Partially digested food, known as chyme, enters the jejunum from the duodenum. As chyme enters the jejunum, it is mixed by segmentations, or localized smooth muscle contractions in the walls of the jejunum. These segmentations help to circulate chyme and increase its contact with the walls of the jejunum. The walls of the jejunum are folded many times over to increase its surface area and allow it to absorb nutrients. Each epithelial cell on the surface of the jejunum contains microscopic folds of cell membrane called microvilli that create tiny pockets and increase the contact between the cells and chyme. The entire wall of the jejunum is also folded into microscopic finger-like ridges known as villi that form larger pockets and further increase the surface area of the jejunum. At the macroscopic level, the inner surface of the jejunum contains many wrinkles of tissue known as circular folds, which create even more pockets for chyme and further increase the surface area available for absorption. Thus, the entire structure of the jejunum is optimized for the absorption of nutrients from chyme. By the time chyme has passed through the jejunum and enters the ileum, around 90% of all available nutrients have been absorbed into the body.