Now that we have supplied our tissues with oxygen and removed carbon dioxide, we need to bring them some fuel for metabolism in the form of organic compounds. Figure 8.12 in your textbook provides a good overview of this topic.
Digestion is the process of converting food to monomers.
The primary site of absorption of the products of digestion in vertebrates is the small intestine. Folds called villi vastly increase the surface area of the small intestine. The small intestine is a good example of a columnar epithelium. New epithelial cells are born in the small intestine every day. They replace those that have died.