SMALL INTESTINE

The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pancreatic duct to aid in digestion. The small intestine is about 18 feet (6.5 meters) long and folds many times to fit in the abdomen. Although it is longer than the large intestine, it is called the small intestine because it is narrower in diameter.

The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum, the shortest, is where preparation for absorption through small finger-like protrusions called villi begins.[2] The jejunum is specialized for the absorption through its lining by enterocytes: small nutrient particles which have been previously digested by enzymes in the duodenum. The main function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion that were not absorbed by the jejunum.

The small intestine is divided into three structural parts.

• The duodenum is a short structure ranging from 20 cm (7.9 inches) to 25 cm (9.8 inches) in length, and shaped like a “C”. It surrounds the head of the pancreas. It receives gastric chyme from the stomach, together with digestive juices from the pancreas (digestive enzymes) and the liver (bile). The digestive enzymes break down proteins and bile emulsifies fats into micelles. The duodenum contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in gastric chyme.
• The jejunum is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m long, and contains the circular folds, and intestinal villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The suspensory muscle of duodenum marks the division between the duodenum and the jejunum.
• The ileum: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients. The ileum joins to the cecum of the large intestine at the ileocecal junction.

• SMALL INTESTINE
  • Serosa; serous coat
  • Subserosa; subserous layer
  • Muscular layer; muscular coat
    • Longitudinal layer; long pitch helicoidal layer
    • Circular layer; short pitch helicoidal layer
  • Circular folds Kerckringi
  • Submucosa
  • Mucosa; mucous membrane
    • Muscularis mucosae
    • Intestinal villi
    • Intestinal glands
    • Solitary lymphoid nodules
    • Aggregated lymphoid nodules Peyeri
  • Duodenum
    • Superior part
      • Ampulla; duodenal cap
    • Superior duodenal flexure
    • Descending part
    • Inferior duodenal flexure
    • Inferior part; horizontal part; transverse part
    • Ascending part
    • Duodenojejunal flexure
    • Hidden part of duodenum
    • Suspensory muscle of duodenum; suspensory ligament of duodenum
      • Phrenicocoeliac part
      • Coeliacoduodenal part
    • Longitudinal fold of duodenum
    • Major duodenal papilla
    • Minor duodenal papilla
    • Duodenal glands
  • Jejunum
  • Ileum
    • Terminal ileum
    • (Ileal diverticulum)