Small intestine Flashcards
What is the main function of small intestine?
main site of digestion in body
absorb nutrients, salt and water
Lengths of SI?
6m long, 3.5m diameter duodenum - 25cm jejunum 2.5m ileum 3.75m same basic histology in all 3 lots of muscle tone shortens the intestine and length depends on its function at the time
What is the mesentery?
substrate for nerve and blood supply for the small intestine
throws small intestine into folds to hold its correct position in the abdomen
What is the SI epithelium like?
serosal surface on outside to intestines can move over each other
external wall of long/circular muscle
internal mucosa arranged in circular folds and covered in 1mm tall villi, invaginations of Crypts of Lieberkuhn
Submucosa to provide blood supply/nerve plexuses in intestinal wall
Describe SI villi?
motile, rich blood and lymph (fat) supply for absorption of digested nutrients
innervation by submucosal plexus
simple epithelium with 90% enterocytes (columnar absorptive cells)
What are the cell types of the SI?
mucosa lined with simple columnar epithelium
- mainly enterocytes (absorption)
- goblet cells
- enterendocrine cells
Crypts of Lieberkuhn
- Paneth cells
- stem cells
What are enterocytes ?
most abundant SI cell type
tall, columnar with microvilli and basal nucleus
for absorption and transport of substances
lifespan of 1-6 days
What are microvilli?
- 0.5-1.5 micrometres
- make up brush border
- several thousand per cell
- surface covered in glycocalyx which is attached to the cytoskeleton
What is glycocalyx?
rich carbohydrate layer on apical membrane of microvilli serving to protect from digestional lumen, allows absorption
traps layers of water and mucous (unstirred layer) that regulates rate of absorption from intestinal lumen, regulate what can cause damage contains enzyme for digestion
Internal SA of small intestine?
internal SA of 0.4m2
fold increase it to 200m2 (x500)
What are goblet cells?
2nd most abundant epithelial cell type
contain mucus granules that accumulate apically
(mucus = glycoprotein)
increase abundance along length of bowel to aid passage of bolus as absorption makes food less lubricated
What are enteroendocrine cells?
chromaffin cells/columnar epithelial cells
lower parts of crypts
secrete hormones to influence gut motility (change depending on location and what have eaten)
What are Paneth cells?
in bases of crypts:
- contain large acidophilic granules (stain red)
- contain antibacterial enzyme lysozyme (protects stem cells)
- glycoproteins and Zn
- engulf some bacteria and protozoa (immune cells that protect SCs needed to maintain gut epithelia)
- may have a role in regulating intestinal flora
What is the epithelial lifespan?
enterocytes/goblet cells short life span of 36hrs
continually replaced by dividing stem cells in the crypts
- undifferentiated cells which remain capable of cell division to replace cells which die
- migrate up to tip of villus and replace older cells that die by apoptosis
- pluripotent
- memory of location in gut
Describe epithelial migration
- diving stem cells in crypts move to surface and tips of villi
- at tip, cells become senescent and are sloughed into the lumen of the intestine to be digested and reabsorbed
- rapid turnover
- most cell types is wks/mnths
Why is turnover rapid?
- enterocytes are first line of defense against GI pathogens and can be directly affected by toxic substances in the stomach
- agents may interfere with cell function and diminish metabolic rate
rapid turnover so lesions and damage short lived
What is the effect of interrupting the escalator-like transit of enterocytes via radiation?
impaired production of new cells leads to severe intestinal dysfunction
- lack absorption
- gut disintegrates
What is the action of cholera enterotoxin?
- leads to prolonged opening of chloride channels in the small intestine allowing uncontrolled secretion of water as Cl- moved into gut lumen followed by water
- dehydration and death
treat via rehydration, cholera bacteria will clear and epithelium replaced
people die due to lack of clean water –> reinfected
What is characteristic of the duodenum?
Brunners glands
- submucosal coiled tubular mucus glands
- secrete alkaline fluid into gut lumen
- open into base of crypts
- neutralise acid secretion from stomach, protection
- optimise pH for pancreatic digestive enzymes
What is characteristic of the jejunum?
plicae circularis/valves of Kerckring = large, numerous folds in submucosa
also in duodenum and ileum but here they are taller, thinner and more frequent (frills)