The jejunum is the middle section of the small intestine in vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms middle intestine or mid-gut may be used instead of jejunum. The jejunum lies between the duodenum and the ileum. The change from the duodenum to the jejunum is usually defined as the Duodenojejunal flexure and is attached, and thus "hung up", to the ventricle by the ligament of Treitz. In adult humans, the small intestine is usually between 5.5 and 6m long, 2.5m of which is the jejunum. The pH in the jejunum is usually between 7 and 9 (neutral or slightly alkaline). If the jejunum is impacted by blunt force the emesis reflex will be initiated. The jejunum and the ileum are suspended by mesentery which gives the bowel great mobility within the abdomen. It also contains circular and longitudinal smooth muscle which helps to move food along by a process known as peristalsis. The jejunum is the second portion of the small intestine, and it has a lining which is specialized in the absorption of monosaccharides (fully digested carbohydrates) and amino acids (fully digested proteins). The proteins have been broken down in the stomach by acid and an enzyme called pepsin into amino acids. The carbohydrates are broken down in the duodenum by enzymes from the pancreas and liver into sugars. Fats are broken down in the duodenum by "lipase" from the pancreas into fatty acids. Amino acid, sugar, fatty acid particles, vitamins, minerals, electrolytes and water are small enough to soak into the villi of the jejunum and drop into the blood stream. The blood takes all these nutrients to all the other parts of the body to provide fuel to do their jobs. The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut contents. The epithelial cells which line these villi possess even larger numbers of micro villi. The transport of nutrients across epithelial cells through the jejunum and ileum includes the passive transport of sugar fructose and the active transport of amino acids, small peptides, vitamins, and most glucose. The villi in the jejunum are much longer than in the duodenum or ileum. The jejunum contains very few Brunner's glands (found in the duodenum) or Peyer's patches (found in the ileum). However, there are a few jejunal lymph nodes suspended in its mesentery. The jejunum has many large circular folds in its submucosa called plicae circulares which increase the surface area for nutrient absorption.