Lecture 22- The small intestine Flashcards
What does the small in small intestine refer to?
The diameter, it is just 3-4 cm
What are the three main parts of the small intestine + the key function of each?
- Duodenum: receiving chamber after stomach
- Jejunum: most absorption takes place
- Ileum: is the longest and empties into the large intestine
What is the approximate total length of the small intestine?
6m
Why is the small intestine coiled?
So we are not freakishly tall
Where is the duodenum (1st segment of small intestine) located in terms of the peritoneal cavity? How does this differ from the other two segments?
- Duodenum is retroperitoneal (behind +outside fixed to posterior wall)
- Jejunum + Ileum need to move, they are therefore intraperitoneal
What are the Jejunum and ileum anchored by? What does this do?
A mesentery (double layer of visceral peritoneum), prevents tube getting tangled.
What does the mesentery contain?
- Arteries
- Veins
- Nerves
- Lymphatics
What’s delivered to the small intestine?
Partially digested food material (chyme)
What is required in the small intestine due to the acidity of chyme?
Need to…
Protect epithelium via
- Mucous secreting cells
- Glands in submucosa (specifically in duodenum)
Neutralize pH
-Bicarbonate (HCO3-) from pancreas duct cells
What is the small intestine specialized for and what does this mean it needs?
- Absorption
- Large surface area
What are the permanent large folds in the small intestine called? What is their structure?
- Plicae Circulares (Circular Folds)
- Core of submucosa (with overlying mucosa)
Aside from increase surface area what is an additional function of the Plicae Circulares?
Slow passage of material through as causes it to
spiral around giving more time for digestion+ absorption
What is found on the surface of each plicae circulares?
- The mucosa is folded to form finger like projections called villi
- These further increase surface area for absorption
What does the Muscularis mucosae do in regards to the villi?
Contract to cause villi to move backwards and forwards and thus exposing them to more of the luminal contents
What is the structure of villi?
- Core of each villi is made of FCT (lamina propria)
- Contains vessels
What are the two types of vessels found in the villi and their respective functions?
- Lymph lacteal (products of fat digestion)
- Capillary network (products of protein and carbohydrate digestion e.g. amino acids, monosaccharides)
Where does nutrient rich, deoxygenated from the small intestine go to?
- Absorbed into the mesenteric veins
- These drain to the hepatic portal vein (bring nutrients to the liver and detox blood)
Where do lymph lacteals drain?
Into the cisterna chyli, thoracic duct and then left subclavian vein (reentering blood vascular space)
What is a special feature of the epithelium in the small intestine?
- It is simple columnar
- Microvilli on the apical surface increase surface area further forming the brush border.
What is found at the brush border giving it a fuzzy appearance?
Glycocalyx:
Network of Glycoproteins forming branched filaments
which tether enzymes to the brush border
What function do the enzymes tethered to the brush border perform?
Contact digestion
How does the plasma membrane of the small intestine act as a barrier for absorption?
- Is selectively permeable
- For things that can’t just pass through has protein channels which act as transporters
What are the two pathways for absorption in the small intestine?
- Transcellular pathway= from lumen of small intestine across the epithelial cell
- Paracellular pathway= goes between the cells
What dictates what can pass through the paracellular pathway for absorption?
- Tight junctions
- These only allow the passing through of very small molecules
What are the 5 cell types found in the epithelium of the small intestine + basic functions?
- Enterocytes: Absorptive cells
- Goblet cells: Secrete mucous for protection
- Stem cells: Make all cell types by dividing and replacing cells further up
- Paneth cells: granules, antibacterial enzymes (lysozyme, defensins)
- Endocrine cells: secrete hormones into blood supply not the lumen
What is the intestinal Crypt? What cell types does it contain?
- Little glands at base of the villi
- Contain the stem cells, Paneth cells
What are the four ways that surface area for absorption is increased in the small intestine?
- 1: Long (~6m in length!)
- 2: Fold the submucosa – permanent folds = plicae circulares
- 3: Fold the mucosa (not the Muscularis mucosa) = villi
- Fold the apical plasma membrane of the epithelium = microvilli
