance Breaks
Three common disturbances show how the compartments interact.
Dehydration. You lose water and sodium together — sweating, vomiting, diarrhea. ECF volume drops, blood pressure falls, ADH rises, and aldosterone rises to retain both. If you drink only water without electrolytes, you can correct volume but dilute your sodium, causing hyponatremia — low blood sodium, which lets water flood into cells and can cause brain swelling.
Edema. Too much fluid in the interstitial space, usually because something disturbs the balance at the capillary. The classic causes: high venous pressure (heart failure pushes fluid out at the venous end of capillaries), low plasma protein (liver disease drops albumin, so water leaks out), or blocked lymphatic drainage. Edema is interstitial fluid, not blood — the volume is in the wrong ECF subcompartment.
Hypokalemia and hyperkalemia. Plasma potassium has a tight normal range (~3.5–5.0 mEq/L) because nerve and muscle cells depend on the K⁺ gradient. Vomiting and diuretics drop it (hypokalemia, causing weakness and arrhythmias); kidney failure and severe acidosis raise it (hyperkalemia, also causing arrhythmias). Small changes in plasma potassium have outsized effects because the resting membrane potential of every excitable cell tracks it.
Getting Help
The kidneys are the central regulator of fluid and electrolyte balance — every hormone above acts on the nephron. See how nephrons filter blood for the mechanism behind the controls. For more physiology walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
Fluid and electrolyte balance is mostly about two compartments — ICF and ECF — and the ions that dominate each. Sodium and chloride drive ECF volume; potassium and phosphate dominate the ICF. Water moves between them by osmosis, so anything that changes plasma osmolarity (especially sodium) shifts water in or out of cells. Three hormones — ADH, aldosterone, and ANP — manage the system at the kidneys. Most clinical disturbances of fluid and electrolyte balance can be diagnosed by figuring out which compartment is too full or too empty, and which ion has shifted.
