In the physiological case, you are generating the voltage from a charge imbalance, making it a concentration cell. There is no actual oxidation-reduction reaction taking place, so you omit the standard reaction potential (E˚). You can think of the Nernst Equation as the net voltage being a sum of the reaction voltage and the voltage from the difference in concentration of the two half cells: Eobserved = E˚redox reaction - RT/nF ln relative concentration. In general, the chemical reaction contributes a substantial amount (on the Voltage scale) and the concentration differences contributes a small amount (on the mV scale). Because the physiology example has no actual redox reaction, the voltages are small (on the mV scale) compared to chemical reactions.
You can also plug 1 in for z in the physiology example.
