Explain how the structure of the ileum is adapted to allow efficient absorption of the products of digestion into the blood.

Villi on the ileal wall greatly increase the surface area available for absorption. The larger surface area allows more digested products to be taken up and the absorption occurs more rapidly. Each villus contains a dense network of capillaries, providing a good blood supply that keeps the concentration of nutrients in the blood low, maintaining a steep concentration gradient that favours diffusion. The epithelial cells of the villi are only one cell thick, so the diffusion distance for nutrients is very short, allowing them to move quickly into the bloodstream.