Small Intestine

The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. The small intestine is divided into 3 segments: the duodenum, the jejunum, and the ileum. Like the entire GI tract, the walls of the small intestine have several layers: an inner absorptive mucosal layer (which is made up of an epithelium, lamina propria, and muscularis mucosa) and submucosal, muscular, and serosal layers. The arterial supply to the small intestine is via branches of the superior mesenteric artery, and veins drain into the hepatic portal system. The small intestine is innervated by the ANS.

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Development

  • Embryologic origins:
    • Proximal duodenum (above the ampulla of Vater): develops from the foregut
    • Distal duodenum, jejunum, and ileum: develop from midgut
  • Development occurs primarily between the 5th and 10th weeks of fetal life
  • Ileum elongates rapidly → forms a U-shaped fold called the primary intestinal loop, which:
    • Grows around the developing superior mesenteric artery (its blood supply)
    • Herniates (protrudes) into the umbilical cord
    • Is connected to the yolk sac via the vitelline duct traveling through the umbilical cord
  • As the abdomen develops, the intestines return to the abdominal cavity
  • Undergoes a 270-degree counterclockwise rotation around the superior mesenteric artery (SMA)  → results in normal anatomic locations for the GI tract

Gross Anatomy

The small intestine is a long tubular structure in the abdomen that is responsible for approximately 90% of nutrient absorption.

General characteristics

  • Connects the stomach at its proximal end to the colon at its distal end
  • Average length: 6–7 m
  • Has 3 parts:
    1. Duodenum 
    2. Jejunum
    3. Ileum
  • Fills the majority of the mid and lower abdominal cavity
Small intestine and its parts

Small intestine and its parts

Image: “2417 Small IntestineN” by OpenStax College. License: CC BY 4.0

Duodenum

General characteristics and anatomic relations:

  • The 1st portion of the small intestines
  • Approximately 25 cm in length
  • C-shaped, curving around the head of the pancreas
  • Receives contents from the stomach at its proximal end
  • Empties contents into the jejunum at its distal end
  • Receives secretions from the pancreatic and bile ducts 

Parts of the duodenum:

The duodenum consists of 4 parts (from proximal to distal):

  • Superior: 
    • Begins at the pyloric valve (distalmost portion of the stomach)
    • Located at spinal level L1
    • Intraperitoneal
    • Most duodenal ulcers occur in this part
  • Descending:
    • Located at spinal level L1–L3
    • Retroperitoneal (duodenum has moved posterior to the peritoneal membrane)
    • Contains connections to the pancreas and bile duct via the major and minor duodenal papilla
    • Major duodenal papilla: 
      • Opening of the hepatopancreatic ampulla (i.e., ampulla of Vater, the combination of the common bile and main pancreatic ducts) into the duodenal lumen to allow entry of bile and pancreatic juice 
      • Papilla contains the hepatopancreatic sphincter (i.e., sphincter of Oddi), which regulates secretions
    • Minor duodenal papilla: 
      • Opening of the accessory pancreatic duct
      • Located just superior to the major papilla
      • Allows pancreatic juice to be released into the duodenum without bile
  • Transverse (sometimes referred to as the horizontal part): 
    • Located at spinal level L3
    • The longest section (10–12 cm)
    • Retroperitoneal
  • Ascending:
    • Located at spinal level L2–L3
    • Retroperitoneal
    • Terminates at the duodenojejunal flexure: 
      • Fixed to the posterior abdominal wall by the ligament of Treitz
      • Marks the end of the duodenum and beginning of the jejunum

Jejunum and ileum

The jejunum and ileum make up a majority of the small intestine as a long winding tube filling a large portion of the abdominal cavity. Both are completely intraperitoneal (within the peritoneal cavity).

  • Jejunum: 
    • Makes up the proximal ⅖ of the small intestine after the duodenum 
    • Approximately 2.5 m in length
    • Begins in the LUQ of the abdomen
    • Progresses inferiorly and medially
  • Ileum: 
    • Makes up the distal ⅗ of the small intestines 
    • Approximately 3.5 m in length
    • Primarily in the lower half of the abdomen
    • Ends at the ileocecal junction in the RLQ, where it empties into the cecum (1st portion of the large intestine)
  • Mesentery: 
    • A fold of peritoneum that suspends the jejunum and ileum from the posterior abdominal cavity
    • Allows the jejunum and ileum significant mobility within the abdominal cavity
  • Transition between jejunum and ileum:
    • Gradual (as opposed to a discrete anatomic landmark)
    • Different segments are defined by differences in microscopic anatomy noted most prominently when comparing the proximal jejunum and the distal ileum

Microscopic Anatomy

Similar to other segments of the GI tract, the layers of the small intestinal wall (from the inner lumen outward) are mucosa → submucosa → muscular layer → serosa. The walls have several different types of folds.

