BIOL 472 - Mammalian Physiology - Topic II - Diffusion, Ionic Concentration, Electrochemical Potential

Penn State - Fall 2026 - Dr. James Strauss

150 mmol / L

Standard concentration of Potassium (K+) in mammalian intracellular fluid

15.0 mmol/L

Standard concentration of Sodium (Na+) in mammalian intracellular fluid

9.0 mmol/L

Standard concentration of Chloride (Cl-) in mammalian intracellular fluid

5.5 mmol/L

Standard concentration of Potassium (K+) in mammalian extracellular fluid

150 mmol/L

Standard concentration of Sodium (Na+) in mammalian extracellular fluid

125 mmol/L

Standard concentration of Chloride (Cl-) in mammalian extracellular fluid

300 mOsm

Standard osmolarity of mammalian blood plasma

-70 mV

Resting membrane potential of a standard mammalian cell

Brain Ischemia

Condition in which low oxygen shuts down Na+/K+ ATPase pumps, so the ion balance is disturbed, water flows into cells, neurons swell, are damaged, and may burst

Concentration Gradient

Gradient that is much more influential in the movement of ions under normal circumstances

Non-ionic, impermeable species

What species can be completely ignored following Gibbs-Donnan principles?

[Cation]inside x [Anion]inside = [Cation]outside x [Anion]outside

Gibbs-Donnan Equation

P = RT (delta[cation] + delta[anion] + delta[A-])

Osmotic Hydrostatic Pressure Equation

Ideal gas constant, .082 L atm / K mole

R in the osmotic hydrostatic pressure equation

3 Na+ out, 2 K+ in

What movement of ions is caused by Na+/K+ ATPase pump?

3 Na+ bind to transporter, ATP binds, phosphate cleaved, binds to pump, conformational change results, Na+ released, K+ binds to outside of transporter, phosphate released, conformational change, K+ released inside

Na+/K+ ATPase Pump Steps

Around or more than 60%

How much of a given somatic cell’s energy is used to operate Na+/K+ ATPase pump?

+60 mV

Equilibrium potential for Sodium (Na+)

-90 mV

Equilibrium potential for Potassium (K+)

-70 mV

Equilibrium potential for Chloride (Cl-)

E(ion) = (RT/Fz) ln([ion]out / [ion]in)

Nernst Equation

E(ion) = (61.5/z) log([ion]out/[ion]in)

Nernst Equation at Body Temperature

8.31 joules / moleK

R in the Nernst Equation

9.64E4 joules/V

F in the Nernst Equation

Ion valence

z in the Nernst Equation

Vmv = (RT/F) ln((P[K+]out + P[Na+]out + P[Cl-]in)/(P[K+]in + P[Na+]in + P[Cl-]out)

Goldmann Field Equation

Vmv = 61.5 log((P[K+]out + P[Na+]out + P[Cl-]in)/(P[K+]in + P[Na+]in + P[Cl-]out)

Goldmann Field Equation at Body Temperature

1.0

Permeability coefficient of Potassium (K+)

0.45

Permeability coefficient of Chloride (Cl-)

0.04

Permeability coefficient of Sodium (Na+) at rest

20.0

Permeability coefficient of Sodium (Na+) in an excited state

Potassium (K+) has the highest permeability coefficient at rest because there are more non-gated channels for Potassium than any other ion

What ion’s permeability coefficient is highest at rest and why?

H gate is open, while M and N gates are closed

What are the states of H, M, and N gates at rest?

H gate closing, because it blocks sodium (Na+) from continuing to enter the cell

What action of what gate is most important in stopping depolarization, and why?

M gate

What gate is slowly opening during depolarization to allow sodium in-flow?

N gate

What gate is slowly opening during depolarization to allow potassium out-flow?

H and M gates

Which gates are associated with sodium channels?

N gate slowly closing

What action of which gate is responsible for the absolute refractory period / post-hyperpolarization overshoot?

Absolute Refractory Period

Period following depolarization where achieving a second action potential is impossible, no matter the strength of the stimulus

Relative Refractory Period

Period following repolarization where achieving a second action potential is possible, but only in the presence of an extremely strong stimulus

+30 mV

The membrane potential reached at peak depolarization

Chronaxie

Time needed to achieve 2x reobase voltage

Reobase

Minimum intensity of electrical (in mV) that still produces a response in excitable tissue

Utilization Time

Time necessary to produce reobase response

Excitability

1 / Chronaxie

Low reobase and short chronaxie

Qualities of highly excitable tissue, in terms of values derived from the strength duration curve

Less Excitable Tissue

What does a right shift in the strength duration curve indicate?

More Excitable Tissue

What does a left shift in the strength duration durve indicate?

Blocks Na+ channels so that nerves cannot produce an action potential, ultimately leading to cardiac and respiratory failure

Fugu (Tetrodotoxin)

While the body’s extracellular fluid is already high in sodium, it is low in potassium. Injecting this high concentration of potassium ions into the pig’s bloodstream raised the concentration of potassium in its extracellular fluid greatly, which upset the typical electrochemical gradient, not allowing potassium to leak out of cells the way it usually can. This causes an inability for cells to repolarize, which stops the heart

Why did the pig that medical students infused with KCl instead of NaCl convulse and die from heart palpatations?