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)
What characterises the ileum?
lots of Peyers patches
- clusters of lymph nodules in submucosa
- prime immune system against intestinal bacteria
- located to prevent fewer bacteria colonising ileum from colon
What are the lines of defence of the ileum?
Peyers patches
bactericidal Paneth cells
rapid cell turnover
What are 3 functions of gut motility?
1 - mix ingested food with digestive secretions and enzymes
2 - facilitate contact between contents of intestine and intestinal mucosa for absorption
3 - propel contents along alimentary tract
What is segmentation?
MIX contents of lumen
- stationary contraction of circular muscles at intervals
- more frequent in duodenum than ileum to mix chyme with bile/pancreatic enzymes
- net movement to colon (but both direction)
What is peristalsis?
PROPEL chyme to colon
- sequential contraction of smooth circular muscle rings
- waves travel 10cm
- more coordinated than segmentation
Describe the migrating motor complex?
cycles of smooth muscle contractions when fasting
cycle = contraction of adjacent segments of small intestine
-start in stomach and migrate to colon
- prevent migration of colon bacterial to ileum and cleans intestine of residual food
- less frequent and ordered in the fed state
Describe digestion in the duodenum?
alkaline environment
digestive enzymes and bile from pancreatic/bile duct
duodenal epithelium produces own digestive enzymes
digestion in lumen (segmentation) and in contact with membrane (brush border)
What is primary and secondary active transport?
1 - direct coupling to ATP
2 - uses secondary energy source from [] gradient set up previously (eg. Na gradient set up by NaK ATPase pump
Describe digestion of carbohydrates in the gut?
- starts in mouth by salivary a-amylase
- destroyed in stomach by acid pH
- most digestion of carbs is in small intestine
- pancreatic a-amylase secreted into duodenum in response to a meal
- needs Cl- for optimum activity (released in pancreatic juice) and neutral/slightly alkaline pH (Brunners glands in duodenum)
- acts mainly in lumen, also adsorbs to brush border
What is luminal digestion of carbs?
to oligosaccharides/disaccharides
Where is digestion of simple carbs and amylase products?
at membrane of small intestine by brush border enzymes (maltase, lactase, sucrase)
then taken into enterocytes
Describe the absorption of glucose/galactose?
by secondary active transport
- use energy from allowing Na back in along [] gradient to pump monosaccharide against [] gradient
- use SGLT-1 carrier protein
- allows high M in enterocyte
Describe absorption of fructose?
by facilitated diffusion
- use GLUT-5 carrier protein on apical membrane
- low [] fructose in cell
What is the function of GLUT-2?
facilitates exit at basolateral membrane by facilitated transport
increase absorption capacity
Describe protein digestion?
- in stomach by pepsin
- pepsin inactivated in alkaline duodenum so pancreatic proteases are secreted as precursors
- trypsin activated by enterokinase on duodenal brush border
4, trypsin then activates other proteases
Describe activation of trypsinogen?
released as inactive precursor from pancreas and activated by enterokinase (so does not digest pancreas)
trypsin further catalyses its own conversion and that of other precursors to own inactive form
Describe the absorption of proteins?
- brush border peptidase break down large peptides
- AAs absorbed by facilitated diffusion and 2ry active transport
- di/tri peptides absorbed using carrier proteins
- cytoplasmic peptidases break thse down before cross basolateral membrane so mostly AA absorbed into blood
Describe digestion of lipids?
4 steps
- secretion of bile and lipase from pancreas
- emulsify by bile
- enzymatic hydrolysis of ester links
- solubilisation of lipolytic products in bile salt micelles
What is emulsification?
- bile salts aid emulsifying fats into suspension of lipid droplets (1micrometre wide)
- increases SA for digestion of TGs by pancreatic lipase to 2 FAs and a monoglyceride
What aids the efficiency of lipase and how?
colipase complexes with pancreatic lipase prevents bile salts from displacing lipase from fat droplet colipase secreted inactive from lipase active form
What is phospholipase A2?
hydrolyses fatty acids at 2 position in phospholipids
leading to lyso-phospholipids and free fatty acids
What is pancreatic cholesterol esterase?
hydrolyses cholesterol ester to free cholesterol and fatty acid
What is the structure of a bile salt?
- steroid nucleus that is planar, amphipathic
- hydrophobic side (nucleus and methyl) dissolve in fat
- hydrophilic side (hydroxyl, carboxylm peptide bonds) dissolve in water
What are bile salt micelles?
micelle = hydrophilic head regions in contact with solvent sequester hydrophobic tail regions in micelle centre
in SI, mixed micelles form when water insoluble monoglycerides from lipolysis are solubilised by forming a core, stabilised by bile salts
Describe the absorption of lipids?
micelles are transported across the unstirred layer and present MGs and FAs to brush border
- bile salts absorbed in ileum and transported back to liver for recycling via enterohepatic circulation
- lipid absorption complete by middle jejunum
How are lipids that are absorbed by enterocytes resynthesised into TGs?
MGs and FAs are resynthesised into TGs by:
monoglyceride acylation MAJOR
phosphatidic acid pathway MINOR
What is monoglyceride acylation?
FAs bind to apical membrane and FABP (binding proteins) transfer FAs from apical membrane to sER
here FAs are esterified to DGs and TGs
What is the phosphatidic pathway?
TGs are made from CoA fatty acids and a-glycerophosphate
How are fats transported from enterocytes?
- lipoprotein particles are made in enterocytes as an emulsion
- 80-90% TGs, 8-9% phospholipids, 2% chol, 2% protein
- chylomicrons are transported to Golgi and secreted across BM by exocytosis
- too big to enter blood capillaries of villi so enter lacteals
How is food passed to colon from ileum?
separated by ileocaecal sphincter
- relax and contracts to control food passage
- prevent back flow of bacteria to ileum
Where is most absorbtion?
at jejunum
also digestion and duodenum and jejunum
