Extracellular Ions Concentration and Their effect on Electrical Properties of Cells

The survival and function of cells depend on the maintenance of an electrochemical gradient across the plasma membrane. This gradient is the result of an unequal distribution of ions between the extracellular fluid (ECF) and the intracellular fluid (ICF), combined with the selective permeability of the cell membrane.

Key Ion Concentrations

Ion	Extracellular (ECF)	Intracellular (ICF)	Ratio (out:in)
Sodium (Na+)	145 mmol/L	12 mmol/L	12:1
Potassium (K+)	4.5 mmol/L	140 mmol/L	1:30
Calcium (Ca2+)	2.4 mmol/L	0.0001 mmol/L	24,000:1
Chloride (Cl-)	105 mmol/L	5-10 mmol/L	15:1

The Biophysical Foundation

The Nernst equation

The Nernst equation calculates the Equilibrium Potential (E) - the theoretical voltage at which the electrical force exactly balances the chemical diffusion force for a single ion. At body temperature (37 degrees Celsius), (n = charge):

$E={61.5 \over n}\times log{(conc.in) \over (conc.out)}$

Results for sodium & potassium-

Na⁺ : +60 mV

K⁺: -90 mV ( the cell is most permeable to K+ at rest)

The Goldman-Hodgkin-Katz (GHK) equation

In a cell, multiple ions are moving at once. The Resting Membrane Potential (Vm) is determined by the GHK equation, which accounts for the permeability (P) of each ion.

$Vm=61\times log{Pk[Kout]+PNa[Naout]+PCl[Clin] \over Pk[Kin]+PNa[Nain]+PCl[Clout]}$

P_k is roughly 40 times higher than P_Na. This is why the resting potential (-70 to -90 mV) is so close to the equilibrium potential of Potassium.

Effect Of Extracellular Concentration Changes

Hyperkalemia

increased extracellular K⁺, When ECF K⁺ increases, the concentration gradient decreases. According to the Nernst equation, this makes Ek less negative (more positive).

• Effect: The resting membrane potential depolarizes (moves closer to threshold).
• Clinical Result: Initially, cells become hyper-excitable, but eventually, they cannot reset after an action potential, leading to cardiac arrest.

Hypocalcemia

Decreased extracellular Ca²⁺, Calcium ions stabilize the voltage-gated sodium channels.

• Effect: Low ECF Ca²⁺ lowers the threshold for activation of Na⁺ channels.
• Clinical Result: Tetany (muscle spasms) and hyper-reflexia because nerves fire too easily.

Maintenance: the Na⁺ / K⁺ ATPasa

The gradients would eventually disappear due to "leaky" channels if not for the Sodium-Potassium Pump.

• It pumps 3 Na⁺ out and 2 K⁺ in.
• It is electrogenic, meaning it contributes directly to the negativity of the cell (-5 mV).

Sources

• https://www.google.com/search?q=https://www.physio-pedia.com/Resting_Membrane_Potential
• https://www.ncbi.nlm.nih.gov/books/NBK538338/
• Lehninger principles of Biochemistry