Small Intestine Flashcards by Urvi Sonawane

3 sections of small intestine+ lengths

What connects them

Duodenum (25cm)
Jejenum (2.5m)
Ileum (3.75m)
Mesentery= throws small intestine into folds, connective tissue for nerve+ blood supply

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Layers of digestive epithelium (outside to in)

Serosa- allows intestine outside layers to move over itself
Longitudinal muscle- runs along gut
Circular muscle- contracts+ closes off lumen
Submucosa- provides layer of tissue for blood supply + nerves within intestinal wall sit
Mucosa- arranged in circular folds (invaginations called Crypts of Lieberkühn + covered in villi (increase SA)

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Villi

Motile
Rich blood supply
Lymph drainage
Good innervation from submucosal plexus (nerve network in submucosa)= susceptible to nervous control
1 cell thick epithelium made of enterocytes (columnar absorptive cells)

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Cell types of small intestine in
Mucosa- type of epithelium?
Crypts of Lieberkühn

Mucosa- simple columnar epithelium made of 
Primarily enterocytes (absorptive, covered in microvilli, most abundant, frequent renewal)
Goblet cells (mucous secreting)
Enteroendocrine cells (hormone secreting)

Crypts of Lieberkühn
Paneth cells (antibacterial, protect stem cells)
Stem cells (migrate up villus, pluripotent for cell renewal)

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Enterocytes
Site of nucleus?
Specialisation?
Lifespan?
Microvilli covered in? Function?

Basal nucleus
Absorption+ transport of substances
1-6 days
Microvilli covered in glycocalyx- hairy bits on microvilli, rich carbohydrate layer on apical membrane.
Function= protection from lumen but also allows for absorption. Traps ‘unstirred layer’ of water+ mucus= regulates absorption rate from lumen. Also contains enzymes helpful for digestion+ trap water

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How much larger is small intestine SA than just cylinder

500 fold increase (0.4m^2 to 200 m^2)

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Goblet cells
Contain?
Spread of goblet cells along small intestine?

2nd most abundant cell type
Contain mucous containing granules
Mucous
Increase frequency along small intestine because absorb more water from food= becomes more solid= need more mucus to help it)

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Mucous purpose

Large glycoprotein that facilitates passage of material through bowel

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Enteroendocrine cells
Where found+ spread?
Function

Scattered amongst absorptive cells
Found in lower part of crypts
Hormone secretion to influence gut motility

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Paneth cells
Found where?
Functions?

REAPP

Only found at the bases of the crypts
Regulate intestinal flora
Engulf some bacteria
Contain large acidophilic granules containing lyzozyme (antibacterial enzyme)+ glycoproteins+ zinc (co-factor)
Protect from protozoa
Protect stem cells which are found at the base of the crypt

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Stem cells

Cell feature?

Undifferentiated cells that remain capable of cell division to replenish surface epithelium by differentiation
Divide by mitosis
Migrate up to tip of villus= replace older cells that become senescent and are sloughed into lumen= digested+ reabsorbed
Large nucleus

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Why such rapid turnover of cells in epithelium?

What happens if turnover= inhibited?

Enterocytes= first line of defence against GI pathogens+ may be affected by toxic substances in diet= effect of agents that interfere with cell function etc= diminished
Any lesions= short-lived
Sever intestinal dysfunction

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Cholera enterotoxin
What happens?
Treatment?

Prolonged opening of Cl channels in small intestine= uncontrolled water secretion= dehydration+ death
Treatment= rehydration, epithelium will be replaced

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What can distinguish the duodenum apart?

Brunner’s glands
Secrete alkaline fluid into gut lumen into base of crypts
Help neutralise potential damage to cells
Optimal enzymes in ileum
Neutralise acidic chyme from small intestine+ protects epithelium

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What can distinguish the jejunum apart?

Presence of plicae circulares/ valves of Kercking
Many large folds in submucosa
Also in duodenum+ ileum but here they are taller, thinner, more frequent
Leads to increase SA

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What can distinguish the ileum apart?

Peyer’s patches
Clusters of lymph nodules in submucosa
Defence against intestinal bacteria
Prevent bacteria from colon from migrating up into small intestine

Immune system against intestinal bacteria?

Peyer’s patches in ileum
Bactericidal Paneth cells
Rapid cell turnover

3 functions of small intestine motility

To mix ingested food with digestive secretions+ enzymes
Facilitate contact between intestine contents+ intestinal mucosa (because different enzymes at wall for other digestion steps)
Propel intestinal contents along ailmentary tract

Migrating Motor Complex
When does it happen?
What does each cycle involve?
Begin+ end?
Purpose?

During fasting
Contraction of adjacent segments of small intestine
Begin in stomach along small intestine, when terminal ileum reached, next one begins at duodenum
Prevents migration of colonic bacteria+ prevents accumulation of residue

Digestion in duodenum
What controls what can move into duodenum?
Environment?
What happens?
What's produced?

