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2.8 BRS Gastroenterology > Small Bowel > Flashcards

Small Bowel Flashcards

1
Q

What is the function of the small bowel?

A

Absorb nutrients, salt and water

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2
Q

What are the 3 sections of the small bowel?

A
  1. Duodenum - 25cm
  2. Jejunum - 2.5m
  3. Ileum - 3.75m
    - total: 6m long
    - diameter: 3.5cm
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3
Q

What is the mesentery?

A

Fold of membrane attached to the bowels
—> Suspends small and large bowel from posterior
abdominal wall (anchors)
—> Conduit for blood and lymphatic vessels

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4
Q

Which 5 blood vessels pass through the mesentery?

A
  1. Middle colic artery
  2. Right colic artery
  3. Ileocolic artery
  4. Superior mesenteric artery
  5. Jejunal and ileal arteries
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5
Q

What are the 4 layers of small bowel tissue?

A
  1. Serosa —> outermost
  2. Muscular - longitudinal
    - circular
  3. Submucosa
  4. Mucosa
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6
Q

How does the small bowel have such a large surface area?

A

Plicae circulares —> inner lining folds
- thicker in jejunum
Villi on plicae circulares
Microvilli on villi

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7
Q

What are villi?

A

Projections
- motile
- rich blood supply
rich lymph drainage
- innervated by submucosal plexus
- simple epithelium —> 1 cell thick
- separated by crypts

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8
Q

What are the 5 cell types in the small bowel?

A

Villi lining:
1. Enterocytes (most) —> absorption
2. Goblet cells —> secrete mucus
3. Enteroendocrine cells

Crypts of Lieberkühn lining:
4. Paneth cells
5. Stem cells (at bottom)

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9
Q

What are enterocytes?

A

Tall columnar cells for absorption and transport
- most abundant
- lifespan: 1-6 days
- villi and microvilli increase surface area from 0.4m2
to 200m2 (x500)

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10
Q

What are microvilli?

A

Projection on villi —> brush border
- 0.5-1.5µm tall
- 1000s per villi
- coated in glycocalyx

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11
Q

What is glycocalyx

A

Carbohydrate-rich coating of microvilli
- protection of microvilli from digestion in lumen
- traps water and mucus —> unstirred layer
- regulates rate of absorption

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12
Q

What are goblet cells?

A

Mucus-secreting cells —> mucus granules (apical side)
- 2nd most abundant
- mucus = large glycoprotein facilitating passage of
material through bowel
- abundance increases along bowel

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13
Q

What are enteroendocrine cells?

A

Hormone-secreting columnar epithelial cells
- usually in lower crypt
- hormones —> gut motility

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14
Q

What are paneth cells?

A

Immune cells at base of crypts
- granules - large + acidophilic
—> lysozyme (antibacterial) - protect stem
cells
—> glycoproteins and zinc - trace metal for
enzymes
- engulf some bacteria and protozoa
- may regulate gut flora

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15
Q

What are small bowel stem cells?

A

Undifferentiated cells that differentiate to replace cells that die (pluripotent)
- continually divide via mitosis
- cell dies —> digested and reabsorbed —> stem cell
differentiates —> migrates to top of villus

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16
Q

Why do enterocytes have rapid turnover?

A

Constantly damaged
- first line against pathogens
- toxic substances in diet
- issues with turnover —> severe intestinal
dysfunction

17
Q

What are Brunner’s glands?

A

Mucus glands secreting alkaline fluid
- in submucosal layer
- coiled and tubular
- open into base of crypts
- alkaline —> neutralises acidic chyme
- protects small bowel
- optimises pH for pancreatic enzymes

18
Q

What are the 3 differences between the jejunum and ileum?

A
  1. Jejunum —> thicker wall ∵ more + thicker plicae
    ciculares
  2. Ileum —> Peyer’s Patches
  3. Jejunum —> longer arterial arcades
19
Q

Why does the small bowel need motility? (3)

A
  1. Mix food with secretions and enzymes
  2. Facilitate contact between contents and mucosa
  3. Move contents along bowel
20
Q

What are the 3 types of small bowel motility?

A
  1. Segmentation —> mixing
    • stationary contraction of circular muscles at
      intervals (faster in duodenum)
    • pushes chyme in both directions but net
      movement to colon
  2. Peristalsis —> propelling
    • sequential contraction of adjacent rings of muscle
    • waves —> around 10cm
  3. Migrating motor complex
    • cycles of contractions from stomach —> small
      intestine —> colon (restart at duodenum)
    • prevents migration of colonic bacteria into ileum
21
Q

How does digestion occur in the duodenum?

A
  • alkaline environment
  • pancreatic juice in via MPD (main pancreatic duct)
    bile in via CBD (common bile duct)
  • enzymes from duodenal epithelium
22
Q

How are carbohydrates digested? (3)

A
  1. Mouth —> salivary α-amylase
    - little digestion ∵ destroyed by stomach acid
  2. Small bowel lumen —> pancreatic α-amylase
    - secreted into duodenum
    - conditions —> Cl- (optimum)
    —> neutral pH
  3. Brush border —> simple carb digestion
    —> absorption
    - glucose —> SGLT-1 (2 active t)
    galactose —> SGLT-1 (2 active t)
    fructose —> GLUT-5 (fac diff)
    - to blood via GLUT-2
  • absorb up to 10kg simple sugars/day
  • monosaccharides: glucose, fructose, galactose
    disaccharides: sucrose, maltose, dextrose
    complex: starch, cellulose, pectin
23
Q

How are proteins digested? (3)

A
  1. Stomach —> pepsin
    - inactivated in alkaline duodenum
  2. Small bowel lumen —> 5 proteases
    - trypsinogen to trypsin via
    enterokinase on duodenal
    brush border
    - trypsin activates others
  3. Brush border —> PepT1 (H+/oligopeptide co-t)
    - absorb small AA chains —> digested
    to single AAs via cytoplasmic
    peptidases in enterocytes
  • proteins hydrolysed to single amino acids and
    oligopeptides (AA)n
24
Q

How are lipids digested? (7)

A
  1. Emulsification: fat globule —> droplets
    • bile salts
    • pancreatic lipases
  2. Enzymatic hydrolysis
    • colipase complexes —> prevent bile salts
      displacing lipases
  3. Solubisation: produce micelles
  4. Enter enterocytes —> fatty acids and monglycerides
    released
  5. Triglyceride synthesis - monoglyceride acylation
    - phosphatidic acid pathway
  6. Chylomicron formation - in golgi
    - 80-90% triglyceride
    8-9% phospholipid
    2% cholesterol
    2% protein
    trace carbohydrate
  7. Exocytosis of chylomicron to lacteal
25
Q

What is the ileocaecal valve?

A

Valve separating ileum and colon
- muscles relax and contract —> control passage
—> prevent backflow

2.8 BRS Gastroenterology (13 decks)

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