Small Intestine

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The small intestine is the next part of the gastrointestinal tract following the stomach. It receives chyle and assists in further digestion and absorption. The small intestine is split into 3 parts, the duodenum (1st 25cm approx.), jejunum (next 2/5) and ileum. We will deal with each of these parts in turn. With the exception of the first part of the duodenum the small intestine is a midgut structure. 

Gross Anatomy



The duodenum is the first part of the small intestine, receiving the chyle passed out from the pylorus of the stomach. It is split into 4 parts, D1-D4. They form a C shape between the levels of L1-L3. D1 is connected to the liver by the hepatoduodenal ligament. It lies superior to the pylorus. It then sweeps down and encloses the head of the pancreas. In D2 the ampulla of Vater is found, the opening for biliary and pancreatic duct secretions to mix with food entering from the stomach. D3 crosses left over the IVC and aorta to lie posterior to the SMA and SMV and below the pancreas. D4 then prturdes anteriorly to form the duodenjejeunal flexure and continues as the jejunum.


Blood supply is the superior pancreatico-duodenal artery (a branch of the gastroduodenal artery) to the level of D2 and the inferior pancreatico-duodenal artery (a branch of the superior mesenteric artery) after. This is because of the fact that up to the level of D2 the duodenum is a foregut structure, whereas it then becomes a midgut structure. The venous supply follows that of the artery.




 The duodenojejunal flexure continues as the jejunum. It continues into the ileum and there are some differences between the two structures that can be seen at laparotomy (see the quick look box below). The blood supply comes from the superior mesenteric artery, an anterior branch of the abdominal aorta. The jejunoileum is a mobile structure tethered by a mesentery in which branches of the SMA run. They form vascular loops called arcades, which give off vasa recta, straight arteries suppling the bowel wall. The venous drainage is via the superior mesenteric vein (which later meets the splenic vein forming the portal vein).

Clinical Anatomy

Duodenal Ulcer


D1 is often the site of duodenal ulcers, which can perforate or cause massive bleeding if the ulcer erodes into the gastroduodenal artery. This is often due to H. pylori infection or NSAIDs. If the ulcer perforates it can lead to widespread peritonitis and may need surgery to under-run the ulcer and washout the peritoneal cavity. If the perforation seals itself off, a conservative approach can be considered with careful monitoring and the use of intravenous proton pump inhibitors.


Meckel’s diverticulum


This is a vestigial remnant of the vitello-intestinal duct, which during embryological development connects the midgut to the yolk sac. The rule of 2’s is well-described; it is found in 2% of individuals, is located 2 feet from the ileocaecal valve, 2/3 have ectopic mucosa (usually gastric or pancreatic), 2% are symptomatic and is 2 inches long. They can cause obstruction, become inflamed or ulcerate and can often mimic appendicitis in presentation. During laparoscopy, if a normal appendix is seen with symptoms, it is imperative to “walk the small bowel”, i.e. to visually inspect it to ensure that the patient does not have a Meckel’s diverticulum. They can also be found within inguinal hernias (a Littre’s hernia).


Small bowel obstruction


Small bowel loops can become twisted, especially in patients who have had previous surgery and formed adhesions or bands. They often present with intractable vomiting, dehydration, abdominal distension and absolute constipation. On examination the abdomen is markedly distended, often with a resonant percussion note. The semi-pathognomonic “tinkling bowel sounds” are often replaced merely by an absence of sound whatsoever. A plain radiograph can often show distended bowel loops, however most patients will often have a CT scan to determine the point at where the obstruction has occurred. This can be managed conservatively, with bowel rest. This involves the use of an NG tube with frequent aspiration, intravenous fluids and the patient being NBM. Quite often a trial period of 24-48hours is used but if the patient fails to improve, then surgery is often required to relieve the obstruction. This can extend from a simple division of a band adhesion to bowel resection and stoma formation.

Quick Anatomy

Key Facts

Differences between the jejunum and ileum



Long vasa recta

Short vasa recta

Fewer arcades

More arcades

Thicker wall

Thinner wall

More numerous plicae circulares

Less plicae circulares



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Monkhouse S. Clinical Anatomy. Master Medicine, Second edition Churchill Livingstone. Elsevier. 2007

Gosling et al. Human Anatomy.  Fourth Edition. Mosby Publishing 2002

Chen et al. Nonsurgical management of partial adhesive small-bowel obstruction with oral therapy: a randomized controlled trial. CMAJ November 8, 2005 vol. 173 no. 10