Ca²⁺

Calcium (Ca²⁺) is a divalent cation. It plays an important role in physiological processes such as muscle contraction and synaptic transmission. Calcium is also an intracellular messenger.

The extracellular concentration of Ca²⁺ is about 2 mM. The intracellular concentration of Ca²⁺ is about 70 nM.

CI⁻

The main anion (negatively charged ion) of the extracellular fluid.

Cloride (Cl⁻) plays an important role in several physiological processes such as the action potential of skeletal muscle cells, CO₂ transport in blood (via Cl⁻/bicarbonate exchange across the plasma membrane of red blood cells), and many other processes.

The extracellular concentration of Cl⁻ is about 110 mM. The intracellular concentration of Cl⁻ is about 10 mM.

The rate of movement of a substance across an interface. The interface could be the plasma membrane (separating the intracellular or extracellular fluid compartments), an epithelial sheet separating two compartments, or where two solutions of different composition meet.

Flux is reported as a rate. It is the amount of substance that moves across a given interface per unit time.

Influx
Efflux
Unidirectional flux
Net flux

An equation used to calculate the equilibrium potential (V_eq.) of an ion. The equilibrium potential for an ion is also referred to as the Nernst potential for that ion. It is the membrane potential at which no net movement of the ion in question occurs across the membrane.



where V_eq. is the equilibrium potential, R is the universal gas constant, T is the temperature in Kelvin, z is the valence of the ionic species, F is the Faraday's constant, and [X]_o and [X]_i are the extracellular and intracellular, respectively, concentrations of the ion in question.

Resting Membrane Potential - Nernst Equilibrium Potential
Derivation of the Nernst Equation

K⁺

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

Potassium (K⁺) plays an important role in the action potential of neurons and muscle cells.

The extracellular concentration of K⁺ is about 4 mM. The intracellular concentration of K⁺ is about 150 mM.

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.