Folds of the intestinal walls

Circular folds (known as plicae):

  • Large folds of intestinal wall projecting into the lumen
  • Contain both mucosa and submucosa
  • Permanent folds (i.e., not obliterated when the intestine is distended, which is different from rugae in the stomach, which are obliterated as the stomach distends)
  • Locations:
    • Begin approximately 6 cm into the duodenum
    • Most prominent in the jejunum
    • Decrease as you move farther down the ileum
  • Function: 
    • Mixes chyme 
    • ↑ Surface area required for:
      • Contact digestion
      • Absorption

Intestinal villi: 

  • Finger-like projections of the mucosa only (epithelium and lamina propria)
  • Location: off the entire lining of the small intestines, including off plicae folds
  • Function: significantly ↑ surface area
Layers and folds in the intestinal walls

Layers and folds in the intestinal walls

Image by Lecturio.

Layers of the intestinal walls

Mucosa:

  • Consists of 3 sublayers:
    1. Epithelium (innermost lining):
      • Enterocytes (also called absorptive cells): simple columnar cells primarily responsible for contact digestion and absorption
      • Goblet cells: secrete mucus
      • Enteroendocrine cells: secrete hormones into the blood
      • Cells are connected via tight junctions
      • Have a brush border: microvilli on the absorptive surface, which ↑ surface area and contain membrane-bound digestive enzymes 
    2. Lamina propria, which contains:
      • Vasculature: arterioles, veins, and capillary networks, which absorb a majority of the nutrients except for fats
      • Lymphatic vessels known as lacteals: absorb a majority of the fat and fat-soluble vitamins
    3. Muscularis mucosa: 
      • Thin layer of smooth muscle
      • Controls movement of the villi to ↑ mixing
  • Crypts of Lieberkuhn:
    • Pores at the base of the villi that open into tubular glands (similar to gastric glands)
    • Upper ½ contains enterocytes and goblet cells
    • Lower ½ contains:
      • Stem cells: replace enterocytes and goblet cells every 3–6 days 
      • Paneth cells: secrete defensins and lysozyme (which protect against bacterial infection)
    • Located throughout the small intestines
  • Peyer patches: 
    • Nodules of lymphatic tissue found within the lamina propria and submucosa
    • Most abundant in the ileum

Submucosa:

  • Loose connective tissue
  • Contains larger vessels
  • Brunner glands: 
    • Produce alkaline mucus that protects the mucosa from damage caused by stomach acid
    • Found only within the duodenum
  • Meissner plexus: 
    • Ganglia of the ANS
    • Control the muscularis mucosa (independently of the muscular layer of the intestines)

Muscular layer: 

The muscular layer is made up of 2 layers of smooth muscle that mix and move chyme along the tract.

  • Circular layer (inner layer)
  • Longitudinal layer (outer layer)
  • Auerbach (myenteric) plexus: 
    • Ganglia of the autonomic nervous system, which controls the muscular layer
    • Located between the 2 layers of smooth muscle

Serosa: 

  • Made up of connective tissue
  • Joins with mesentery or peritoneum
Histology of the small intestine

Histology of the small intestine:
(a): The absorptive surface of the small intestine is vastly enlarged by the presence of circular folds, villi, and microvilli.
(b): Micrograph of the circular folds: Note that the folds contain both the mucosa and the submucosa.
(c): Micrograph of the villi: Note that the villi contain only the epithelial and lamina propria layers of the mucosa; the muscularis mucosa is visible as a “pink line” along the left edge of the slide.
(d): Electron micrograph of the microvilli: from left to right—light microscope, ×56; light microscope, ×508; electron microscope, ×196,000

Image: “Histology of the Small Intestine” by Phil Schatz. License: CC BY 4.0

Anatomic Differences between the Jejunum and Ileum

Characteristic differences between the jejunum and ileum are summarized in the table.

Table: Anatomic differences between the jejunum and ileum
JejunumIleum
Diameter 4 cm (at the most proximal end) 2 cm (at the most distal end)
Wall thickness Thicker Thinner
Fat in mesentery Less More
Circular folds Many, best developed Some in the proximal ileum, very few distally
Lymphoid tissue/Peyer patches Few Many

Neurovasculature

Arterial supply

  • Duodenum: superior and inferior pancreaticoduodenal arteries
  • Jejunum and ileum: jejunal and ileal arteries (branch off the SMA)
    • Run within the mesentery from the SMA toward the intestines
    • Form anastomotic loops with each other, known as arcades 
    • Vasa recta: small straight arteries off the arcades that supply the intestinal wall

Venous drainage

  • Veins running adjacent to the arterial supply
  • Drain into the superior mesenteric vein (SMV)
  • The SMV unites with the splenic vein to form the hepatic portal vein → brings nutrients absorbed through the gut to the liver

Lymphatics

Lacteals within the intestinal villi drain into:

  • Pancreaticoduodenal nodes (duodenum only)
  • Superior mesenteric nodes (all 3 segments)

Innervation

The small intestine is innervated by the ANS.

Parasympathetic innervation (stimulatory): 

  • Posterior trunk of the vagus nerve
  • ↑ Secretions, motility, and blood flow
  • Relaxation of sphincters
Parasympathetic innervation of the GI tract

Parasympathetic innervation of the GI tract
CN: cranial nerve

Image by Lecturio.

