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BRS GASTROENTEROLOGY > Small Bowel > Flashcards

Small Bowel Flashcards

1
Q

What is the function of the small bowel?

A

To absorb nutrients, salt and water

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2
Q

Label the diagram.

A
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3
Q

What is the length and diameter of the small bowel?

A

6m long
3.5 cm in diameter

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4
Q

How long is the duodenum?

A

25cm

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5
Q

How long is the jejenum?

A

2.5m

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6
Q

How long is the ileum?

A

3.75m

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7
Q

What is the transition like between the DJI?

A

no sudden transition
they all have same basic histological organisation

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8
Q

What is the function of the mesentery?

A

Suspends small & large bowel from posterior abdominal wall
- anchoring them in place
- whilst still allowing some movement

Provides a conduit for blood vessels, nerves & lymphatic vessels.

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9
Q

Label the arteries in the mesentery.

A
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10
Q

What is the digestive epithelium like?

A

plicae circulares-> vilus

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11
Q

Describe the structure of the villus.

A

Endocrine cells
Absorptive cells (enterocytes)
Cells Paneth

Villus
Epithelium
crypt
brush border

Intestinal gland

Lacteal
blood capillaries

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12
Q

Where do villi occur?

A

Onl occur in the small intestine

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13
Q

How would you describe villi.

A

Motile

Have a rich blood supply & lymph drainage for absorption of digested nutrients

Have good innervation from the submucosal plexus.

Have a simple epithelium (1 cell thick)

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14
Q

What cells dominate the villi?

A

enterocytes (columnar absorptive cells)

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15
Q

Label this diagram.

A
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16
Q

What is the villi (mucosa) lined with?

A

simple columnar epithelium consisting of:
- primarily enterocytes (absorptive cells)
- scattered goblet cells
- enteroendocrine cells

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17
Q

What is the crypt called?

A

Crypts of Lieberkuhn

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18
Q

What does the crypt of lieberkuhn epithelium include?

A

Paneth cells
Stem cells

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19
Q

What is the most abundant cell in the small bowel?

A

Enterocytes

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20
Q

What are enterocytes?

A

Tall columnar cells with microvilli & a basal nucleus.

Specialised for absorption & transport of substances.

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21
Q

What is the lifespan of enterocytes?

A

1-6 days

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22
Q

Describe the surface area of small bowel.

A

Cylindrical internal surface area of small bowel is 0.4m2.

Folds, villi & microvilli ↑ surface area to ~200m2 (size of a tennis court)
- At least a 500 fold ↑

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23
Q

What are microvilli?

A

Microvilli (~0.5-1.5μm high) make up the “brush border”.

Several thousand microvilli per cell

Surface of microvilli covered with glycocalyx

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24
Q

What is glycocalyx?

A

Rich carbohydrate layer on apical membrane

Serves as protection from digestional lumen yet allows for absorption.

Traps a layer of water & mucous known as “unstirred layer”

Regulates rate of absorption from intestinal lumen

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25
Q

What is the 2nd most abundant epithelial cell type?

A

goblet cells

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26
Q

What is inside goblet cells?

A

Mucous containing granules accumulate at apical end of
cell, causing ‘goblet’ shape.

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27
Q

What is mucous?

A

Mucous → large glycoprotein that facilitates passage of material through bowel.

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28
Q

How does goblet cell abundance change through the small bowel?

A

abundance of goblet cells along entire length of bowel
↓ in duodenum
↑ in colon

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29
Q

What are enteroendocrine cells?

A

Columnar epithelial cells

scattered among enterocytes

most often found in lower part of crypts

Hormone secreting
- e.g. to influence gut motility (CCK)

  • In older text books → referred to as chromaffin cells (affinity for chromium/silver salts).
30
Q

What are paneth cells?

A

Found only in the bases of crypts

Contain large, acidophilic granules

Granules contain:
- antibacterial enzyme lysozyme (protects stem cells)
- Glycoproteins & zinc (essential trace metal for a no. of enzymes)

Also engulf some bacteria & protozoa

May have a role in regulating intestinal flora

31
Q

What are stem cells in the small bowel?

A

Undifferentiated cells which remain capable of cell division to replace cells which die

Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium

Continually divide by mitosis

Migrate up to tip of villus, replacing older cells that die by apoptosis
- They are to digested and reabsorbed

Differentiate into various cell types (pluripotent)

32
Q

Enterocytes & goblet cells of small bowel have a short life span (about 36 hrs)
Rapid turnover contrasts with lifespan of weeks/months for other epithelial cell types (e.g. lung, blood vessels)
Q. Why?

A

Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet.

Effects of agents which interfere with cell function, metabolic rate etc will be diminished.

Any lesions will be short-lived.

If escalator-like transit of enterocytes is interrupted through impaired production of new cells (e.g. radiation) severe intestinal dysfunction will occur

33
Q

How is the duodenum distinguishable?

A

Distinguished by presence of Brunner’s glands

34
Q

What are brunner’s glands?

A

Submucosal coiled tubular mucous glands secreting alkaline fluid
- Open into the base of the crypts

35
Q

What do the alkaline secretion of brunner’s glands do?

A

Neutralizes acidic chyme from stomach, protecting proximal small bowel
Help optimise pH for action of pancreatic digestive enzymes

36
Q

What is the difference between the jejunum and ileum?

A

Jejunum=
thick wall
plicae circulares

ileum=
not as prominent plicae circulares
thin wall
payer’ patches
smooth mucous membrane

37
Q

What are the functions of small intestine motility?

A

To mix ingested food with digestive secretions & enzymes

To facilitate contact between contents of intestine & the intestinal mucosa

To propel intestinal contents along alimentary tract

38
Q

What are the types of small bowel motility?

