The small intestine is a hollow intraperitoneal organ that develops from the distal foregut and midgut. It extends from the pylorus of the stomach to the ileocecal junction and is subdivided into the duodenum, the jejunum, and ileum. With the exception of the proximal duodenum, which is supplied by the celiac trunk, the main arterial supply for the small intestine is provided by branches of the superior mesenteric artery. The veins of the small intestine drain into the portal vein and the lymphatics eventually drain into the superior mesenteric and celiac lymph nodes. The small intestine is innervated by the sympathetic and parasympathetic nervous system, as well as the myenteric plexus and submucous plexus of the enteric nervous system. The mucosa of the small intestine contains many folds (i.e., plicae circulares, intestinal villi, and microvilli), which greatly increase its absorptive surface area. It also contains intestinal glands (crypts of Lieberkuhn) made up of enterocytes, which reabsorb nutrients, and other specialized cells of the small intestine (e.g., stem cells, Paneth cells, goblet cells, enteroendocrine cells). Pancreatic secretions and bile collect in the duodenum and break down chyme into sugars, amino acids, and fatty acids. Absorption of micronutrients and water predominantly occurs in the jejunum. Absorption of vitamin B12 and bile acids occurs in the terminal ileum.
- Middle portion of the gastrointestinal tract situated between the pylorus proximally and the ileocecal junction distally
- Length (adult): ∼ 6.5 m (21 ft)
- Subdivisions: duodenum, jejunum, and ileum
- Embryology: develops from the distal foregut and midgut
- Enzymatic breakdown of chyme into monosaccharides, amino acids, and fatty acids (predominantly in the duodenum)
- Absorption of nutrients and water (predominantly in the jejunum and ileum)
- Innate immune system and adaptive immune system function
- First and widest part of the small intestine
- C-shaped: surrounds the head of the pancreas
- Located mainly within the epigastric and umbilical regions of the abdomen
|Parts of the duodenum|
|Embryological origin||Important anatomy||Clinical significance|
|First part of the duodenum (superior) (duodenal bulb)|| || |
|Second part of the duodenum (descending)|| || || |
|Third part of the duodenum (horizontal)|| |
|4th (ascending) part|| || |
Only the 1st part of the duodenum is intraperitoneal. The 2nd–4th parts are retroperitoneal.
- Second part of the small intestine
- Located mainly in the LUQ of the abdomen
- Final and narrowest part of the small intestine
- Located mainly in the RLQ of the abdomen
- Separated from the large intestine at the ileocecal junction by the ileocecal valve:
Swallowed foreign objects are most likely to become lodged in the narrowest parts of the small intestine: the pylorus, the DJ flexure, and the ileocecal junction.
Vasculature, lymphatics, and innervation of the small intestine
|Vasculature, lymphatics, and innervation of the small intestine|
|Arteries|| || || |
|Lymphatics|| || |
Functional bowel obstruction, or paralytic ileus, occurs most commonly as a postoperative complication. Mechanical bowel obstruction of the small bowel occurs most commonly as a result of postoperative bowel adhesions.
The four histological layers of the small intestine are the same as the layers of the gastrointestinal tract.
Microscopic anatomy of the small intestine
|Microscopic anatomy of the small intestine |
|Parts of the small intestine||Mucosa||Submucosa||Muscularis propria||Serosa|
|Duodenum|| || || || |
|Jejunum|| || |
IgA is the main antibody of the intestines and is often called “secretory IgA” because it is also found in tears, saliva, mucus, and breast milk.
Specialized cells of the small intestine
|Specialized cells of the small intestine|
|Cell||Secretory product||Location and characteristics||Function|
|Enterocytes|| || |
|G cells|| |
|Paneth cells|| || |
|Goblet cells|| || || |
|Stem cells|| || || |
The diagnosis of celiac disease is confirmed via endoscopy of the duodenum. Biopsy specimens show villous atrophy, crypt hyperplasia, and intraepithelial lymphocytic infiltration.
- Neutralization of acidic chyme
- Breakdown of food by digestive pancreatic enzymes and bile
- See also “Secretory and regulatory products of the gastrointestinal tract”.
Jejunum and ileum
Absorption of water and nutrients by enterocytes
- Glucose and amino acids are cotransported with Na+ into enterocytes.
- Water is absorbed along the osmotic gradient created by the absorption of Na+.
- Bile acids emulsify fats to form micelles, which further breakdown fats into monoglycerides that are then absorbed by enterocytes.
- Most nutrients are absorbed in the jejunum.
- Vitamin B12 and bile salts are absorbed in the terminal ileum (i.e., enterohepatic circulation).
Innate immunity conferred by:
- Epithelium, which forms a physical barrier against microbial infection
- Macrophages and dendritic cells within the submucosa
- Lysozymes and defensins secreted by Paneth cells
Adaptive immunity conferred by:
- Peyer patches containing T and B lymphocytes and M cells
- Innate immunity conferred by:
- Absorption of water and nutrients by enterocytes
Vitamin B12 must be bound to intrinsic factor to be absorbed.
Dude Is Just Feeling Ill, Bro: The Duodenum is the site of Iron (Fe2+) absorption. The Jejunum is the primary site of Folate absorption. The terminal Ileum is the site of vitamin B12 (dependent on intrinsic factor) and Bile salt absorption.
- Foregut: duodenum proximal to the major duodenal papilla
- Midgut: distal duodenum, jejunum, and ileum
- See “Embryology” in “Gastrointestinal tract” for further details.
Duodenal atresia occurs when the closed duodenum fails to recanalize during development. It is frequently accompanied by further anomalies, especially Down syndrome, and appears on abdominal x-ray as a double bubble sign.
Jejunal and ileal atresia result from vascular accidents in utero, which cause ischemia, necrosis, and reabsorption of segments of the small intestine.
Meckel diverticulum is the most common congenital gastrointestinal tract anomaly and can result in intussusception, volvulus, or obstruction in the terminal ileum.
- Malabsorption syndromes
- Developmental disorders