Overview
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
DUODENUM
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.
JEJUNUM/ILEUM
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 |
|
Jejunum |
Ileum |
Long vasa recta |
Short vasa recta |
Fewer arcades |
More arcades |
Thicker wall |
Thinner wall |
More numerous plicae
circulares |
Less plicae circulares |
Aide-Memoire
Summary
References
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