A

Segmentation (mixing)
Peristalsis (propelling)
Migration Motor Complex

39
Q

What is segmentation?

A

Mixes contents of lumen

Occurs by stationary contraction of circular muscles at intervals.

More frequent contractions in duodenum cf. ileum

Allow pancreatic enzymes & bile to mix with chyme

Although chyme moves in both directions, net effect is movement
→ colon

40
Q

What is peristalsis?

A

Involves sequential contraction of adjacent rings of smooth muscle

Propels chyme towards colon

Most waves of peristalsis only travel about 10cm

Segmentation & peristalsis result in chyme being segmented, mixed & propelled → colon

41
Q

What is migrating motor complex?

A

Cycles of smooth muscle contractions sweeping through gut

Begin in stomach → small intestine → colon → next wave starts in
duodenum

Prevents migration of colonic bacteria into ileum

42
Q

Label this diagram.

A
43
Q

What is digestion like in the duodenum?

A

Digestion in small bowel occurs in an alkaline environment

Pancreatic digestive enzymes & bile enter duodenum from MPD & CBD (common bile duct)

Duodenal epithelium also produces its own digestive enzymes

Digestion occurs in lumen & in contact with the membrane

44
Q

How much fo the western diet is carbohydrates?

A

roughly 50%

45
Q

Where does digestion start?

A

In mouth by salivary alpha amylase

46
Q

ow is salivary amylase destroyed?

A

in stomach by acid pH

47
Q

Where does most digestion of carbs happen?

A

In the small intestine

48
Q

What are examples of simple carbohydrates monosaccharides?

A

glucose and fructose

49
Q

What are examples of simple carbohydrates disaccharides?

A

sucrose and maltose

50
Q

What are examples of complex carbohydrates and what are they?

A

starch, cellulose, pectins
-> sugars bonded together to form a chain

51
Q

What is pancreatic alpha-amylase? (5 things)

A

Secreted into duodenum in response to a meal

Continues digestion of starch & glycogen in small bowel (started by salivary amylase)

Needs Cl- for optimum activity & neutral/slightly alkaline pH

Acts mainly in lumen (some also adsorbs to brush border)

Digestion of amylase products & simple carbohydrates occurs at the brush border

52
Q

Are you happy with this diagram?

A

Circled nos= approx. % of substrate hydrolysed by each brush border enzyme

53
Q

What are the types of alpha-amylase?

A

alpha-dextrins
Maltotriose
Maltose

54
Q

How are each carbohydrates absorbed?

A

Absorption of glucose & galactose is by 2ary active transport
- Carrier protein = SGLT-1 on apical membrane

Absorption of fructose is by facilitated diffusion.
- Carrier protein = GLUT-5 on apical membrane

GLUT-2 facilitates exit at basolateral membrane

Human small intestine can absorb 10kg of simple sugars/day

55
Q

What are the steps of digestion of carbs?

A
  1. Digestion of starch in lumen
  2. digestion of oligosaccharides at brush border
  3. absorption of monosaccharides
56
Q

Where does protein digestion start?

A

in lumen of stomach by pepsin

57
Q

When is pepsin from the stomach inactivated?

A

in alkaline duodenum

58
Q

How are proteases secreted for digestion of protein in small bowel?

A

5x pancreatic proteases secreted as precursors in the lumen of small bowel e.g., trypsinogen

59
Q

How is trpysin activated?

A

Trypsin is activated by enterokinase

60
Q

Where is enterokinase located?

A

on duodenal brush border

61
Q

What does trypsin do?

A

activates other proteases

hydrolyse proteins → single amino acids (AA) & oligopeptides (AA)

62
Q

What progressively hydrolysis amino acid chains to amino acids?

A

peptidases at brush borders of enterocytes

Enterocytes directly absorb some of small (AA)n via action of H+/oligopeptide cotransporter PepT1

These small peptides are digested to AAs by peptidases in cytoplasm of enterocytes

This is the action of luminal, brush-border & cytosolic peptidases

63
Q

Are lipids soluble?

A

No,they are poorly soluble in water

64
Q

What does the lipid’s solubility affect?

A

Makes it more complicated to digest

65
Q

What are the stages of digestion of lipids?

A
  1. Secretion of bile salts & pancreatic lipases
  2. Emulsification (↑s surface area for digestion)
  3. Enzymatic hydrolysis of ester linkages
    - Colipase complexes with lipase – prevents bile salts displacing
    lipase from fat droplet
  4. Solubilisation of lipolytic products in bile salt micelles
66
Q

How are lipids different to AA and simple sugars?

A

lipids transformed as absorbed via enterocytes

67
Q

Describe the absorption of lipids.

A

Fatty acids (FAs) & monoglycerides (MG) leave micelles and enter enterocytes

FAs & MG resynthesized into triglycerides (TGS) by 2x pathways:
- Monoglyceride acylation (major)
- Phosphatidic acid pathway (minor)

Chylomicrons - lipoprotein particles synthesised as an emulsion (80-90% TGs, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrate) in Golgi apparatus

68
Q

ow are chylomicrons secreted?

A

Chylomicrons secreted across basement membrane by exocytosis

Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel

69
Q

How do you know which side is closer to the blood,. apical or basolateral?

A

think basolateral= blood

70
Q

How is the ileum separated from the colon?

A

Ileocaecal valve

71
Q

What control passage of material into colon?

A

Relaxation and contraction

72
Q

What does relaxation and contraction also help?

A

prevent back flow of bacteria into ileum

BRS GASTROENTEROLOGY (25 decks)

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