- The longest part of the alimentary canal
- Divided into three regions
Duodenum
- Approximately 25 cm long and is C shaped
- Except for the first part it is retroperitoneal
- Divided into four parts
- First part is continuation of the pylorus and runs transversely
- Second part run vertically in front of the hilum of the right
kidney
- Third part run horizontally below the pancreas
- Fourth part runs upward to the duodenojejunal junction
- Ligament of Treitz connects the DJ flexure to the right crus of the
diaphragm
- Duodenal papilla is found on the medial wall of the second part
- Site of entry of the common bile duct and pancreatic duct
- Blood supply is from:
- Superior pancreaticoduodenal artery - branch of the gastroduodenal
artery
- Inferior pancreaticoduodenal artery - branch of the superior
mesenteric artery
- Venous drainage is into the portal and superior mesenteric vein
- Lymphatic drainage is to the coeliac and superior mesenteric nodes
Jejunum and ileum
- The jejunum and ileum are about 6m long
- Proximal 40% is the jejunum
- Jejunum begins at the duodenojejunal junction
- Ileum ends at the ileocaecal valve
- Connected to the posterior abdominal wall by the small bowel
mesentery
- Blood supply is from the superior mesenteric artery
- Branches form arcades within the mesentery
- Lower part if the ileum is supplied by the ileocolic artery
- Venous drainage is into the superior mesenteric vein
- Lymphatic drainage is into the superior mesenteric nodes
Histology
- Highly modified for absorption
- Structures that maximize surface area include
- Plicae circulares
- Deep permanent folds of the mucosa and submucosa
- Increase surface area and slow the movement of chyme
- This provides more time for absorption and digestion to occur
- Villi
- Finger-like extensions of the mucosa
- Absorptive epithelial cells line their surface
- Within the core of each villus is the lamina propria
- Contains blood capillaries and a lacteal
- Microvilli
- Tiny projections of the plasma membrane of each absorptive
epithelial cell
- Give the cell’s luminal surface a fuzzy appearance known as the
brush border
- Membrane bound enzymes are embedded in the brush border and
function in nutrient breakdown
- Small intestine does contain the 4 basic layers
- Epithelium is an unremarkable simple columnar with goblet cells
- Epithelial invaginations are known as intestinal glands (crypts of
Lieberkuhn)
- They contain, among other things, absorptive cells, immune cells and
neuroendocrine cells
- Submucosa is unremarkable except in the proximal duodenum and
terminal ileum
- Proximal duodenal submucosa contains alkaline mucus glands
(Brunner's glands)
- Terminal ileal submucosa contains Peyer’s patches
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