The small intestine is the longest part of the digestive system. It extends from the stomach (pylorus) to the large intestine (cecum) and consists of three parts: duodenum, jejunum and ileum. The main functions of the small intestine are to complete digestion of food and to absorb nutrients.
Dysfunction of the small intestine can bring you some uneasy experiences such as diarrhea while travelling or worse, on a date. This article will discuss the anatomy, function and neurovasculature supply of the small intestines.
|Definition||A part of the alimentary tract which extends from the stomach (pyloric orifice) to the large intestine (ileal orifice)|
|Parts||Duodenum, jejunum, ileum|
|Blood supply||Arteries: celiac trunk, superior mesenteric artery
Veins: hepatic portal vein, superior mesenteric vein
|Innervation||Parasympathetic: vagus nerve (CN X) (through the submucosal (Meissner’s) and myenteric (Auerbach’s) nervous plexuses)
Sympathetic: Thoracic splanchnic nerves
|Function||Final stages of food digestion
Absorption of nutrients and water
|Clinical relations||Diarrhea, obstructive disorders, infectious diseases, neoplastic growths, congenital conditions, duodenal ulcer|
- Blood vessels
- Clinical relations
The small intestine is divided into the duodenum, jejunum, and ileum. Together these can extend up to six meters in length. All three parts are covered with the greater omentum anteriorly. The duodenum has both intraperitoneal and retroperitoneal parts, while the jejunum and ileum are entirely intraperitoneal organs. As the small intestine is the main site for the final stages of food digestion and its absorption, its gross and microanatomy are adjusted to that function.
The duodenum by definition is the first part of the small intestine. It extends from the pyloric sphincter of the stomach, wraps around the head of the pancreas in a C-shape and ends at duodenojejunal flexure. This flexure is attached to the posterior abdominal wall by a peritoneal fold called the suspensory muscle (ligament) of duodenum, also called the ligament of Treitz.
The duodenum has four parts: superior (duodenal bulb/ampulla), descending, horizontal and ascending. Among several features of the duodenum, we’ll list the two most important:
- The superior part (duodenal bulb/ampulla) is the only intraperitoneal part, as the hepatoduodenal ligament and greater omentum attach to it.
- The descending part of the duodenum has an opening called the major duodenal papilla (tubercle of Vater). The papilla contains the hepatopancreatic sphincter (sphincter of Oddi, Glissons’ sphincter) which regulates the emptying of the bile from the hepatopancreatic ampulla.
The jejunum is the second part of the small intestine. It begins at the duodenojejunal flexure and is found in the upper left quadrant of the abdomen. The jejunum is entirely intraperitoneal as the mesentery proper attaches it to the posterior abdominal wall.
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There is no clear line of demarcation between the jejunum and ileum, but there are some anatomical and histological differences that distinguish them:
- The jejunum represents the proximal two-fifths of the jejunum-ileum continuum
- The wall of the jejunum is thicker and its lumen is wider than in ileum
- The jejunum contains more prominent circular folds of Kerckring
The ileum is the last and longest part of the small intestine. It is found in the lower right quadrant of the abdomen, although the terminal ileum can extend into the pelvic cavity. The ileum terminates at the ileal orifice (ileocecal junction) where the cecum of the large intestine begins.
At the ileocecal junction, the lamina muscularis of the ileum protrudes into the lumen of the cecum forming a structure called the ileocecal fold. These muscular fibers form a muscular ring within the fold called the ileocecal sphincter which controls the emptying of ileal content into the large intestine.
Histologically, the small intestine has four layers. From internal to external, they are mucosa, submucosa, muscularis externa, and serosa. These layers are easy to remember using the mnemonic M.S.M.S. There are several unique features in the small intestine, which act to significantly increase its absorptive surface:
- Circular folds (valves of Kerckring, plicae circulares) are the transverse folds of mucosa found predominantly in the distal duodenum and proximal jejunum
- Intestinal villi are fingerlike extensions of intestinal mucosa which project into the lumen of the small intestine. Between the villi are intestinal glands (crypts of Lieberkuhn) which secrete intestinal juice rich in digestive enzymes.
- Microvilli are projections found on the apical surface of each intestinal cell (enterocyte)
There are also features of the small intestine which are segment-specific:
- Peyer's patches are part of gastrointestinal associated lymphoid tissue (GALT). They are found in ileum.
- Brunner glands are found in the submucosa of the duodenum. They produce mucus rich in alkalines which protects the duodenum from the corrosive effects of gastric acid.
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The main functions of the small intestine are secretion and absorption. The epithelial cells of the small intestine secrete enzymes which digest chyme into the smallest particles, making them available for absorption. Concurrently the duodenum functions to mix food with bile and pancreatic enzymes to continue the digestion of carbohydrates, fats, and proteins.
Concerning absorption, carbohydrates and proteins are absorbed in the duodenum and jejunum respectively. The jejunum also functions to absorb most fats. The ileum function involves absorption of vitamin B12, bile salts and all digestion products which were not absorbed in duodenum and jejunum. All three small intestine segments absorb water and electrolytes.
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The jejunum and ileum are supplied by 15-18 branches of the SMA called the jejunal and ileal arteries. They anastomose with each other to form arterial arcades which send numerous straight arteries (vasa recta) to the jejunum and ileum.
The small intestines drain into the hepatic portal vein. The lymph of the small intestine is drained into the superior mesenteric lymph nodes.
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The small intestine is innervated by branches of the vagus nerve (CN X) and thoracic splanchnic nerves. Their nerve branches extend throughout the entire length of the small intestine in the form of two plexuses:
- Submucosal nervous plexus (of Meissner) found in the submucosa of the small intestine and contains only parasympathetic input from the vagus nerve (CN X)
- Myenteric nervous plexus (of Auerbach) located in the muscularis externa of the small intestine, contains both sympathetic and parasympathetic nerve fibers
Learn to differentiate between these layers really easily using a menmonic! Just memorise 'SMP & MAPS', which stands for:
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- Diarrhea is the frequent passage of unformed stool. In most cases, the diarrhea is caused by microorganisms such as Escherichia coli, Salmonella and Shigella.
- Obstructive disorders including paralytic ileus, a hernia or volvulus are common but can become complicated.
- Infectious diseases such as tapeworm, tropical sprue or giardiasis are rare but severe when left untreated. More common infections such as the adenovirus or salmonella are seen in the west.
- Neoplastic growths may include gastrointestinal stromal tumors (GIST), lymphomas and sarcomas.
- Developmental, congenital or genetic conditions include pyloric stenosis, duodenal atresia and gastroschisis.
- Other conditions or a miscellaneous mixture of diseases that can affect the small intestine include Crohn’s disease, coeliac disease, gastric dumping syndrome and irritable bowel syndrome to name but a few.
An ulcer is a defect of intestinal mucosa. Ulcers can appear in the stomach and/or duodenum, usually due to a bacterial infection of the pylorus of the stomach with Helicobacter pylori combined with the corrosive effect of gastric acid and pepsin.
Duodenal peptic ulcers are usually found in the ampulla of duodenum. They appear more often within the younger population, contrasting with gastric ulcers which are more prevalent in the older population. The most common symptom of duodenal ulcers is the presence of burning pain in the epigastric region of the abdomen around 2-3 hours after a meal. Depending on the severity of ulceration, pain can be followed by nausea and vomiting.