Small intestine (Small bowel)

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.

Gross anatomy

Overview [1]

Anatomical subdivisions

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
1st (Superior) part (duodenal bulb)
  • Intraperitoneal
  • The gastroduodenal artery and common bile duct lie posterior to the duodenal bulb.
2nd (Descending) part
3rd (Horizontal) part
  • Anterior relation: superior mesenteric vessels
  • Posterior relation: aorta
  • SMA syndrome
4th (Ascending) part

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

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

Duodenum Jejunum Ileum
Arteries
Veins
Lymphatics
Innervation

Bowel obstruction is an interruption of the normal passage of bowel contents either due to a functional (e.g., ileus) or mechanical cause. 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.

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 [2]
Parts of the small intestine Mucosa Submucosa Muscularis propria Serosa
Duodenum
Jejunum
Ileum

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
Cell Secretory product Location and characteristics Function
Enterocytes
  • None
  • Absorb nutrients
  • Transport nutrients to the bloodstream

Enteroendocrine cells

G cells
D cells
I cells
  • Cholecystokinin
S cells
  • Secretin
K cells
  • Gastric inhibitory polypeptide
Mo cells
  • Motilin
Paneth cells
  • Lysozymes and defensins destroy bacteria.
  • TNF-α regulates the immune response to pathogens.
Goblet cells
  • Mucous
  • Glycoproteins
  • Lubricates the GIT
  • Traps bacteria
  • Aids binding of immunoglobulins to pathogens
Stem cells
  • None
  • 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.

Function

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.

Embryology

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.

Clinical significance

last updated 11/13/2019
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