GI tract - Small intestine Flashcards
How long is the small intestine?
Approximately 6.5m.
Where does the small intestine start and where does it end?
It starts at the pylorus of the stomach.
It ends at the ileocaecal junction.
What are the three divisions of the small intestine?
1) Duodenum
2) Jejenum
3) Ileum
What are the divisions of the duodenum?
1) Superior
2) Descending
3) Inferior
4) Ascending
It forms a ‘C-shape’, that is around 25cm long and wraps around the head of the pancreas.
‘Duodenum’ is derived from the latin words, ‘duodenum digitorum’, meaning twelve finger lengths.
D1 - Superior
Vertebral level?
L1.
This section is known as ‘the cap’. It ascends upwards towards the pylorus of the stomach, and is connected to the liver by the hepatoduodenal ligament. This area is most common site of duodenal ulceration.
The initial 3cm of the superior duodenum is covered anteriorly and posteriorly by visceral peritoneum, with the remainder, retroperitoneal (only covered anteriorly).
D2 - decending
Vertebral level?
L1 - L3.
The descending portion curves inferiorly around the head of the pancreas. It lies posteriorly to the transverse colon, and anterior to the right kidney.
Internally, the descending duodenum is marked by the major duodenal papilla - the opening at which bile and pancreatic secretions to enter from the ampulla of Vater (hepatopancreatic ampulla).
D3 - inferior
Vertebral level?
The inferior duodenum travels laterally to the left, crossing over the inferior vena cava and aorta. It is located inferiorly to the pancreas, and posteriorly to the superior mesenteric artery and vein.
D4 - Ascending
Vertebral level?
After the duodenum crosses the aorta, it ascends and curves anteriorly to join the jejenum at a sharp turn known as the duodenojejunal flexure.
Located at the duodenojejunal junction is a slip of muscle called the suspensory muscle of the duodenum. Contraction of the muscle widens the angle of the flexure, and aids movement of the intestinal contents into the jejunum.
Clinical relevance - duodenal ulcers
A duodenal ulcer is the erosion of the mucosa in the duodenum. It may also be described as a peptic ulcer (although this term can also be used to refer to ulcerations in the stomach). Duodenal ulcers are most likely to occure in the superior portion of the duodenum.
The most common causes of duodenal ulcers are Heliobacter pylori infection and chronic NSAID therapy.
An ulcer in itself can be quite painful, but is not particularly troublesome and can be treated medically. However, if the ulcer progresses to create a complete perforation through the bowel wall, this is a surgical emergency, and usually warrants immediate repair. A perforation may be complicated by:
1) Inflammation of the peritoneum (peritonitis) - causing damage to the surrounding viscera, such as the liver, pancreas and gall bladder.
2) Erosion of the gastroduodenal artery - causing haemorrhage and potential hypovolemic shock.
Jejenum and Ileum
These are the two distal parts of the small intestine. In contrast to the duodenum, they are intraperitoneal.
They are attach to the posterior abdominal wall by the mesentery.
The jejunum begins at the duodenaljejunal flexure. There is no clear external demarcation between the jejunum and the ileum - although the two are macroscopically different. The ileum ends at the ileocaecal junction.
At this junction, the ileum invaginates into cecum to form the ileocecal valve. Although it is not developed enough to control movement of material from the ileum to the cecum, it can prevent reflux of material back into the ileum.
Clinical relevance: characteristic features between the jejunum and ileum
During surgery it is important to distinguish between jejunum and the ileum. See image below.
Vasculature and lymphatics of the duodenum
The arterial supply is derived from two sources:
1) Proximal to the major duodenal papilla - supplied by the gastroduodenal artery (branch of the common hepatic artery from the coeliac trunk).
2) Distal to the major duodenal papilla - supplied by the inferior pancreaticoduodenal artery (branch of the superior mesenteric artery).
This transition is important - it marks the change from the embryological foregut to midgut. The veins of the duodenum follow the major arteries and drain into the hepatic portal vein.
Lymphatic drainage is to the pancreaticoduodenal and superior mesenteric nodes.
Vasculature and lymphatics of the Jejunum and Ileum
The arterial supply to the jejunum is from the superior mesenteric artery.
The superior meseneteric artery arises from the aorta at the level of the L1 vertebrae, immediately inferior to coeliac trunk. It moves in between layers of the mesentery, splitting into approximately 20 branches. Theses branches anastomose to form loops, called arcades. From the arcades, long and straight arteries arise, called vasa recta.
The venous drainage is via the superior mesenteric vein. It unites with the splenic vein at the neck of the pancreas to the hepatic portal vein.
Lymphatic drainage is via the superior mesenteric nodes.
Clinical relevance - ileocaecal valve
The ilecaecal valve represents the separation between the small and large intestines. Its main function is to prevent the reflux of enteric fluid from the colon into the small intestine. It is also used as a landmark during colonoscopy, indicating that the limit of the colon has been reached at that a complete colonoscopy has been performed.
The ileocaecal valve is also important in the setting of large bowel obstruction. Should the ileocaecal valve be competent, a closed loop obstruction can occur and cause bowel perforation. Should the ileocaecal valve be incompetent (i.e. allow backflow of enteric contents into the small bowel) then the situation is less emergent and the trajectory of the obstruction is less rapid.
