For an idea on how it works, think of when you take a bath, if you stay too long your fingers wrinkle. They wrinkle because your body has more salt than the bath water, so your body salt diffuses into the tub. Just like if there is some chemical that in the tub that is more than in your body, it will diffuse into your body. So, until there is a balance it will diffuse across the membrane. And since all chemicals have a "size" you use different membranes to allow different chemicals to pass through.
Atleast this is my old grade 11 chemisty understanding talking
