Small intestine

Last updated: September 26, 2022

Summary

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.

Organ fact sheet: small intestine

Gross anatomy

Overview ^[1]

Characteristics
- 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
Function
- 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

Anatomical subdivisions

Parts of the small intestine

Duodenum

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
	Peritoneal relations	Embryological origin	Important anatomy	Clinical significance
First part of the duodenum (superior) (duodenal bulb)	Intraperitoneal	Foregut	The gastroduodenal artery and common bile duct lie posterior to the duodenal bulb.	Most common site of duodenal ulcers A posterior duodenal ulcer may erode the gastroduodenal artery and cause an upper GI bleed. Attached to the liver via the hepatoduodenal ligament
Second part of the duodenum (descending)	Retroperitoneal	Proximal to the ampulla of Vater: foregut Distal to the ampulla of Vater: midgut	Contains: Ampulla of Vater Major duodenal papilla: protrusion of the ampulla of Vater into the duodenum Sphincter of Oddi Minor duodenal papilla: opening of the accessory pancreatic duct	The major duodenal papilla is cannulated during ERCP. Ampulla of Vater sphincterotomy is performed for impacted distal choledocholithiasis.
Third part of the duodenum (horizontal)		Midgut	Anterior relation: superior mesenteric vessels Posterior relation: aorta	SMA syndrome
4^th (ascending) part		Midgut	Duodenojejunal flexure (DJ flexure) Junction between the ascending duodenum and the jejunum Located on the left side of the L2 vertebra Ligament of Treitz: avascular peritoneal fold that fixes the DJ flexure to the posterior abdominal wall	DJ flexure on the right side of the L2 vertebra indicates intestinal malrotation.

Only the 1^st part of the duodenum is intraperitoneal. The 2^nd–4^th parts are retroperitoneal.

Pancreatic regional anatomy Extrahepatic biliary tree Characteristic structures of the duodenum Endoscopic view of the major duodenal papilla

Jejunum

Second part of the small intestine
Located mainly in the LUQ of the abdomen
Intraperitoneal

Ileum

Final and narrowest part of the small intestine
Located mainly in the RLQ of the abdomen
Intraperitoneal
Separated from the large intestine at the ileocecal junction by the ileocecal valve:
- Muscular sphincter; that regulates the passage of fluid and nutrients from the ileum into the cecum and prohibits reflux
- Provides a mechanical barrier to bacterial migration into the small intestine

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
	Duodenum	Jejunum	Ileum
Arteries	Proximal to the major duodenal papilla (foregut derivatives): branches of the celiac trunk Gastroduodenal artery Superior pancreaticoduodenal artery Distal to the major duodenal papilla (midgut derivatives): branches of the superior mesenteric artery (inferior pancreaticoduodenal artery)	Jejunal branches of the superior mesenteric artery → intermesenteric arterial anastomoses → vasa recta (intestines)	Ileal branches of the superior mesenteric artery → intermesenteric arterial anastomoses → vasa recta Ileocolic artery supplies the distal ileum.
Veins	Superior pancreaticoduodenal vein → portal vein Inferior pancreaticoduodenal vein → SMV → portal vein	SMV → portal vein
Lymphatics	Celiac and superior mesenteric nodes	Superior mesenteric nodes
Innervation	Celiac plexus Parasympathetic: vagus nerves Sympathetic: greater and lesser splanchnic nerves	Superior mesenteric plexus Parasympathetic: vagus nerves Sympathetic: greater and lesser splanchnic nerves

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.
Arterial supply of the gastrointestinal tract Arterial supply of the duodenum, pancreas, and spleen Tributaries of the hepatic portal vein

