Small Intestine Flashcards
What are the 3 parts of the small intestine?
duodenum
jejunum
ileum
What is the small intestine?
primary site of digestion and absorption in GI tract
Motility in Small Intestine
What are the two predominant motility paradigms in small intestine?
- segmentation
- migrating mobility complex
Motility in Small Intestine
What is segmentation and what does it do?
alternating contractions and relaxations of adjacent sections of small intestine
ensures thorough mixing during processing of chyme, and helps move chyme toward large intestine
Motility in Small Intestine
What does the migrating mobility complex do?
moves luminal contents along small intestine, in the period between meals
Motility in Small Intestine
What is the primary motility during a meal?
segmentation
Motility in Small Intestine
What is segmentation initiated by? (3)
- distension of lumen
- presence of enterogastrone gastrin
- parasympathetic input (sympathetic input decreases segmentation)
Motility in Small Intestine
When does segmentation get replaced by migrating mobility complex?
after absorption of a meal
Motility in Small Intestine
Describe the process of migrating motility complex.
- begins at duodenal-gastric junction
- consists of weak peristaltic contractions that travel for a short distance
- second wave begins slightly more distally than initiation site of first wave and travels slightly further
- ~2 hrs to travel from stomach to large intestine
- cycle repeats itself until ingestion of another meal initiates segmentation again
What are the two structures at the ileocecal junction?
- ileocecal valve
- ileocecal sphincter
What does the ileocecal valve do?
contents of ileum can push through the valve…
cecal contents pushing backward, close the valve ‘flaps’, preventing reverse flow
What controls the ileocecal sphincter?
neuronal and hormonal control
What causes relaxation of the ileocecal sphincter?
distension on ileal side
What causes constriction of the ileocecal sphincter?
pressure on cecal side
What also inhibits the ileocecal valve?
gastrin
What does the mucosal epithelium of small intestine secrete?
~1.5 L of aqueous salt and mucus solution (succus entericus)
What does aqueous salt and mucus solution (succus entericus) do? (3)
- lubricates passage through lumen
- protects mucosa from acid injury
- H2O molecules required for hydrolysis reactions
What is the stimulus for aqueous salt and mucus solution (succus entericus) production and release?
presence of luminal chyme
What are fats digested by?
completely hydrolyzed by pancreatic lipase
What are the absorbable components of fats?
monoglycerides and free fatty acids (FFAs)
How are proteins digested?
reduced to small peptide chains and some unitary amino acids by pancreatic proteolytic enzymes
peptide fragments require further hydrolysis by aminopeptidases in epithelial brush border prior to their absorption
How are carbohydrates digested?
broken into disaccharides and some monosaccharides
further hydrolyzed by disaccharidases in epithelial brush border
What are the adaptations of the small intestine for absorption?
- circular folds or ridges
- villi
- brush border
What are the circular folds or ridges of the small intestine?
on inner surface of small intestine – increase surface area 3x
What are villi?
microscopic projections extending into lumen – increase surface area further 10x
What is the brush border of the small intestine?
visible only by electron microscopy – increase luminal surface area further 20x
each epithelium has between 3000-6000 hair-like microvilli projecting into lumen
What are the 5 main parts of a villus?
- epithelial and mucous cells
- connective tissue core (lamina propria)
- network of capillaries
- lymphatic vessel (central lacteal)
- crypts of Lieberkuhn