Physio - Transport Flashcards
What are the nominal intracellular and extracellular concentrations for Na+, K+, and Cl- ions?
What is the relative membrane permeability to intra/extracellular ions when a cell is at resting membrane potential?
How to calculate Net Rate of Diffusion of Non-Electrolytes?
- The higher the [con] difference (Ca - Cb), the greater the amount of substance crossing the membrane per unit time
J = PA (Ca-Cb)
- J = Net rate of diffusion (mmol/sec)
- P = Permeability (cm/sec)
- A = Surface Area for Diffusion (cm^2)
- Ca-Cb = concentration (mmol/L)
How do electrolytes diffuse across membrane?
Basics:
- PM = impermeable to ions
- Ions pass thru water-filled pores (ion channels) w/out energy
Na+ Selective Membrane:
- Na+ moves down its [con] gradient
- potential difference across membrane = generated
- Movement stops at electrochemical equilibrium
- chemical + electrical driving forces = equal + opposite
What is an Electrochemical Equilibrium Potential?
Example of Potassium
-
Electrical Potential
- difference moves K+ into cells
-
Concentration gradient
- moves K+ out of cell
RMP = permeable to potassium (via leaking channels)
Basics:
- Equilibrium = reached when chemical gradient + electrical gradient balance each other
- Unique eq value for each ion
- calculated via Nerst Equation
What is the Nernst Equation?
Nernst Equation (Equilibrium potential)
- electrical potential across the membrane that is needed to counterbalance the movement of an ion due to the [con] gradient
E(ion) = - (RT/zF)*ln([ion]in/[ion]out)
- E = electrical potential
- R = universal gas constant 2cal/mol/K
- T = temperature in K
- z = valence of ion
- ie: divide by 2 for ions w/ 2 valance e- like Ca2+ or Cl2-
- F = Faraday constant 23cal/mV/mol
What happens to Ek during hyperkalemia?
- Inside = more positive
- influx of K+ from blood
- Depolarization
- electrical gradient into cell > concentration gradient out of cell
Potassium: RMP and EK+ are not at equilibrium, but the net force is small.
- Due to leak channels, cells are VERY sensitive to K changes
- Clinical example: Ischemia causes cardiac arrhythmias
Sodium: RMP and ENa+ are not at equilibrium and the net force is large.
- Due to zero Na conductance, cells are not sensitive to Na changes at rest
- Clinical example: Hypernatremia causes water retention but no change in RMP
Chloride: RMP and ECl- are the same in magnitude and charge.
- Chloride ions are at (or close) to equilibrium
- Clinical relevance: Inhibitory potentials as small in size
What influences Resting Membrane Potential?
- Equilibrium potential of each ion = weighted
- Average of the weighted potentials determines the membrane potential
Goldman Equation:
Em = (RT/F)*ln([PKo + PNaout + PClin]/[PKin + PNain + Clout])
- Em = resting membrane potential
- P = permeability
Note: K contribued MOST for RMP
RMP Summary
Basics:
- Characteristic for all living cells
- Imbalane of ions across membrane
- More (+) outside; More (-) inside cell
- creates difference in electrical charge
- Electro-chemical gradient
- potential energy
- measure = mV
- Measure always INSIDE in relation to OUTSIDE
- Mainly determined by K+
- weighted average of the individual eq potentials
Note:
- In most excitable cells, K+ rather than Na+ plays the dominant role in determining the resting membrane potential because plasma membrane is more permeable to K+ than it is to Na+
Role of the Na-K-Pump for the RMP
- Small direct effect due to electrogenicity of the pump
- physiologically irrelevant
- Indirect, long-term effect
- physiologically crucial
Inhibition of Na-K pump:
- Intracellular Na will increase
- Intracellular K will decrease
- The cell will depolarize
- The membrane potential will reach zero
- The cell will become unresponsive and die
