Before you can understand the physiology you need to understand the structure. Lots of integration (mixing) in systems and between systems doesn"t operate on its own. Membrane is a lot more permeable to potassium than sodium more inside the cell sodium more outside the cell. Move down concentration gradient high to low potassium wants to go out sodium wants to go in. Ions move down electrical gradient as well depending on their charge so if positive wants to move to negative. Ions have to go through a selective channel filter in channel that only allow certain ions through it. Sodium will lost some of its water through its filter binding to the water then gain the water back filters deal more with binding not with size. Disregard signs when talking about membrane potential just look at number greatest potential between +30 and -80 is -80.