Na⁺

The main cation (positively charged ion) of the extracellular fluid.

Sodium (Na⁺) plays an important role in several physiological processes such as the action potential of neurons and muscle cells, secondary active, sodium-coupled transport of ions, nutrients, neurotransmitters across the plasma membrane of cells, and many other processes.

The extracellular concentration of Na⁺ is about 145 mM. The intracellular concentration of Na⁺ is about 15 mM.