After digestion the next step is absorption done by cells of the mucous membrane of the intestine. For this task a large absorption surface is an advantage. How is it possible in the small internal space of the body of a pluricellular organism to present a large intestinal surface?
Evolution tried to solve this problem in two ways. The simplest is the long and tubular shape of the bowels (approximately eight meters in extension), making possible that numerous small intestine loops fold closely. More efficient solutions are the intestinal villi and the microvilli of the mucosal membrane cells. The intestinal wall is not smooth. The mucous membrane, together with its submucosa, projects inside the gut lumen like glove fingers forming invaginations and villi that multiply the available surface for absorption. In addition the epithelial cells that cover these villi have themselves numerous hairlike projections called microvilli on the external face (lumen face) of their plasma membrane. The absorptive area of the intestines is thus increased hundreds of times with these solutions. In the jejunum and ileum there are folds that have the function of increasing the absorption surface too.
