The property of a membrane where some substances can cross while others cant

The membrane is selectively permeable because substances do not cross it indiscriminately. Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot.

Can you include the paragraph too ?

Na+ loss through sweet plus the dilution has lowered na+ concentration outside of neurons, altering the na+ equilibrium potential. sodium exists more outside the cells than inside. when na+ is lost by sweat plus the dilution( because they drink a lot of water) changes the concentration outside the cell, forcing the water to enter the cell and attempt to balance the concentration inside/outside.the problem is that the abundance of water will cause the cells to swell. most cells can adapt, however, the brain cells can't that's why the runner has confusion and can't control the muscles.  if not treated and this occurs in less than 48 hours, it can be immediately fatal .