Microscopic anatomy

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 ^[2]
Parts of the small intestine	Mucosa	Submucosa	Muscularis propria	Serosa
Duodenum	Simple columnar epithelium Structures that increase the absorptive surface area Plicae circulares: transverse folds of the mucosa and submucosa (visible to the naked eye) and most prominent in the jejunum Intestinal villi Finger-like projections of mucosa Shortest in the duodenum Tallest in the jejunum Lined by simple columnar epithelium and the specialized cells of the small intestine Each villus has an arteriole, venule, and lymphatic channel at its core. Microvilli Minute projections from the epithelial cells of the villi Give the small intestinal mucosal surface a “brush border” appearance Have a superficial layer of glycocalyx that enables binding of nutrients and enzymes Crypts of Lieberkuhn Intestinal glands Lie within the lamina propria Flanked by two adjacent villi Lined by specialized cells of the small intestine	Brunner glands Lined by columnar cells that secrete mucus and HCO^3- Secretions neutralize acidic chyme from the stomach. Meissner plexus	Inner circular smooth muscle and outer longitudinal smooth muscle layers Auerbach plexus lies between the two layers of smooth muscle.	Mesothelium of the visceral peritoneum
Jejunum	Similar to duodenal mucosa Plicae circulares are most prominent in the jejunum.	No glands Meissner plexus
Ileum	Similar to duodenal mucosa, with thinning out of plicae circulares Peyer patches Aggregates of nonencapsulated lymphoid tissue within the lamina propria and submucosa (GALT of the ileum) Contain specialized cells: Microfold cells (M cells): endocytose antigens and deliver them to antigen-presenting cells B lymphocytes in germinal centers become plasma cells and secrete IgA, which prevents pathogens from binding to and invading the intestinal mucosa.	No glands Meissner plexus

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.

Histological layers of the gastrointestinal tract Duodenum Plicae circulares Plicae circulares (valves of Kerckring) Brush border in enterocyte Intestinal villi and crypts of Lieberkuhn Brunner glands Jejunum Jejunum Peyer patches

Specialized cells of the small intestine

Specialized cells of the small intestine
Cell		Secretory product	Location and characteristics	Function
Enterocytes		None	Simple columnar epithelium Basal nuclei Lining villi and crypts of Lieberkuhn Most common cell type of the intestinal mucosa	Absorb nutrients Transport nutrients to the bloodstream
Enteroendocrine cells	G cells	Gastrin	Crypts of Lieberkuhn (lower half)	See “Secretory and regulatory products of the gastrointestinal tract”.
	D cells	Somatostatin
	I cells	Cholecystokinin
	S cells	Secretin
	K cells	Gastric inhibitory polypeptide
	Mo cells	Motilin
Paneth cells		Lysozyme Defensin TNF-α	Crypts of Lieberkuhn (base) More common in jejunum and ileum Contain acidophilic granules	Lysozymes and defensins destroy bacteria. TNF-α regulates the immune response to pathogens.
Goblet cells		Mucous Glycoproteins	Villi Crypts of Lieberkuhn Most abundantly present in the small intestine	Lubricates the GIT Traps bacteria Aids binding of immunoglobulins to pathogens
Stem cells		None	Crypts of Lieberkuhn (lower half)	Proliferate and differentiate into the various cells of the small intestine Repair mucosal injuries

The diagnosis of celiac disease is confirmed via endoscopy of the duodenum. Biopsy specimens show villous atrophy, crypt hyperplasia, and intraepithelial lymphocytic infiltration.
Chalk Talk: Regulation of gastric acid secretion Phases of gastric acid secretion Paneth cells Goblet cells in the small intestine

Function

Duodenum
- 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).
- Immunity ^[3]
  - 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
    - IgA

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.

Intestinal resorption Intestinal reabsorption of glucose

Embryology

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.

Primitive gut (4th week of development) Endodermal development Intestinal rotation

Clinical significance

Infections
Malabsorption syndromes
Developmental disorders
- Malrotation
- Duodenal atresia and stenosis
- Jejunal atresia
- Meckel diverticulum
Malignancies
Others

References

Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences ; 2016
Normal histology of the small intestine. http://www.pathologyoutlines.com/topic/smallbowelnormalhistology.html. Updated: February 9, 2018. Accessed: February 22, 2019.
Santaolalla R, Fukata M, Abreu MT. Innate immunity in the small intestine.. Current opinion in gastroenterology. 2011; 27 (2): p.125-31.doi: 10.1097/MOG.0b013e3283438dea . | Open in Read by QxMD

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