Ions and electrolytes physiology

Author: Scott A. Sheffield MS

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More than 70% of our body substance is fluid. Mostly, this fluid consists of various ions and substances dissolved or suspended in water. There are two major compartments in which the fluid is stored:

  • Intracellular fluid (ICF), which refers to the fluids bound within our body cells and it amounts to about one third of body fluids
  • Extracellular fluid (ECF), which comprises the fluids outside of the cells

Among other things, bodily fluids contain various ions. An ion is an atom or molecule of a certain chemical element that is electrically charged. Positively charged ions are called cations, while negatively charged ions are called anions. If ions are water soluble, they dissolve in the water and create electrolytes.

The composition of intracellular and extracellular fluids differs significantly. Intracellular space consists mostly of potassium, magnesium, and phosphate ions. In turn, extracellular fluid consists predominantly of ions of sodium, chloride and bicarbonate, as well as different compounds and molecules for cells’ nutrition, such as oxygen, glucose, fatty acids and amino acids.

Key termsDefinition
IonElectrically charged atom or molecule
AnionNegatively charged ion
CationPositively charged ion
ElectrolyteA substance within which ions are dissolved. It can be decomposed into ions with electrolysis.

Intracellular vs. extracellular ions

The ions in the body are measured by calculating molarity and expressing it in millimoles (mM) per liter. Normal values of ions in the body are important in order to maintain essential physiological processes that depend on ions, such as muscle contraction and nerve excitation and neural impulse conduction.

Sodium (Na+) is the main extracellular cation, while chloride (Cl-) is the main anion. On the other hand, the main intracellular cation is potassium (K+). Here is the summary of the physiological values of intracellular and extracellular ions and cations:

IonIntracellular concentration (mmol/L)Extracellular concentration (mmol/L)

The physiological balance of intracellular and extracellular concentrations of sodium and potassium in particular is important for generating the electrical potential of cell membranes of excitable cells (neurons and muscles). During action potentials, the concentration of these and other ions in the cell and outside of it changes, enabling the nerve cells to send impulses and muscle cells to contract.


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