Intestine
A regular human usually has 20-feet long small intestines and about one inch in diameter. Their smooth, velvety tissue is divided into three parts: the duodenum, the jejunum and the ileum.
The main function of the small intestine regards the absorption of nutrients, and this process happens with the help of villi, which is a finger-like tissue covering the intestines. This structure is also covered by even smaller tissue called microvilli, and both create a surface area big enough for minerals and other helpful substances to be absorbed.