Whether oxygen diffuses from the alveoli into the blood or not is dependent upon? 1- partial pressure of nitrogen in lungs
2-partial pressure of O2 in lungs
3- partial pressure of 02 in lungs and the blood
4-partial pressure of all gases in the atmosphere and the blood
5-Partial pressure of O2 and CO2 in blood
6- partial pressure of nitrogen in the atmosphere and blood

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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1. is geographic distribution

2 is increasing complexity

3 is transitional forms