The small intestine (human)
The small intestine leads from the stomach to the large intestine. It consists of three segments: the duodenum, the jejunum and the ileum. Its total length is 20 feet, and its diameter varies between 2.5 and 4 cm. The small intestine is a hollow muscular tube. It has transverse (across its width) folds called plica. These plicae are covered with little fingerlike projections called villi. The plicae and the villi serve to increase the surface area of the small intestine, so that there is greater absorptive area. The more absorption occurring in a section of intestine, the more plicae and villi will be present.
The duodenum lies directly beyond the stomach. It is ca. 1 foot long. It receives the liquefied food (aka chyme) through the pyloric sphincter of the stomach. Bile from the gall bladder and pancreatic enzymes from the pancreas are added to the mix in this segment. The alkaline pancreatic juices, in addition to aiding in digestion, also serve to neutralize the acidity of the chyme (since the stomach has added a large amount of acid). Some absorption of nutrients begins towards the end of the duodenum. There are no plicae and almost no villi in the first part of the duodenum, but they increase in size and density towards the end.
The jejunum is the second segment of the small intestine. It is 8 feet long, and it is here that most of the nutrient absorption occurs. It has many plicae and a dense covering of villi.
The ileum is the last segment of the small intestine, and is 11 feet long. Most of the nutrient absorption is complete before intestinal contents reach the middle of the ileum, and by the end of this segment there are no more plicae and only scattered small villi. Some water absorption occurs here, but one of the main functions of the ileum is to act as a buffer zone protecting the small intestine from the bacteria that flourish in the large intestine. There are large clumps of lymphatic tissue called Peyer's patches under the ileal mucosa. These are the 'bases' from which the immune system fights back any bacteria that attempt to invade the nice, nutrient-rich small intestine. The ileum ends at the ileocecal valve, which joins it to the cecum, the first portion of the large intestine.