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Y1 Alimentary > Small intestine > Flashcards

Small intestine Flashcards

1
Q

What are the three sections of the small intestine?

A

Duodenum
Mesentery
Jejunum

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2
Q

Length of duodenum

A

25cm approx

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3
Q

Which is the longest section of the small intestine?

A

Ileum

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4
Q

Which section does most of the absorption take place in?

A

Jejunum

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5
Q

Which layer are the major blood supplies and nerves found?

A

Submucosal layer

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6
Q

What are the invaginations in the small intestine called?

A

The Crypts of Leiberkuhn

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7
Q

What cell types are found in the mucosal layer of the small intestine?

A

Enterocytes (mainly)
Goblet cells
Enteroendocrine cells

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8
Q

What cell types are found in the crypts of leiberkuhn?

A

Paneth Cells

Stem cells to make new epithelial cells

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9
Q

What are enterocytes for? How long to they live?

A

Absorption and transport cells

1-6 days lifespan

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10
Q

What structures do enterocytes have?

A

Microvilli that make up the “brush border” and increase surface area and covered in glycocalyx. There are several thousand microvilli per cell

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11
Q

What are glycocalyx?

A
  • a rich carbohydrate layer on apical membrane that serves as protection from the digestional lumen but allows for absorption
  • It also traps a layer of water known as the unstirred layer that regulates the rate of absorption from intestinal lumen
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12
Q

What are goblet cells

A

Mucous containing granules that accumulate at the apical end of a cell causing a goblet shape. There is a greater abundance of goblet cells the further along the bowel you get as more water is absorbed so the food bolus gets drier and needs more mucus to lubricate transport

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13
Q

What is mucous?

A

Large glycoprotein that facilitates passage of material through the bowel

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14
Q

What are enteroendocrine cells

A

Columnar epithelial cells that are scattered among the absorptive cells. In the intestines they are often found in the lower part of the crypts and are hormone secreting (eg to influence gut motility)

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15
Q

What are the qualities of villi?

A
  • They only occur in the small intestine
  • They are motile and have a rich blood supply and lymph drainage for absorption of digested nutrients
  • they have good innervation from the submucosal plexus
  • have simple epithelium (1 cell thick) like the rest of the intestine and dominated by enterocytes
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16
Q

What are Paneth cells?

A

They are only found in the bases of crypts and contain large acidophilic grandules containing:
-lysozyme
-glycoproteins and zinc
and also engulf some bacteria and protozoa. They also may have a role in regulating intestinal flora

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17
Q

Why is zinc required in paneth cells?

A

It is an essential trace metal for a number of enzymes

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18
Q

What are the stem cells in the GI tract for?

A

Replenish surface epithelium by dividing by mitosis and migrating up to the tip of the villus and replacing older cells that die by apoptosis. They differentiate into various cell types - pluripotent

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19
Q

What are the steps of stem cells replacing epithelial cells in villus?

A

An ‘escalator’ of epithelial migration -
dividing stem cells in the crypts move to surface and tips of villi.
At the villus tips, cells become senescent, sloughed into the lumen of the intestine and digested and reabsorbed.

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20
Q

Why is there such a rapid turnover of enterocytes?

A

Because they are suceptible to toxins/damage/bacteria due to their job of absorbing stuff taken in, a rapid turnover of cells makes the effect of the toxins/bacteria minimal and any lesions are short lived. EXAMPLE CHOLERA

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21
Q

How does cholera cause disease and death?

A

Cholera enterotoxin results in prolonged opening of chloride channels in small intestine allowing uncontrolled secretion of water. This means bodily fluids move freely into lumen and out through intestine leading to rapid and massive dehydration and death.

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22
Q

What is the treatment for cholera?

A

Treatment is rehydration and if kept alive for 3 days the patient normally gets better as cholera bacterium removed and epithelium replaced however it is hard to get clean non cholera infected water in areas where cholera is prevalent eg natural disaster zones or developing countries

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23
Q

What differs the duodenum in the small intestine?

A

Presence of Brunner’s glands

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24
Q

What are Brunner’s glands?

A

Submucosal coiled tubular mucous glands secreting alkaline fluid that open into the base of crypts. The alkaline secretions of brunner’s glands neutralise acidic chyme from the stomach and protect the proximal small intestine and also help optimise the pH for pancreatic digestive enzymes

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25
Q

What differs in jejunum in the small intestine?

A

Presence of numerous large folds in the submuosa called pilae cirulares or valves of Kerckring. They are also present in other parts of small intestine but here they tend to be taller, thinner and more frequent

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26
Q

What differs in the ileum?

A

Lots of Peyer’s patches which are large clusters of lymph nodules in the submucosa. They are prime immune system against intestinal bacteria and are well positioned to prevent bacteria from colon migrating up into small intestine

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27
Q

What are the immune system mechanisms against intestinal bacteria?

A

Peyer’s patches
Bactericidal Paneth cells
Rapid cell turnover

28
Q

What are the functions of small intestine motility?

A
  • Mix ingested food with digestive secretions and enzymes
  • Facilitate contact between contents of intestine and the intestinal mucosa
  • Propel intestinal contents along alimentary tract
29
Q

How does segmentation work in the small intestine?

A

MIXES contents of the lumen by stational contraction of circular muscles at intervals. More contractions in duodenum than ilium so pancreatic enzymes and bile can mix with chyme, net movement toward colon

30
Q

How does peristalsis work in the small intestine?

