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Small Intestine

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

Location and Surface Anatomy of the Small Intestine

The small intestine is part of the digestive tract that connects orally with the pylorus and leads aborally at the ostium ileale (Bauhin’s valve) into the colon. The small intestine is highly convoluted in the abdomen. It is connected to the abdominal wall via mesentery through which run all types of vessels. The small intestine is located intraperitoneally and is divided anatomically into:

  • Duodenum, which is continuous with the gastric pylorus, is where the common bile duct (ductus choledochus) and the pancreatic duct lead into the major duodenal papilla.
  • Jejunum, which is the middle section constituting about two-fifths of the small intestine.
  • Ileum, which is the terminal section ending in the right iliac fossa in the colon. The ileocecal junction consists of ostium ileale (Bauhin’s valve).
Small Intestine

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

Structure of the Small Intestine

The small intestine is similar to the rest of the gastrointestinal tract in structural layout. However, it presents important anatomical features related to its function. Here are the individual layers listed from the outside to the inside.

Layers of the Alimentary Canal

Image: “Layers of the Alimentary Canal” by Goran tek-en. License: CC BY-Sa 3.0

The serous membrane (serosa) and subserous membrane (subserosa): The serosa corresponds to the visceral peritoneum. The subserosa corresponds to loosely arranged connective tissue between the visceral peritoneum and the muscle layer. In parts of the duodenum where the small intestine is not completely surrounded by the peritoneum, thus located in a secondary retroperitoneal position, the connective tissue is called tunica adventitia.

The muscular tunic (muscle layer): The muscular tunic consists of an external and an internal circular muscular layer. The inner tunic is significantly more pronounced in the small intestine. The myenteric plexus (Auerbach’s plexus) lies between these layers. It belongs to the enteric nervous system (ENS) and controls intestinal peristalsis. The muscular tunic is responsible for the transport of food via peristaltic contractions.

The submucosal tela (submucosa): The submucosa consists of loose connective tissue and numerous elastic fibers. It contains blood vessels (especially small arteries and veins), lymphatic vessels and nerve cells of the plexus submucosus (Meissner’s plexus) arranged in ganglia. The нито искам дplexus submucosa is also part of the enteric nervous system (ENS) and is responsible for intestinal peristalsis and secretory regulation.

The mucous membrane (mucosa): The mucosa is the inner (luminal) layer of the small intestine and is further divided into 3 laminae. The lamina muscularis mucosae is a thin layer of muscle cells that facilitates autonomous movement of the mucous membrane. Its inner adjacent layer is known as lamina propria mucosae, a layer of connective tissue that contains capillaries, nerve endings, and lymphatic vessels. The innermost layer coats the lumen of the small intestine and is known as lamina epithelialis mucosae. It consists of a single-layered cylindrical epithelium. The columnar epithelial cells known as enterocytes are studded with microvilli.

Hint: The small intestinal wall is known for rapid cell division. The epithelium is replaced every 24–72 hours.

The mucosa is the most variable layer of the gastrointestinal tract and is very well adapted to the respective organs. The mucous membrane of the small intestine provides a large surface for optimal water and nutrient exchange.

The plicae circulares are folds in the mucous membrane visible to the naked eye. They extend about 1 cm deep into the lumen and include the submucosa. These folds carry 0.2 mm high intestinal villi protruding from the mucosa. They significantly increase the surface and thereby improve nutrient reabsorption. The smaller crypts of Lieberkühn (intestinal glands) between the villi run into the glandular ducts and are the site where the cell division is initiated in the small intestinal epithelium.

Note: It is easy to distinguish the colon from the small intestine based on the villi, which are only found in the small intestine typically.


Anatomical illustration using traditional media to display the duodenum.

