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Alimentary > Small Intestine > Flashcards

Small Intestine Flashcards

1
Q

What is the function of the SI

A

Absorb nutrients salt and water

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2
Q

How long is the small intestine and what is the diameter and what is the surface area

A

6m long

  1. 5cm diameter
  2. 4m^2 cyclindrical, total 200m^2 (x500 increase)
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3
Q

What are the lengths of the duodenum, jejunum and the ileum

A

duodenum - 25cm
jejunum - 2.5m
Ileum - 3.75cm

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4
Q

What is the mesentery

A

Fan shaped connective tissue
Shapes the small intestine into folds
Supports the blood supply

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5
Q

Describe the histological organisation of the duodenum jejunum and ileum

A

All have the same basic organisation

No sudden transition between them

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6
Q

Describe the structure of the digestive epithelium

A
  1. Serosa
  2. Longitudinal muscle
  3. Circular muscle
    4.submucosa
  4. Mucosa
  5. Circular folds (internal mucosa)
    Mucosa covered villi (1mm) that increase SA
    Invaginations - crypts of Lieberkuhn
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7
Q

Describe the villi

A

Only found in the SI
motile with a rich blood supply and lymph drainage for absorption
Good innervation from the submucosal plexus (responsive to nervous control)
Simple squamous epithelium dominated by enterocytes (columnar)

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8
Q

What does the epithelium include in the crypts of lieberkuhn

A

Paneth and stem cells

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9
Q

What cells are contained in the SI mucosa

A

enterocytes
goblet cells
enteroendocrine cells

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10
Q

Describe the enterocytes

A

Most abundant cells in the intestine
Tall columnar cells with microvilli and a basal nucleus
Absorption and transport
Short lifespan (1-6 days)

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11
Q

What are the kinds of cell junctions in the enterocytes

A

Tight junctions

Desmosomes

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12
Q

Describe microvilli

A

0.5-1.5 micrometers
Several thousand per cell
Covered with glycocalyx

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13
Q

What is glycocalyx

A

Rich carb layer on the apical membrane
Protection from the lumen but allows absorption
Traps a layer of water and mucous (unstirred layer) regulating absorption
Attached to the cytoskeleton

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14
Q

Describe the goblet cells

A

2nd most abundant cell type
Mucous containing granules that accumulate at the apical end (shape)
mucous is a large glycoprotein that facilitates passage
The abundance of goblet cells increase as you travel down the bowel

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15
Q

Describe the enteroendocrine cells / chromaffin cells

A

Columnar epithelial
Most found in the lower part of the crypts
Secretes hormone
Affinity for chromium/silver salts

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16
Q

Describe the paneth cells

A 
Bases of crypts 
Large acidophilic granules
Contains lysozyme for stem cell protection and glycoproteins and zinc for enzymes
Engulfs bacteria and protozoa 
(role in regulating intestinal flora)
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17
Q

Describe turnover in the epithelium

A

Enterocytes and goblet cells have a short lifespan of 36 hours
Cells become senescent and sloughed into the lumen of the intestine, digested and reabsorbed
Continuously replaced by pluripotent stem cells in the crypts that migrate up the villus to replace the cells that undergo apoptosis
Such a large turnover is energy intensive (30% of energy)

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18
Q

Why does the SI epithelium have such a rapid turnover

A

Enterocytes may be affected by toxic substances in the diet
They may interfere with cell function, metabolism etc.
Lesions will be short lived
Interruption of turnover will cause severe intestinal dysfunction

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19
Q

Describe the cholera enterotoxin

A

Results in prolonged opening of the chloride channels in the small intestine allowing uncontrolled secretion of water
Rapid, massive dehydration and death

20
Q

What is the treatment for the cholera enterotoxin

A

Rehydration

Cholera bacteria will clear and the epithelium will be replaced

21
Q

What are the distinguishing features of the duodenum, jejunum and ileum

A

duodenum - Brunners glands
Jejunum - numerous large folds in the submucosa called plicae circulates/ valves of kerckring that are taller, thinner and more frequent
Ileum - lots of peyers patches that prevent bacteria migrating up to the SI

22
Q

What are the brunners glands

A

Submucosal coiled tubular mucous glands that secrete alkaline fluid
Opens into the base of the crypts
Neutralises the acidic chyme from the stomach to protect the SI
helps optimise pH for action of pancreatic digestive enzymes

23
Q

What is the mechanism of immunity against intestinal bacteria

A

peyer’s patches
Bactericidal paneth cells
Rapid cell turnover

24
Q

What are the 3 functions of SI motility

A

Mix ingested food with digestive secretions and enzymes
Facilitate contact between contents of intestine and intestinal mucosa
Propel intestinal contents along the tract

25
Q

Describe segmentation in the SI

A

Stationary contraction of circular muscles at intervals
More frequent in the duodenum compared to the ileum, allowing pancreatic enzymes and bile to mix with chyme
Chyme moves in both directions but net is towards the colon