A

Sequential contraction of adjacent rings of smooth muscle propels chyme towards the colon - most waves only about 10cm so short.
This and segmentation lead to chyme propelled toward colon

31
Q

What is the migrating motor complex

A

Cycles of smooth muscle contractions of adjacent segments of small intestine IN FASTING beginning in stomach toward colon. Prevents migration of colonic bacteria into ileum and may clean the intestine of residual food. Also occur in fed state- less ordered and less frequent

32
Q

Is the small intestine an acidic or alkaline environment?

A

Alkaline

33
Q

How do digestive enzymes and bile enter the duodenum?

A

Pancreatic duct and bile duct. The duodenal epithelium also produes it’s own digestive enzymes.

34
Q

What enzyme begins carbohydrate digestion in the mouth?

A

Salivary alpha amylase

35
Q

What enzyme does the duodenum secrete for carb digestion? What does it do? What conditions does it need?

A

Pancreatic alpha amylase continues digestion of starch and glycogen that started in the mouth
Needs Cl- for optimum activity and a neutral or slightly alkaline pH

36
Q

How is an alkaline pH maintained in the small intestine?

A

Brunner’s glands in duodenum

37
Q

Where does pancreatic alpha amylase act? Where are products digested?

A

Mainly in the lumen and also adsorbs to the brush border. Digestion of amylase products and simple carbs occurs at membrane

38
Q

How is glucose and galactose absorbed in the small intestine?

A

Secondary active transport - chemical protein and electrochemical gradient. Carrier protein SGLT-1 on apial membrane.

39
Q

How is fructose absorbed in the small intestine?

A

Facillitated diffusion using carrier protein GLUT-5 on apical membrane

40
Q

What does GLUT-2 do?

A

Facillitates exit at the basolateral membrane

41
Q

What does trypsin do and how is it activated?

A

It activates other proteases as pepsin is inactivated by the alkaline environment
Trypsin is activated by enterokinases, an enzyme on the duodenal brush border

42
Q

How are amino acids absorbed in the small intestine?

A

Facillitated diffusion and secondary active transport

43
Q

What happens to peptidases before they are absorbed in the small intestine?

A

Brush border peptidases break them down

44
Q

What are the 4 stage processes in the small intestine for digestion of lipids?

A
  • secretion of bile and lipases
  • emulsificaiton
  • enzymatic hydrolysis of ester linkages
  • solubilizaiton of lipolytic products in bile salt micelles
45
Q

What makes lipids more complicated to digest?

A

They are poorly soluble in water

46
Q

What do bile salts do

A

Facilitate the emulsification of fat into a suspension of lipid droplets, increasing SA for digestion and allows pancreatic lipase to split triglycerides

47
Q

What is a triglyceride broken down into?

A

2 Fatty acids and a monoglyceride at fat/water interface

48
Q

What is the process of lipid digestion?

A

Lipase breaks down triglycerides into monoglycerides and free fatty acids. Pancreatic lipase complexes with colipase prevents bile salts from displacing lipase from the fat droplet

49
Q

What are 2 other important lipid enzymes

A

Phospholipase A2

Pancreatic cholesterol esterase

50
Q

What does phospholipase A2 do?

A

Hydrolyses fatty acids at the 2 position in many phospholipids resulting in lysopohospholipids and free fatty acids

51
Q

What does pancreatic cholesterol esterase do?

A

Hydrolyses cholesterol ester to free cholesterol and fatty acid

52
Q

What is the structure of a bile salt molecule?

A

Steroid nucleus, planar - two faces, amphipathic.
The hydrophobic face dissolves in fat (nucleus and methyl) and the hydrophillic face (hydroxyl and carboxyl) dissolves in water

53
Q

What are bile salt micelles

A

hydrophilli head region in contat with solvent, hydrophobic tail regions in the micelle centre

54
Q

What are mixed micelles?

A

water insoluble monoglycerides from lipolysis that are solubilised by forming a core and stabilised by bile salts

55
Q

How are micelles absorbed?

A

Bile salts are absorbed in the ileum but the lipid absorption is in the middle of the jejunum

56
Q

Where do the bile salts go after micelles absorbed?

A

Back to liver for recyling - enterohepatic circulation

57
Q

Why are micelles used in lipid absorption?

A

They are absorbed so much quicker than emulsion, they allow transport across the unstirred layer and present the fatty acids and monoglycerides to the brush border

58
Q

When in the enterocytes, how are lipids metabolised?

A

They resynthesize into triglycerides by 2 pathways - monoglyceride acylation (major) and phosphatidic acid pathway (minor)

59
Q

What happens in monoglyceride acylation?

A

Fatty acid binds to apical membrane
Fatty acid binding proteins (FABP) facilliate transfor of fatty acids from apical membrane to the smooth ER
In SER, fatty acids esterified into diglycerides and triglyerides

60
Q

What happens in the phosphatidic acid pathway?

A

Triglycerides are synthesised from CoA fatty acid and alpha glycerophosphate

61
Q

What are chylomicrons?

A

Lipoprotein particles synthesised in enteroytes as an emulsion of 80-90% triglyeride, 8-9% phospholipid, 2% cholesterol and 2% protein with trace carbs

62
Q

What happens to chylomicrons made in enterocytes?

A

Transported to golgi and secreted across the basement membrane by exocytosis. Too big to enter the blood capillaries of villi so instead enter lacteals

63
Q

What are lacteals

A

Lymph channels

64
Q

How is the ileum seperated from the colon?

A

Ileocaecal sphincter

65
Q

What does relaxation and contraction do in the ileum?

A

Controls passage of material into colon

Prevents backflow of bacteria into the ileum

Y1 Alimentary (21 decks)

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