The area of the membrane matters
One more thing, before we get too enthusiastic about our new equations. We can imagine a gallon of salt water and a gallon of distilled water with a membrane (the partially permeable kind) between them, and we know that the thickness of the membrane is taken care of as a part of P ... but how wide and how tall is the membrane?? It will make a big difference if the membrane is 1 square centimeter or 1 square meter in area!! In fact, I would be willing to take bets that if the membrane has 10 times the area, it will let 10 times more stuff through (that would be a linear relationship).
This issue becomes pretty important when we start thinking about scaling and things like the surface area of the lungs.
Small Membrane to left
Large Membrane on right
Copyright University of Maryland, 2007
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