Image: “Anatomical illustration using traditional media to display the duodenum.” by Luke Guthmann. License: CC BY-Sa 3.0

The duodenum (about 30 cm long) is the first section of the small intestine that runs around the pancreatic head in a semi-circular C-shape connecting the pyloric orifice with the jejunum. The duodenum is divided into four parts:

  • Pars superior,  which is approx. 5 cm long and located intraperitoneally, has a large lumen initially and is known as ampulla duodeni, or duodenal cap, which is located directly ventral from the pancreaticoduodenal artery. Thus, a duodenal ulcer perforating the duodenal wall in this section can cause severe bleeding. 
  • Pars descendens in the duodenum is the secondary retroperitoneal. The orifice of the shared excretory duct of the ductus choledochus (bile duct) and the ductus pancreaticus (pancreatic duct) is located at the major duodenal papilla (papilla of Vater). The minor duodenal papilla (papilla of Santorini) is considered an anatomical variation and is an additional orifice of the pancreatic duct.
  • Pars horizontalis runs horizontally and is occasionally called Pars inferior.
  • Pars ascendens is where the duodenum leads into the intraperitoneal jejunum. This junction known as the duodenojejunal flexure forms the end of the upper gastrointestinal tract.

Attention: The shift between intra- and secondary retroperitoneal position of the duodenum is a popular exam question.

Jejunum and Ileum

The lower gastrointestinal tract begins with the 3–5 m long intestinal sections jejunum and ileum. The small intestine lies intraperitoneally in these two sections. Jejunum and ileum are suspended from the abdominal wall by the mesentery, which also ensures the vascular supply, and thereby, they have specific mobility inside the abdominal cavity. They are bound by the colon on three sides.

Jejunum und Ileum

The mucous membrane of the small intestine changes along its course. In the aboral direction, the plicae circulares start to level out until the ileum where they are no longer detectable. Similarly, the villi on the plicae become shorter whereas the crypts run deeper towards the end of the ileum. The Peyer’s patches (or aggregated lymphoid nodules) in the lamina propria are unique to the ileum. These lymphoid follicles are detectable microscopically as protrusions in the ileum without deep folds.


Hint: The differences in mucosal structure between jejunum and ileum are frequently asked in examinations.

Characteristics of jejunum and ileum

Feature Colour Diameter Wall thickness Blood supply Fat in mesentery Circular folds Lymphoid nodules
 Jejunum  Red 4 cm Thicker More Less Many Few
 Proximal ileum  ↓  ↓  ↓  ↓  ↓  ↓  ↓
 Distal ileum  Pink 2 cm Thinner Less More Few Many

Vasculature and Nerve Supply of the Small Intestine

The arterial supply to the small intestine varies:

  • Duodenum is vascularized by the coeliac trunk and the superior mesenteric artery. The anterior and posterior branches of the superior pancreaticoduodenal artery, arising in the gastroduodenal artery (a branch of the hepatic artery of the celiac trunk), supply the upper sections of the duodenum. The retroduodenal artery emerging from the gastroduodenal artery supplies the dorsal segments of the duodenum. The inferior pancreaticoduodenal artery with its anterior and posterior branches emerging from the superior mesenteric artery supply the lower sections of the duodenum.
  • Jejunum is supplied by the jejunal arteries arising from the superior mesenteric artery, running inside the mesentery and vertically along the arteriae rectae into the intestinal wall.
  • Ileum is supplied by the iliac arteries emerging from the superior mesenteric artery. They run inside the mesentery similar to the arteries of the jejunum.

The venous drainage occurs parallel to the arteries. The mesenteric superior vein fuses with the splenic vein and together form the vena portae (portal vein). Thus, the blood from the small intestine, similar to that of all the unpaired abdominal organs, is transported to the liver.

In the small intestine, lymph is first drained into numerous lymph nodes located in the mesentery, and further via the nodi lymphoidei mesenterici superiores into the intestinal trunk and the cisterna chyli, where the thoracic duct originates. The thoracic duct is the largest lymphatic vessel in the human body.

The motility and secretion of the small intestine are regulated by the enteric nervous system but can occur autonomously. Nevertheless, the vegetative nervous system influences the ENS via sympathetic and parasympathetic nervous system.

The sympathetic nervous system innervates the small intestine with the major splanchnic nerve which, after synapsis in the celiac ganglion, runs into the duodenum and after synapsis in the superior mesenteric ganglion, to the jejunum and ileum. The sympathetic nervous system inhibits the glandular secretion and the movement of the digestive tract muscles.

Nerve fibers of the truncus vagalis posterior (N. vagus) innervate the small intestine as part of the parasympathetic nervous system. The nerve fibers synapse in the intestinal wall. The parasympathetic nervous system triggers intestinal secretion and motility.