26
Q

What is the migrating motor complex

A

Fasting = cycles of smooth muscle contractions. Each cycle is contraction of adjacent segments of the SI
Stomach -> SI -> Colon
ileum -> next contraction in the duodenum
Prevents migration of colonic bacteria to the ileum
Fed state = less ordered and less frequent

27
Q

Describe digestion in the duodenum

A

Occurs in an alkaline environment
Digestive enzymes and bile from the pancreatic duct + bile duct
Produces its own digestive enzymes
Occurs both at the lumen and in contact with the membrane

28
Q

Describe the following mechanisms: Passive diffusion, facilitated diffusion, primary active transport, secondary active transport

A

Passive - no energy with the gradient
faciliated - with the gradient through a carrier protein
Primary active - against the gradient through a carrier protein after hydrolysis of ATP
Secondary active - Against the gradient through a protein with an electrochemical gradient

29
Q

Describe the digestion of carbs

A

50% of ingested calories
Salivary alpha-amylase digests in the mouth (destroyed in the stomach)
Most digestion is in the SI

30
Q

What is alpha-amylase and what does it require

A

Enzyme secreted into the duodenum lumen in response to a meal
Continues digestion of starch and glycogen in the SI
Requires Cl- for optimum activity and a neutral/slightly alkaline pH (found in pancreatic secretions)

31
Q

Where does digestion of amylase and products and simple carbohydrates occur

A

Membrane of the duodenum

32
Q

Explain the process of carb digestion

A

complex carbs e.g. starch are broken down by pancreatic amylase
The disaccharides and oligosaccharides produced are converted to monosaccharides at the membrane by maltase, lactase, sucrase etc.
Enters the enterocyte

33
Q

Describe the absorption of carbs

A

glucose and galactose = Secondary active transport with SGLT-1 carrier protein n the apical membrane

fructose = facilitated diffusion with GLUT-5 carrier protein on the apical membrane

GLUT-2 facilities exit at the basolateral membrane
10kg of sugar a day

34
Q

Describe peristalsis in the SI

A

Sequential contraction of adjacent rings of smooth muscle

Propels chyme and mixes It

35
Q

Describe the digestion of proteins

A

Begins in the stomach by pepsin
Inactivation of pepsin in the alkaline duodenum
Pancreatic proteases are secreted as precursors

36
Q

What is trypsin

A

Activated by enterokinase (brush border)

Trypsin can activate other enzymes such as other trypsin and proteases

37
Q

Explain the process of protein absorption

A
  1. Pepsin breaks down proteins in to smaller peptide chains
  2. brush border peptidases break them down more
  3. amino acids are absorbed by facilitated diffusion and secondary active transport
  4. Di and tri peptides are absorbed by different carrier proteins
  5. Cytoplasmic peptidases break down the di and try peptides before they cross the basolateral membrane
38
Q

Describe the digestion of lipids

A

Poorly soluble in water so they are more complex to digest
Four stages:
1. secretion of bile and lipases
2. Emulsification
3. Enzymatic hydrolysis of ester linkages
4. Solubilisation of lipolytic products in bile salt micelles

39
Q

Describe the emulsification of lipids

A

Bile and lipase secreted into the duodenum facilitate emulsification of fat into lipid droplets (1 micrometer)
Increases the surface area for digestion and allows lipase to split TAGs to 2 fatty acids and MAG

40
Q

What is the significance of colipase

A

Pancreatic lipase cannot enter the fat droplet to break down CAGs without forming a complex with cop lipase.
Prevents bile salts from displacing lipase from the fat droplet

41
Q

Give two other important lipid enzymes other than lipase

A

Phospholipase A2 hydrolyses fatty acids at the 2 position in many phospholipids, resulting in lyso-phospholipids and free fatty acids.

Pancreatic cholesterol esterase hydrolyses cholesterol ester to free cholesterol and fatty acid.

42
Q

Describe the bile salt molecule

A

Steroid nucleus with two faces
Amphipathic
Hydrophilic hydroxyl and carboxyl face dissolves inwards (outer layer)
Hydrophobic nucleus and methyl face that dissolves in fat (inner layer)

43
Q

What is a micelle

A

Lipid core surrounded by bile salts with hydrophilic heads in contact with the solvent
Water insoluble
Allows quicker absorption across the unstirred layer
Bile salts transported back to the liver for recycling (enterohepatic)

44
Q

How is the micelle absorbed

A

Bile salts - ileum

Lipid - middle of the jejunum

45
Q

Describe lipid metabolism in enterocytes

A

MAGs and free acids are absorbed by the enterocytes and resynthesises to TAGs

Monoglyceride acylation (major) - FAs bind to the apical membrane and fatty acid binding proteins help transfer the FAs to the smooth ER where they are esterfied

Phosphatidic acid pathway (minor) - TAGs synthesised from CoA fatty acid and alpha glycerophosphate

46
Q

What are chylomicrons

A

Lipoprotein particles synthesised in enterocytes as an emulsion
TAGs, phospholipids, cholesterol, protein
Enters the lacteals

47
Q

How is the ileum separated from the colon

A

ileocaecal sphincter

Relaxation of this controls passage of material into the colon and prevents the back flow of bacteria into the ileum

Alimentary (16 decks)

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