Why is diffusion important in the gastrointestinal tract function and how is the gastrointestinal tract designed to maximise the amount of diffusion that can occur?

As you saw in yesterday's question diffusion is a mechanism by which molecules can move. Increasing surface area increases the opportunity for molecules to move and the gastrointestinal tract has a huge surface area. The gastrointestinal tract is long, which in itself increases surface area. On its surface it has tiny folds, and each of those tiny folds has even smaller folds on their surfaces. All of these folds dramatically increase the surface area available for absorption. As you saw with the respiratory system, the gastrointestinal tract, and the small intestine particular, also has a very rich blood supply that enables absorbed molecules to be quickly collected and transported to the remainder of the body.