Anatomical dissections

Image “Anatomical dissections” by Anatomist90. License: (CC BY-SA 3.0)

Function of the Small Intestine

The small intestine is the key organ involved in nutrient decomposition and the seat of several enzymatic processes catalyzing the breakdown of chymus, a liquid mass of partly digested food. It is also the site of reabsorption of valuable nutrients. Slow peristaltic movements of the small intestine facilitate the close contact between food and mucous membrane before the food is further transported into the large intestine (colon).

Two liters of pancreatic secretions reach the duodenum daily via papilla duodeni major in the pars descendens of the duodenum, combining with the chymus. Due to stomach acid, the chymus has an acidic pH of 2 when entering the duodenum, where it is alkalized by the bicarbonates in the pancreatic secretions.

The neutral pH is essential for the activity of digestive enzymes, which are secreted by the pancreas in an inactive form and are physiologically changed into their active form near the mucous membrane of the small intestine. Activated pancreatic enzymes such as amylase, trypsin, and lipase decompose the food into utilizable nutrients such as monosaccharides and amino acids, which are then absorbed by the small intestinal epithelium.

Additionally, the bile fluid is secreted under hormonal regulation via the ductus choledochus into the lumen of the duodenum where it blends with the chymus. Here, the bile acids digest the fats. Together with the fat-soluble components in the food, they form the so-called micelles, which are reabsorbed by the mucous membrane.

The triglycerides are then transported in chylomicrons via lymphatic vessels. Large amount of bile fluid is reabsorbed by the mucous membrane in the terminal ileum and transported back into the liver via the portal vein. This circulation, which minimizes the demand for fresh synthesis, is called enterohepatic circulation.

The small intestine is crucial for water reabsorption in the human body. Research suggests that up to 80% of water gets reabsorbed in the small intestine, predominantly in the jejunum. However, the epithelium also secretes water in order to balance hypertonic chymus. Therefore, diarrheal diseases represent a great danger in the form of exsiccosis (dehydration).

Furthermore, the small intestine, especially the ileum, plays an important part in immune defense. Lymphatic tissue, particularly the Peyer’s patches encounter antigens in the intestinal lumen, which triggers a targeted immune response to facilitate the screening of beneficial and foreign intestinal bacteria. The whole of the lymphatic tissue is designated as the gut-associated lymphatic tissue (GALT).

Important Diseases of the Small Intestine

Duodenal ulcer

Research suggests that 1% of the western population is affected by duodenal ulcer (ulcus duodeni), which is more frequent than stomach ulcer. It is defined as an erosion of the duodenal wall that breaks through the lamina muscularis mucosae. Ulcers are often caused by chronic infection with Helicobacter pylori or frequent intake of non-steroidal anti-inflammatory drugs such as ibuprofen. They cause severe pain, which is typically relieved by food intake. Complications may lead to bleeding and perforations.

Malassimilation syndrome (food intolerance)

Food intolerance (malassimilation) is another common disease of the small intestine. A distinction is made between malfunction in food decomposition (maldigestion) and malfunction in resorption (malabsorption). Celiac disease is a very severe malabsorption syndrome and is defined as chronic gluten intolerance. It is characterized by severe inflammation and villous atrophy of the small intestine. However, it is relieved with strict gluten-free diet.

Another severe case of malabsorption is lactose malabsorption (lactose intolerance). Nearly 75% of the world’s adult population suffers from lactose intolerance due to a lack of the enzyme lactase. Patients suffer from meteorism and diarrhea when consuming lactose since the milk sugar passes the small intestine undigested and is then fermented by gut bacteria in the large intestine.

Crohn’s Disease

Crohn’s disease is a chronic inflammatory bowel disease (CIBD) of the small intestine. It is characterized by chronic and recurrent inflammation of the intestinal wall. It usually affects separate segments of the ileum and the colon (“skip lesions”) and manifests as a cobblestone-like appearance of the mucosa.

Patients between the ages of 15 and 35 years manifest:

  • Weight loss
  • Fever
  • Abdominal pain
  • Arthritis
  • Skin changes
  • Diarrhea
  • Growth failure
  • Perianal disease or mouth ulcers

In addition, the affected patients carry a high risk of complications such as strictures and fistulae, warranting surgical resection.

Endoskopiebild Iliitis terminalis

Image “Endoskopiebild Iliitis terminalis” by Joachim Guntau. License:  (CC BY-SA 2.0 DE)

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