The walls of the small intestine are folded by what?

The small intestine is the part of the gastrointestinal tract where the absorption of nutrients and minerals occurs. Because of that, surface of the small intestine needs to be increased. The inner wall (mucosa) is lined with epithelial tissue and covered with folds called plicae circulares. From the plicae circulares extend finger-like pieces of tissue called villi. The individual epithelial cells of mucosa also contain finger-like extensions-microvilli. Thus, plicae circulares, villi and microvilli are structures that improve absorption by increasing the surface of small intestine.