Small intestine

The small intestine runs from the pyloric sphincter of the stomach to the ileocecal valve, where it joins the large intestine. It is the longest part of the digestive tract, several meters long, and is called small only because it is narrower than the large intestine. It has three parts: the short, C-shaped duodenum, which receives bile from the liver and digestive juice from the pancreas; the jejunum; and the ileum, the longest section.

Its wall is built for absorption. Circular folds, finger-like villi and the microscopic microvilli on each lining cell, which form the brush border, multiply the absorbing surface hundreds of times. Pancreatic and brush-border enzymes, helped by bile, finish breaking carbohydrates, proteins and fats into sugars, amino acids, fatty acids and monoglycerides, which pass into blood capillaries or lymph vessels in the villi. The nutrient-rich blood then travels through the hepatic portal vein to the liver. The duodenum and jejunum absorb most nutrients, while the ileum takes up vitamin B12 and bile salts. Peristalsis and mixing contractions move the chyme along.

Disease here interferes with absorption. In celiac disease, an immune reaction to gluten flattens the villi, Crohn's disease often inflames the ileum, and loss of a large length of bowel can cause short bowel syndrome.