Sympathetic innervation (inhibitory):

  • Greater, lesser, and least splanchnic nerves forming the superior mesenteric plexus around the SMA
  • Pass to small intestines via periarterial branches
  • ↓ Secretions, motility, and blood flow
Sympathetic innervation of the GI tract

Sympathetic innervation of the GI tract

Image by Lecturio.

Function

The small intestines are the primary site of chemical digestion and absorption of nutrients.

  • Receive:
    • Chyme from the stomach
    • Pancreatic juice and enzymes
    • Bile
  • Secrete intestinal juice, which (along with the pancreatic juice and bile):
    • Neutralizes stomach acid
    • Digests nutrients (carbohydrates, fat, and proteins)
  • Primary site of nutrient absorption
  • Brush border of small intestines:
    • Contains additional digestive enzymes required for contact digestion
    • ↑ Surface area for absorption of nutrients
  • Secrete hormones 
  • Functions of intestinal motility:
    • Mix chyme with bile and digestive juices from the pancreas and intestines
    • Churn chyme, bringing it into contact with the mucosa for contact digestion and absorption
    • Move contents toward the large intestine

Clinical Relevance

  • Intestinal malrotation: congenital GI anomaly that results from failure of the normal primitive gut rotation around mesenteric vessels during embryologic development. Intestinal malrotation can result in a number of anatomic patterns characterized by abnormal location and attachments of the intestines within the abdominal cavity. These anomalies can be clinically silent or may present with a number of complications, the most catastrophic of which is midgut volvulus. 
  • Intussusception: telescoping of the proximal part (intussusceptum) into a distal part (intussuscipiens) of the intestine. Intussusception can cause obstruction and, if untreated, can progress to bowel ischemia. Intussusception is most common in the pediatric population, but it is occasionally encountered in adults. The pediatric individual typically presents with acute cyclical abdominal pain and vomiting, whereas adults present with symptoms of bowel obstruction. 
  • Meckel diverticulum: persistent remnant of the omphalomesenteric (vitelline) duct. Meckel diverticulum is usually located in the antimesenteric border of the ileum. The mucosal lining of the diverticulum may contain heterotopic mucosa (most commonly gastric). Although it is frequently asymptomatic, Meckel diverticulum can cause ulceration and present with lower GI bleeding. Other complications include diverticulitis and small bowel obstruction. 
  • Small bowel obstruction: The normal passage of bowel contents is interrupted either because of a functional decrease in peristalsis or mechanical obstruction from intraluminal or extraluminal mechanical compression. The most common causes in developed countries include postsurgical adhesions, hernias, and malignancies. The most important clinical features include crampy abdominal pain, obstipation (i.e., inability to pass flatus or stool), nausea, and vomiting.
  • Crohn’s disease: form of inflammatory bowel disease characterized by patchy transmural inflammation; any portion of the luminal GI tract can be involved, though the terminal ileum and proximal colon are most commonly affected. The most important clinical features include crampy abdominal pain, diarrhea, fatigue, and weight loss. Complications include fistulas, phlegmon/abscess formation, and perianal disease.
  • Celiac disease: also known as gluten-sensitive enteropathy. Celiac disease is a common immune-mediated inflammatory disease of the small intestine caused by sensitivity to gliadin, a component of gluten. The most important clinical features include diarrhea with malabsorption, crampy abdominal pain, and villous atrophy on histology.

References

  1. Collins, J., Nguyen, A. (2021). Anatomy, abdomen and pelvis, small intestine. StatPearls. Retrieved August 23, 2021, from https://www.statpearls.com/articlelibrary/viewarticle/32127/ 
  2. Saladin, K.S., Miller, L. (2004). Anatomy and Physiology, 3rd ed., pp. 964–965. McGraw-Hill Education.
  3. Peppercorn, M.A., Kane, S.V. (2020). Clinical manifestations, diagnosis, and prognosis of Crohn’s disease in adults. UpToDate. Retrieved August 20, 2021, from https://www.uptodate.com/contents/clinical-manifestations-diagnosis-and-prognosis-of-crohn-disease-in-adults
  4. Peppercorn, M.A., Kane, S.V. (2020). Clinical manifestations, diagnosis, and prognosis of ulcerative colitis in adults. UpToDate. Retrieved August 20, 2021, from https://www.uptodate.com/contents/clinical-manifestations-diagnosis-and-prognosis-of-ulcerative-colitis-in-adults
  5. Drake, R.L., et al. (Ed.) (2020). Regional anatomy. In Gray’s Anatomy for Students, 4th ed. Churchill Livingstone/Elsevier, pp. 309–315.
  6. Schuppan, D., Dieterich, W. (2020). Epidemiology, pathogenesis, and clinical manifestations of celiac disease in adults. UpToDate. Retrieved August 20, 2021, from https://www.uptodate.com/contents/epidemiology-pathogenesis-and-clinical-manifestations-of-celiac-disease-in-adults

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