Digestive enzymes+ bile enter duodenum from pancreatic duct+ bile duct
Pyloric sphincter
Alkaline
Stuff broken down in middle breaks down large molecules which are further broken down at brush border
Duodenal epithelium produces its own digestive enzymes

Types of active transport

Primary- relates to ATP hydrolysis to pump something

Secondary- uses energy second-hand by using concentration gradient that has already been set up

Digestion of carbohydrates
Enzyme?
Requirements?
Where does it act?

Begins with salivary amylase but destroyed in stomach
Pacreatic α-amylase secreted into duodenum
Needs Cl-+ slightly alkali pH provided by Brunner’s glands in duodenum
Acts mainly in lumen
Digestion of amylase products+ simple carbohydrates occurs at membrane

Absorption of glucose+ galactose
Method?
Name of carrier protein taking in glucose+ Na+

Absorption of fructose
Method?
Name of carrier protein

Name of carrier protein facilitating exit from basement membrane

Secondary active transport
Na+/K+ pump at basolateral membrane, Na out, K+ in
SGLT-1 on apical membrane pumping glucose/ galactose in against conc. gradient using sodium gradient

Facilitated diffusion
GLUT-5 on apical membrane

GLUT-2 allows exit of fructose, galactose and glucose

Digestion of proteins
Enzymes
Protease life-span

Begins in stomach with pepsin but it’s inactivated in alkaline duodenum
Pancreatic proteases secreted as precursors- trypsinogen
Trypsin activated by enterokinase located on duodenal brush border so that pancreas doesn’t digest itself
Trypsin then activates other proteases, some lipolytic enzymes and more of itself (+ve feedback)
Short lived because digested themselves

Absorption of proteins What happens before absorption? How are amino acids absorbed? How are di+ tri-peptides absorbed? What happens after?

Brush border peptidases break down larger peptides prior to absorption Facilitated diffusion+ secondary active transport Carrier proteins different from amino acids, cytoplasmic peptidases break down most of them before they cross the basolateral membrane Mostly absorb single amino acids by the end

``` Digestion of lipids Why are they more complicated to digest? 4 stages? What does lipase require? Function? Other lipid enzymes? ```

Poorly soluble in water 1. Secretion of bile+ lipases in active form from pancreas into duodenum which facilitate: 2. Emulsification of fat into lipid droplet suspension to increase SA for lipases to split tryglycerides into two fatty acids+ monoglyceride (still linked at middle) 3. Enzymatic hydrolysis of ester linkages (fats) 4. Solubilisation of lipolytic products in bile salt micelles Lipase requires bile salts+ co-lipase which is secreted as a precursor and only activated when it's in the gut Co-lipase prevents bile salts from displacing lipase from fat droplet Others: Phospholipase A2- hydrolyses fatty acids at 2 position= lysophospholipds+ free fatty acids Pancreatic cholesterol esterase- hydrolyses cholesterol ester= free cholesterol+ fatty acid

Bile salt molecule structure+ function

``` Two faces Flat nature= can be amphipathic Hydrophobic face (nucleus+ methyl) dissolves in fat Hydrophilic face (hydroxyl + carboxyl) dissolves in water ```

Bile salt micelles Structure? Function? Property?

Hydrophilic 'head' regions in contact with surrounding solvent, hydrophobic tail regions in micelle centre Micelles in small intestine= water insoluble monoglycerides from lipolysis= solubilised by forming a core stabilised by bile salts ENterohepatic circulation: Can be transported back to the liver for recycling

Absorption of lipids | Which is quickest method of absorption?

Micelles quicker than emulsion | Micelles allow transport across unstirred layer+ present fatty acids+ monoglycerides to brush border

Where are bile salts absorbed? | Where is lipid absorption usually complete by?

Ileum | Middle of the jejenum

Lipid metabolism Which cell type are monoglycerides + free fatty acids absorbed by? How are they resynthesised into triglycerides? (2 methods) What is formed? Where is it transported?

Enterocytes 1. Monoglyceride acylation- major pathway, Fatty acids bind to apical membrane Fatty acid binding proteins (FABP) facilitate transfer of fatty acids from apical membrane to smooth ER In smooth ER, fatty acids esterified into diglycerides+ triglycerides 2. Phosphatidic acid pathway- minor pathway Form new tryglycerides from acetyl CoA+ α-glycerophosphate Chylomicrons are formed: Chylomicron= Lipoprotein particles synthesised in enterocytes as an emulsion Transported to Golgi+ secreted across basement membrane by exocytosis Too big to enter blood capillaries= enter lacteals (lymph channels) instead

How is the ileum seperated from the colon? | 2 functions?

Ileocaecal sphincter Relaxation+ contraction controls passage of material into colon Prevents back flow of bacteria into ileum