Small Bowel Flashcards
What is the purpose of the small bowel?
To absorb nutrients salt and water
How long is the small bowel?
6m
What is the diameter of the small bowel?
3.5cm
How long is the duodenum?
25cm
How long is the jejunum?
2.5m
How long is the ileum?
3.75m
Is there a sudden transition between parts of the small intestine?
No
Do the parts of the small intestine have similar or different basic histological organisation?
Same
What is the mesentery?
a fold of membrane that attaches the intestine to the wall around the stomach area and holds it in place
What are the functions of the mesentery?
Suspends small & large bowel from posterior abdominal wall:
-anchoring them in place
-whilst still allowing some movement
Provides a conduitfor blood vessels, nerves & lymphatic vessels.
Where do villi occur?
The small intestine
Are villi motile or static?
Motile
Do villi have a rich blood supply and lymphatic drainage?
Yes
Why do villi have a rich blood supply and lymphatic drainage?
for absorption of digested nutrients
Where do villi have good innervation from?
the submucosal plexus.
Do villi have a simple epithelium?
Yes
How thick is the villi epithelium?
One-cell thick
What type of cell is the villi epithelium mostly made up of?
dominated by enterocytes (columnar absorptive cells)
What are the 3 cell types in the villi epithelium?
primarily enterocytes (absorptive cells)
scattered goblet cells
enteroendocrine cells
What are crypts of Lieberkühn?
Pits between villi
What 2 cell types do crypts of Lieberkühn contain?
Paneth cells
Stem cells
What are enterocytes specialised for?
absorption & transport of substances.
What is the lifespan of an enterocyte?
1-6 days
By how much do folds and villi of small intestine increase its surface area?
500 fold increase
What makes up the brush border?
Microvilli
What is the surface of microvilli covered with?
glycocalyx
What is glycocalyx?
rich carbohydrate layer on apical membrane of microvilli
Describe the purpose and function of glycocalyx
serves as protection from digestional lumen
yet allows for absorption.
traps a layer of water & mucous known as “unstirred layer”
- regulates rate of absorption from intestinal lumen
What causes goblet shape of goblet cells?
Mucous containing granules accumulating at apical end of cell
What is mucus?
large glycoprotein that facilitates passage of material through bowel.
Describe how abundance of goblet cells changes along bowel
increasing abundance of goblet cells along entire length of bowel
↓ in duodenum
↑ in colon
Where are enteroendocrine cells most often found?
lower part of crypts.
Where are Paneth cells only found?
The bases of crypts
What do Paneth cells contain and what is the purpose of what they contain?
large, acidophilic granules that contain:
antibacterial enzyme lysozyme (protects stem cells)
Glycoproteins & zinc (essential trace metal for a no. of enzymes)
What do Paneth cells engulf?
Some bacteria and protozoa
What may Paneth cells have a role in?
Regulating intestinal flora
Why are epithelial stem cells essential in the GI tract?
to continually replenish the surface epithelium
What type of ‘potent’ are epithelial stem cells?
Pluripotent
How do epithelial stem cells replace dying cells?
Migrate up to tip of villus, replacing older cells that die by apoptosis
digested and reabsorbed
What are 3 reasons why lifespan of enterocytes is so short?
Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet.
Effects of agents which interfere with cell function, metabolic rate etc will be diminished.
Any lesions will be short-lived.
How is the duodenum distinguished?
presence of Brunner’s glands
What are Brunner’s glands?
Submucosal coiled tubular mucous glands secreting alkaline fluid
What do the alkaline secretions of Brunner’s glands do?
Neutralizes acidic chyme from stomach, protecting proximal small bowel
Help optimise pH for action of pancreatic digestive enzymes.
What are the 3 functions of small intestine motility?
To mix ingested food with digestive secretions & enzymes
To facilitate contact between contents of intestine & the intestinal mucosa
To propel intestinal contents along alimentary tract
What are the 3 things that cause the motility of the small intestine?
- Segmentation (mixing)
- Peristalsis (propelling)
- Migrating motor complex
Describe segmentation of the small intestine?
Mixes contents of lumen
Occurs by stationary contraction of circular muscles at intervals.
More frequent contractions in duodenum cf. ileum
allow pancreatic enzymes & bile to mix with chyme
Although chyme moves in both directions, net effect is movement → colon
Describe peristalsis in the small intestine?
Involves sequential contraction of adjacent rings of smooth muscle
Propels chyme towards colon
Most waves of peristalsis only travel about 10cm
Segmentation & peristalsis result in chyme being segmented, mixed & propelled → colon
Describe the Migrating Motor Complex
Cycles of smooth muscle contractions sweeping through gut
Begin in stomach → small intestine → colon → next wave starts in duodenum
Prevents migration of colonic bacteria into ileum
Does digestion in the small bowel occur in an acidic or alkaline environment?
Alkaline
Where do pancreatic digestive enzymes & bile enter duodenum from?
MPD (main pancreatic duct) & CBD (common bile duct)
Does the duodenal epithelium produce its own digestive enzymes?
Yes
Where does digestion occur in the duodenum?
Digestion occurs in lumen & in contact with the membrane
Where does most of carbohydrate digestion occur?
Small intestine
What does pancreatic alpha amylase need for optimum activity?
Needs Cl- for optimum activity & neutral/slightly alkaline pH
Where does digestion of amylase products & simple carbohydrates occur?
Brush border
Where does pancreatic alpha amylase mainly act?
In the lumen
How are glucose and galactose absorbed?
by 2^o active transport
What carrier protein allows glucose and galactose to be absorbed?
Carrier protein = SGLT-1 on apical membrane
What carrier protein allows fructose to be absorbed?
Carrier protein = GLUT-5 on apical membrane
Describe digestion of proteins?
Protein digestion begins in lumen of stomach by pepsin
-pepsin then inactivated in alkaline duodenum
5x pancreatic proteases secreted as precursors → lumen of small bowel (e.g. trypsinogen)
Trypsin activated by enterokinase
an enzyme located on duodenal brush border
Trypsin → activates other proteases
hydrolyse proteins → single amino acids (AA) & oligopeptides (AA)n
Describe action of luminal, brush-border & cytosolic peptidases in protein digestion.
Variety of peptidases at brush borders of enterocytes progressively hydrolyse (AA)n → AAs
Enterocytes directly absorb some of small (AA)n via action of H+/oligopeptide cotransporter PepT1
These small peptides are digested to AAs by peptidases in cytoplasm of enterocytes
A level lipid digestion
+
What is the purpose of Colipase complexes?
prevents bile salts displacing lipase from fat droplet
What separates the ileum from the colon?
ileocaecal valve
What controls passage of material into colon?
Relaxation & contraction of the ileocaecal valve
What does the ileocaecal valve prevent?
back flow of bacteria into ileum
